Biochemistry

Dr, lJ, Satyanarayana

M . S c,.P h.D .,F.l .C .,F.A .C .B .
Professor of Biochemistry
Siddhartha Medical Colle g e
(NTR University of Health Sciences)

Vijayawada, 4.P., India

Dr, lJ, Chakrapani

M.B .B ,S .M, .S .

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Eiochemistrg

First Published : March 1999
Reprinted: 1999
RevisedReprint: August2000
Reprinted: 2OQO2, 001, 2QO2
Second Revised Edition : June 2002
Reprinted: 2003
RevisedReprint: 2004
RevisedReprint: 2005
Third Revised Edition (multicolour) : 2006
Revised Reprint : 2007

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Prefaceto the Third Edition

The responseto the first and the secondeditionsof my book 'Biochemistry('reprintedseveratlimesin
just 6 years)from the studentsand teachersis simplyoverwhelmingI. wasfloodedwith highly appreciative
lettersfrom all cornersof India and abroad!This givesme immensesatisfactionand encouragemLnint this
academicventure.

I havecorrespondewdith manybiochernistrtyeachersi,nvitingtheir commentsandopinionsfor further
improvingthe book.Most of them havebeenkind enoughto offer constructivesuggestionsI. alsovisited
severacl ollegesandhadpersonailnteractionwith facultymembersandstudentsT. heseexercisess,preadover
the past 6 years,have helpedme to get direct feedbackon my book, besidesrealisingthe additional
requirementsof students.

I havegreatpleasurein presentingthe third edition of my bookwith severalunique/novelfeatures,some
high-lightsof which are listedbelow.
. A thoroughrevisionand updatingof eachchapterwith latestadvances-
. Multicolouredillustrationsfor a betterunderstandingof chemicalstructuresand biochemicarleactions.
. Increasein the font sizeof the text for more pleasanat nd comfortablereading.
o Incorporationof a new Sectionon MolecularBiologyand Biotechnology.
. Additionof ten new chapters-humangenomeproject,genetherapy,bioinformaticsf,ree radicalsand

antioxidantst,issueproteinsand bodyfluids,environmentabl iochemistryg, eneticsi,mmunologyetc.
. An improvedorientationand treatmentof humanbiochemistryin healthand disease.
. Additionof practicalbiochemistryand clinicalbiochemistrylaboratoryin the appendix.

It is true that I represenat selectedgroupof individualsauthoringbooks,havingsometime at disposal,
besideshardwork, determinationand dedicationI. considermyselfan eternallearnerand a regularstudent
of biochemistryH. oweveri,t is beyondmy capabilityto keeptrackof the evergrowinagdvanceisn biochemistry
due to the exponentialSrowth of the subject.And this makesme nervous,wheneverI think of revisingthe
book.I honestlyadmit that I haveto dependon maturereadersfor subsequenetditionsof this book.
AN INVITATION TO READERS

It is not all the time possiblefor me to meetthe readersindividuallyandget their feedbackd,espitemy
ferventwish.Of course,I do write to somepeoplepersonaliyseekingtheir opinions.HoweverI, wish to have
the commentsandsuggestionosf eachoneof the readersof my book.I sincerelyinvitethe readersto feelfree
andwrite to me expressingtheir frank opinions,critical commentsand constructivesuggestions.

DT. U. SATYANARAYANA

trl

I owea deepdebt of gratitudeto my parents,the late Sri U. VenkataSubbaiah,and Smt. Vajramma,for
cultivatingin me the habit of earlyrising.Thewriting of this bookwould neverhavebeenpossiblewithout
this healthyhabit.I am gratefulto Dr. B. S. NarasingaRao(formerDirector,NationalInstituteof Nutrition,
Hyderabadf)or discipliningmy professionallife, and to my eldestbrother Dr. U. Gudaru(formerProfessorof
PowerSystemsW, alchandCollegeof EngineeringS, angli)for discipliningmy personalife.

My elder son, U. Chakrapan(i MBBS)deservesa specialplacein this book. He madea significant
contributionat everystageof its preparation-writing, verification,proof-readingandwhat not. I hadthe rare
privilegeof teachingmy sonashe happenedto be a studentof our collegeA. nda majorpart of this bookwas
writtenwhilehewaslearningbiochemistryT. hus,hewasthe first personto learnthe subjectof biochemistry
from my handwrittenmanuscript.The student-teacherelation (ratherthan the father-son)hashelpedme in
receivinScl onstantfeedbackfrom him and restructure the book in a way an undergraduatestudentwould
expecta biochemistrytextbookto be.

Next,I thank Dr. G. PitcheswaraRao(formerProfessorof Anatomy,SMC,Vijayawadaf)or his constructive
criticism and advice,and Dr. B. Sivakumar(Director,NationalInstitute of Nutrition, Hyderabadf)or his
helpful sugi5lestionosn the microfigures.I am grateful to my nephew,Mr. U. SrinivasaRao,for helping me
in drawingsomefigures.

Last but not least,I thank my wife Krishna Kumari and my youngerson, Amrutpani,without whose
cooperationand encouragementhis book could never have been written. The manuscriptwas carefully
nurturedlike a new born babyand the bookhasnow becomea full-pledgedmemberof our family.

ACKNOWLEDGEMENTTSO THE THIRD EDITION

I am indebtedto a largenumberof friends,pen-friendasndstudentswho helpedmeto reviseandimprove
the qualityof this book.I haveindividuallyandpersonallythankedall of them (whonumbera fewhundreds!).
I onceagainexpressmy gratitudeto them.

I thank my friend and colleagueM, r. M.S.T.JaganMohan,who has helpedme with his frequent
interactionsto improvethe book,andmakeit morestudent-friendlyI .wouldlike to placeon recordmy deep
senseof appreciatiotno my post-graduat(eM.D.)studentsD, r. (Mrs.)U.B.VijayaLakshmiandDr. (Mrs.)Vidya
DesaSi ripadw, hoseperiodicaal cademicinteractionandfeedbachkavecontributedto the improvemenot f the
biomedicaVclinicaasl pectsin somechaptersI. acknowledgtehe helpof my friend,Dr. P. Ramanujam(Reader
in English,AndhraLoyolaCollegeV, ijayawadafo) r his help and encouragemenint revisingthe book.

I expressmy gratitude to Mr. ArunabhaSen, Director, Books & Allied (P) Ltd. Kolkata,for his
wholeheartedsupportand constantencouragemenint revisingthe book in multicolour,and taking all the
painsto bring it out to my satisfactionI. thank Mr. ShyamalBhattacharyafor his excellentpage-makingand
graphics-workin the book.I am indebtedto Mr. PrasenjitHalderfor the coverdesignof this book.

I thank my wife, Krishna Kumari, and my younger son, Amrutpani, for their constantsupport and
encouragementI. am grateful to UppalaAuthor-PublisherInterlinks, Vijayawadaf,or sponsoringand
supportingme to bring out this edition.

DT. U. SAIYANARAYANA

Iiii]

Biochemistry

The term Biochemistrywas introducedby Carl Neubergin 1903.Biochemistrybroadlydealswith the
chemistrvof life and living processesT.hereis no exaggerationin the statement,'Thescopeof biochemistrg
is asuastaslilb itself !' Everyaspectof life-birth, growth,reproduction,aginganddeath,involvesbiochemistry.
For that matter, everymovementof life is packedwith hundredsof biochemicalreactions.Biochemistryis the
mostrapidlydevelopingandmostinnovativesubjectin medicineT. hisbecomesevidentfrom the factthat over
the years,the major shareof Nobel Prizesearmarkedfor Medicineand Physiologyhas gone to researchers
engagedir: biochemistry.

The discipline of biochemistryservesas a torch light to trace the intricate complexicitiesof biology,
besidesunravellingthe chemicaml ysteriesof life.Biochemicarlesearchhasamplydemonstratetdhat all living
things are closelyrelatedat the molecularlevel.Thusbiochemistryis the subjectof unity in the diversified
living kingdom.

Advancesin biochemistryhavetremendousimpacton humanwelfare,andhavelargelybenefitedmankind
and their living styles.Theseincludethe applicationof biochernistryin the laboratoryfor the diagnosisof
diseasesth. e products(insulin,interferon,€rowthhormoneetc.)obtainedfrom geneticengiineeringa,ndthe
possibleuseof genetherapyin the near future.

0rganizationof the Book

This texthook,comprising43 chapters,is orgianizedinto serrensecl:ionsin the heirarchicalorder of
learninSbiochemistry.

. SectionI dealswith the chemicalconstituentsof life-carbohydratesl,ipids,proteinsand amino acids,
nucleicacidsand enzymes.

. SectionII physiologicaclhemistryincludesdigestionand ahsorptionp, lasmaproteins,hemoglobinand
prophyrinsa, nd biologicaloxidation.

. SectionIII incorporatesall the metabolisms(carbohydratelsip, ids,aminoacids,nucleotidesm, inerals)
. Section[V covershormones,organfunction tests,water,electrolyteandacid-basebalancet,issueproteins

and trodi'fluids,and nutrition.
. Section V is exclusivelydevotedto molecularbiologyand biotechnology(DNA-replicationr,ecombination,

ar"lnrepairt,ranscriptionandtranslationr,egulationof geneexpressionre, combinanDt NAandbiotechnology)
. Section VI givesrelevantinformation on current topics such a^shuman genomeproject, genetherapy,

bioirrtormaticsp,rostaglandinsd,iabetesc,ancerA, IDSetc.
. Section VII deals with the basic aspectsfor learning and understandingbiochemistry (bioorganic

chenristryh',iophysicacl hemistrytoolsof biochemistryg, eneticsi,mmunology).
Each chapter in this book is carefully craftedwith colour illustrations, headingsand subheadingsto
facilitatequickunderstandingT.heimportantapplicationosf biochemistryto humanhealthanddiseasaereput
togetheras biomedical/clinicacl oncepts.Icons are usedat appropriateplacesto serveas 'landmarks'.
The origins of biochemicalwords, confusablesin biochemistry,practicalbiochemistryand clinical
biochemistrylaboratory,given in the appendixare novel features.
The briokis so organizedas to equipthe readerswith a comprehensivkenowledgeof biochemistry.

Iiu]

Gontents

SECTION ONE SECTION FIVE
ChemicalConstituentsof Life
MolecularBiologyand Biotechnology
1 > Biomolecualnedsthecell
2 > Carbohydrates 3 24 > DNA-replicarteiocno,mbinaatinodnrepai5r23
3 > Lioids 9
4 > Proteinasndaminaocids 28 25 > Transcriotaionndtranslation 542
5 > Nucleaiccidsandnucleotides 43
6 > Enzymes 69 26 > Regulatioofngeneexpression 566
7 > Vitamins 85
176 27 b RecombinDaNntAandbiotechnolog5y78
SECTION TWO
PhysiologicaBl iochemistry 165 sEcTtcN stx
182
B > Digestioanndabsorption 196 Current Topics
9 > Plasmoaroteins 221
10 > Hemogloabnindporphyrins 28 > Humangenomeproject 619
11 > Biologicaloxidation
29 > Genetherapy 625
SECTION THBEE
30 F Bioinformatics 634

31 p 'lvletaboliosfmxenobioti(cdsetoxificatio6n3)8

32 >' Prostaglandainnsdrelatedcompounds644

33 > Biologicmalembraneasndtransport 650

34 b Freeradicalasndantioxidants 655

35 > Environmenbtaiolchemistry 662

36 l" Insuling,lucosehomeostasis,

anddiabetemsellitus 669

37> Cancer 58s

38> Acquireimd munodeficiency

syndrom(AeIDS) 695

q3 > Metaboliosfmcarbohydrates 241 SECTION SEVEN
244
*4 > Metaboliosfmlioids 285 Basicsto LearnBiochemistrv
330.
F-, Metabolisomfaminoacids 380 39 > Introducttiobnioorgancihcemistry 703
16 > Int6gratioofnmetabolism 387
403 40 > Overvieowf biophysiccahlemistry 708
17 > Metaboliosfmnucleotides
427 41 > Toolsofbiochemistrv 719
1B > Mineramletabolism 453
42 > lmmunology 732
SECTION FOUR 468
Clinical Biochemistrvand Nutrition 487 43 > Genetics 737
502
19 > Hormones APPENDICES 745
20 > Orgafnunctiotensts AnswetrosSelf-assessmEexenrl cises 751
21 > Wateer,lectrolyatned I Abbreviatiuosnesdinthisbook' 756
ll Greeaklphabets
acid-basbeqlance lll Originosl importabnitochemiwcaolrds tJt
22 > Tissuperoteinasndbodyfluids lV Commocnonfusabilnebsiochemistry
23 > Nutrition- V Practicbailochemistry-principles 760
Vl Clinicabliochemislatrbyoratory 764
770

INEEX 773

fi Protuinsand Amino acids 4:
Nucleic acidsand Nucleotides 69

BflomnoXeaeiuldttrhsssCelll

-l- hu living matter is composedof mainly six organiccompounds.lt is believedthat man may

I elements-carbon, hydrogen, oxygenl contain about 100,000 different types of

nitrogen, phosphorus and sulfur. Theseelements molecules although only a few of them have

togetherconstituteabout 90% of the dry weight been characterized.

of the human body. Severalother functionally
importantelementsare also found in the cells. Sornpiex *riomoleeules

Theseinclude Ca, K, Na, Cl, Mg, Fe,Cu, Co, l, The organiccompoundssuch as amino acids,

Zn, F, Mo and Se. nucleotidesand monosaccharidesserve as the

monomeric unitsor building blocksof complex

earbon-a unique element of life biomolecules-proteins,nucleicacids(DNA and

Carbon is the most predominantand versatile RNA) and polysaccharides,respectively.The
elementof life. lt possesseas unique propertyto important biomolecules(macromoleculesw) ith
form infinite number of compounds. This is their respective building blocks and major
attributedto the ability of carbon to form stable functions are given in Table 1.1. As regards
covalentbonds and C-C chainsof unlimited lipids, it may be noted that they are not
length. lt is estimated that about 90% of biopolymersin a strict sense,but majority of
compounds found in living system invariably them containfatty acids.

contain carbon. Structural heirarehy off asn organisnl

Ghemical molecules of li#e The macromolecules(proteins,Iipids,nucleic
acidsand polysaccharidesf)orm supramolecular

Life is composed of lifeless chemical assembl i es(e.g. membranes)w hi ch i n tu r n

molecules. A single cell of the bacterium, organize into organelles,cells, tissues,organs

Escherichiacoli contains about 6.000 different and fi nal l y the w hol e organi sm.

3

BIOCHEMISTFIY

Biomolecule Building block Major functions
1. Protein (repeatingunit)
Fundamenbtasl isofstructuraend
Aminoacids functioonfcell(statiacnddynamficunctions).

2. Deoxyribonucalecidc(DNA) Deoxyribonucleotides fl_eq_oo_.l9s.!i1to9iFr9ryyi{9l1llgt

3. Ribonucleaiccid(RNA) Ribonucleotides Essentiarellqyuirelodrprotebiniosynthesis.
Storagfeormofenergtyomeest hortterm
4. Polysaccharide(glycogen) Monosaccharid(gelsucose) demands.
Storagtoermofenergtyomeeltongterm
5. Lipid Fattyacidsg, lycerol demandsst;ructurcaolmponeonftsmembranes.

Chem*ca! composition of man Prokaryotic and eukaryotic cells

The chemical compositionof a normal man, The cells of the living kingdom may be

weighing65 kg, is given in Table 1.2.Water is divided into two categories

the solventof life and contributesto more than 1. Prokaryotes(Creek : pro - before;karyon-
60"h of the weight. This is followed by protein nucl eus)l ack a w el l defi nednucl eusand possess
( m os t lyin mu s c l e )a n d l i p i d (mo s tl yi n a d i pose relatively simple structure.These include the

tissue).The carbohydratecontent is rather low variousbacteria.
which is in the form of glycogen.

2. E ukaryotes(Greek: eu-true; karyon-

nucleus)possessa well defined nucleusand are

more complex in their structureand function.
The hi gher organi sms(ani mal sand pl ants)are

The cell is the structuraland functional unit composedof eukaryoticcells.
of life. ft may be also regardedas the basic unit
of hiological activity. A comparisonof the characteristicsbetween
prokaryotesand eukaryotesis listed in Table 1.3.

The concept of cell originated from the
c ont r ibut ion so f S c h l e i d e na n d S c h w a n n(1838).
However, it was only after 1940, the
complexitiesof cell structurewere exposed.

Constituent Percent(7") Weight (kg) The human body is composedof about 1014
Water 61.6 40 cells.There are about 250 typesof specialized
Protein 17.0 11 cel{s in- the human body'G.g. erythrocytes,
nerve-cells, muscle cells, B cells of pancreas.
A n eukaryoti ccel l i s general l y10 to 100 pm
in diameter. A diagrammatic representation
of a typical rat liver cell is depicted in

Fig.I.t.

Lipid 13.8 I The plant cell differsfrom an animal cell by
Carbohydrate 6.1
Minerals '| possessinga rigid cell wall (mostlycomposedof

4 cellulose)and chloroplasts.The latter are the
sitesof photosynthesis.

Chapter 1 : BIOMOLECULEASND THE CELL

Characteristic Prokaryotic cell Eukaryoticell
1. Size Smal(lgeneral1ly-10pm)
2. Celml embrane Large(general1ly0-10p0m)
3. Sub-cellular Notweldl efinedD; NAisfound Celilsenvelopebdyaflexiblpelasmma embrane
asnucleoihdi,stoneasreabsent Distinoctrganellaersefound
organelles Mitochondaribasenet,nzymeosf (e.9m. itochondnriuac,leulsy,sosomes)
4, Nucleus energmy etabolisbmountdo Nucleuissweldl efinesdu,rroundbeyda
membrane membranDeN: Aisassociatewdithhistones
5. Energmy etabolism Usuallfyissionandnomitosis Enzymeoslenergmy etabolisamrelocated
0rganelleasndcytoskeleton inmitochondria
6. Celldivision absent
7. Cytoplasm Mitosis
Containosrganelleasndcytoskeleton
(anetwoorkftubuleasndfilaments)

T he c ell c o n s i s tso f w e l l d e fi n e dsu b cel l ul ar N ucl eus

organelles,enveloped by a plasma membrane.
By differential centrifugation of tissue Nucleus is the largestcellular organelle,

homogenate, it is possible to isolate each surroundedbv a double membrane nuclear

cellular organelle in a relatively pure form envelope.The outer membraneis continuous
(Refer Chapter 41). The distribution of major w i th the membranesof endopl asmi creti cul um .

enzymes and metabolic pathways in different At certainintervals,the two nuclearmembranes
c ellular org a n e l l e s i s g i v e n i n th e chapter have nuclearporeswith a diameterof about 90
on enzymes (Refer Fig.6.6). The subcellular nm. Thesepores permit the free passageof the
organellesare briefly describedin the following products synthesizedin the nucleus into the

pages. surroundni g cytopl asm.

Roughendoplasmicreticulum Mitochondrion
Golgiapparatus Plasmamembrane
Vacuole

Ribosomes

Lysosome Peroxisome
Cytoskeleton
Cytosol

Coatedpits

Ftg. 1.1: Diagrammaticrepresentationof a nt liverell.

BIOCHEMISTF|Y

Nucleus contains DNA, the repository of acid cycle, p-oxidation).The matrix enzymes

genetic information. Eukaryotic DNA is also parlicipate in the synthesisof heme and

associatedwith basic protein (histones)in the urea. Mitochondria are the principal producers

ratio of 1 : 1, to form nucleosomesA. n assembly of ATP in the aerobic cells. ATP, the energy

of nucleosomesconstitutes chromatin fibres of currency,generatedin mitochondriais exported

chromosomes(Creek'.chroma - colour; soma - to all partsof the cell to provide energyfor the

body). Thus, a single human chromosomeis cellularwork.

c omoo s e do f a b o u t a m i l l i o n n u c l e osomesT. he The mi tochondri almatri xcontai nsa ci rcul ar

number of chromosomes is a characteristic double stranded DNA (mtDNA), RNA and
feature of the species. Humans have 46 ribosomesT. hus,the mitochondriaare equipped
chromosomesc, ompactlypackedin the nucleus. with an independent protein synthesizing

The nucleusof the eukaryoticcell containsa machinery.It is estimatedthat about 10% of the
dense bodv known as nucleolus.lt is rich in mitochondrial oroteins are produced in the

RNA, particularlythe ribosomal RNA which mitochondria.

entersthe cytosol through nuclear pores. The structureand functions of mitochondria

The ground materialof the nucleus is often closely resemble prokaryotic cells. lt is

referredto as nucleoplasm. lt is rich in enzymes hypothesizedthat mitochondria have evolved

s uch a s D N A p o l y me ra s e s and R N A from aerobicbacteria.Further,it is believedthat

polymerasesT. o the surpriseof biochemistst,he during evolution,the aerobicbacteriadeveloped

enzy m e s o f g l y c o l y s i s ,c i tri c a ci d cycl e and a symbi oti c rel ati onshi p w i th pri mor dial

hexose monophosphateshunt have also been anaerobiceukaryoticcells that ultimatelyled to

detectedin the nucleoplasm. the arrivalof aerobiceukaryotes.

Mitochondria Endoplasmic reticulum

The mitochondria (Creek'. mitos- thread; The network of membraneenclosedspaces
chondros- granule) are the centres for the that extends throughout the cytoplasm
c ell ula rre s p i ra ti o na n d e n e rg yme ta bol i smT. hey c o n s t i t u t e se n d o p l a s m i cr e t i c u l u m ( E R ) .S o m e o f
are regarded as the power housesof the cell these thread-like structuresextend from the
wit h v a ri a b l es i z e a n d s h a p e .Mi t o chondri aare nuclearporesto the plasmamembrane.

rod-like or filamentousbodies, usuallv with A large portion of the ER is studdedwith
ribosomesto give a granularappearancewhich
dimensions of 1.0 x 3 pm. About 2,0O0 is referred ro as rough endoplasmic reticulum.
mitochondria,occupyingabout 1/5thof the total Ribosomes are the factories of protein

c ell v o l u me , a re p re s e n ti n a ty p i ca l cel l .

The mitochondriaare comoosedof a double biosynthesis. During the process of cell
membrane system. The outer membrane is fractionation,rough ERis disruptedto form small
smooth and completelyenvelopsthe organelle. vesiclesknown as microsomes. It may be noted
The inner membrane is folded to form cristae that microsomesas such do not occur in the

(Latin- crests)which occupy a larger surface cel l .

area.The internalchamber of mitochondriais The smooth endoplasmicreticulum does not

referred to as matrix or mitosol. contain ribosomes.lt is involvedin the synthesis

The componentsof electron transportchain of lipids (triacylglycerolsp,hospholipids,sterols)
and oxidative phosphorylation (flavoprotein, and metabolismof drugs,besidessupplyingCa'?.
c y t oc h ro m e sb , c 1 , C , a a n d a 3 a nd coupl i ng for the cel l ul arfuncti ons.

factors)are buried in the inner mitochondrial
membrane.The matrixcontainsseveralenzvmes Golgi apparats,r$

concerned with the energy metabolism of E ukaryoti ccel l s contai n a uni que cl ust erof

c ar boh y d ra te sl i,p i d sa n d a m i n oa c i d s(e.g.,ci tri c membrane vesicles known as dictyosomes

Chapter 1 : BIOMOLECULEASND THE CELL

whic h, in turn , c o n s ti tu teC o l g i a p p a ra tus(or The pH of the lysosomaml atrix is more acidic
Colgi complex).The newly synthesizedproteins (pH < 5) than the cytosol (pH-7) and this

are handed over to the Colgi apparatuswhich facilitatesthe degradationof differentcompounds.

catalysethe addition of carbohydratesl,ipids or The lysosomal enzymes are responsible for

sulfatemoietiesto the proteins.Thesechemical maintaining the cellular compounds in a dynamic

modificationsare necessaryfor the transportof stafe, by their degradationand recycling.The

proteinsacrossthe plasma membrane. degradedproductsleave the lysosomesu, sually

Certainproteinsand enzymesare enclosedin by diffusion, for reutilization by the cell.
membrane vesiclesof Colgi apparatusand Sometimes,however, certain residualproducts,
secreted from the cell after the appropriate rich in lipids and proteins,collectivelyknown as
signals.The digestiveenzymesof pancreasare Iipofuscinaccumulatein the cell. Lipofuscinis
oroducedin this fashion. the agepigmentor wear and tear pigmentwhich
has been implicatedin ageingprocess.

Colgi apparatusare also involved in the The digestiveenzymesof cellularcompounds
membrane synthesis, particularly for the are confinedto the lvsosomesin the best interest
formation of intracellular organelles (e.g.
of the cell. Escapeof theseenzymesinto cytosol
p e r o x i s o m e s l,y s o s o m e s ) . will destroythe functionalmacromoleculeosf tne

Lysosornes cel l and resul t i n many compl i cati ons.The
occurrence of several diseases(e.g. arthritis,

Lysosomesare sphericalvesiclesenveloped musclediseasesa,llergicdisordersh) asbeenpartly

by a single membrane.Lysosomesare regarded attributedto the releaseof lysosomael nzymes.

as the digestivetract of the cell, since they are
actively involved in digestion of cellular Feroxisomes

substances-namely proteins, lipids, carbo- Peroxisomes,also known as microbodies, are
hydratesand nucleic acids. Lysosomael nzymes si ngl e membranecel l ul ar organel l esT. hey are
are categorizedas hydrolases.These include the spherical or oval in shape and contain the
following enzymes(with substratein brackets) enzyme catalase.Catalaseprotectsthe cell from
the toxic effectsof HrO, by converting it to HrO
a-C lucosidase(glycogen)

C a t h e p s i n s( p r o t e i n s ) and Or. Peroxisomesare also involved in tne
Lip a s e s(l i p i d s ) oxidation of long chain fatty acids (> C,s),and
Rib o n u c l e a s e(sR N A ) synthesisof plasmalogenas nd glycolipids.Plants
contain glyoxysomes, a specialized type of

BTOMED|eAL/ CLINICAL COIUCEPTS

A liuing cell is a true representotiueof life with its own organizotion and specialized

lunctions.

Accumulotion oJ lipofuscin,a pigment rich in lipids and proteins, in the cell hasbeen
implicated in ogeing process.

Leokageof lysosomalenzymesinto the cell degrodesseuerolfunctional macromolecules
and this may leod to certain disorders (e.9. arthritis).

rq Zellweger syndrome is a rare diseose characterized by the absence of functional
peroxisomes.

E}IOCHEMISTF|Y

peroxisomes, which are involved in the three types- microtubules, actin filaments and

glyoxylate pathway. intermediatefilaments.The filamentswhich are

Peroxisome biogenesis disorders (PBDs), are polymers of proteins are responsiblefor the
a Broup of rare diseasesinvolving the enzyme structure,shapeand organizationof the cell.
activities of peroxisomes. The biochemical
abnor m ali ti e sa s s o c i a te dw i th P BD s i ncl uoe INTEGRATIOI{ OF
increasedlevels of very long chain fatty acids CELLULAR FUNCTIONS
(C2aand C26)and decreasedconcentrationsof
plasmalogensT. he most severeform of PBDs is The eukaryoticcells performa wide rangeof
Zellweger syndrome, a condition characterized complex reactionsfunctionsto maintaintissues,
by the absenceof functional peroxisomesT. he and for the ultimatewell-beingof the whole
v ic t im sof th i s d i s e a s em a v d i e w i th i n o ne vear organism. For this purpose, the various
after birth. intracellularprocesseasnd biochemicalreactions
are tightly controlledand integrated.Divisionof
a cel l i nto tw o daughtercel l si s good exampl eof

{iytosol and cytoskeleton the orderly occurrenceof an integratedseriesof

The cellular matrix is collectively referredto cellularreactions.
as cytosol. Cytosol is basicallya compartment Apoptosisis the programmedcell death or

containing several enzymes/ metabolitesand cell suicide. This occurs when the cell has

saltsin an aqueousgel like medium.More recent ful fi l l ed i ts bi ol ogi calfuncti ons.A poptosi smay

studies however, indicate that the cytoplasm be regardedas a natural cell death and it differs

actuallycontainsa complex network of protein from the cell death caused by injury due to

filaments, spread throughout, that constitutes radiation,anoxia etc. Programmedcell death is

cytoskeleton.The cytoplasmicfilaments are of a highly regulatedprocess.

1. Life is composed ol lifeless chemical molecules. The complex biomolecules, proteins,

nucleic ocids (DNA and RNA), polysaccharidesand lipids are formed by the monomeric
units amino acids,nucleotides, monosaccharideas nd fotty acids,respectluely.

2 . The cell is the structuroland functional unit of life. The eukoryoticcell consisfsof well

det'inedsubcellulor organelles,enuelopedin a plasma membrane.

3 . The nucleus contoins DNA, the repositoryol genetic int'ormation.DNA, in association

with proteins (histones)f,orms nucleosomeswhich, in turn, make up the chromosomes.
The mitochondria qre the centresfor energy metobolism. They are the principal producers
of ATP which is exported to all parts of the cell to ptouide energy lor cellular work.
5. Endoplosmic reticulum (ER) ts the network of membrane enclosed spoces that extends

throughout the cytoplosm. ER studded with ribosomes, the factorles of protein
biosynfhesis, ts relerred to as rough ER. Golgi opparatus sre a cluster of membrane

uesiclesto uthich the newlg synthesizedproteins are handed ouer for t'urther processing
ond export.

6. Lysosomesare the digestiue bodies ol the cell, actiuely involued in the degradotion of

cellular compounds. Peroxisomescontoln the enzymecatalose that protects the cell lrom
the toxic elfects of HrOr. The cellular ground motrix is referred to as cytosol which, in

fact, is composed of a network ot' protein t'ilaments, the cytoskeleton.
7 . The eukaryoticcellsperform a wide rangeof complex lunctionsin a well coordinatedand

integrated fashion. Apoptosis is the processol programmed cell death or cell suicide.

1^ arbohydratesare the most abundant organic 1. They are the mostabundantdietarysource

\ - m o lec u l e s i n n a tu re . T h e y a re p ri mari l y of energy (a Cal/S)for all organisms.

composedof the elementscarbon, hydrogen and 2. Carbohydratesare precursorsfor many

oxygen.The name carbohydrateliterally means organic compounds(fats,amino acids).
'hydratesof carbon'. Someof the carbohydrates
3. Carbohydrate(sasglycoproteinsand glyco-
pos s es sthe e mp i ri c a lfo rmu l a (C .H 2 O)nw here
n 3 3, satisfyingthat these carbohydratesare in lipids) participate in the structure of cell
fact carbon hydrates.However,there are several membraneand cel l ul ar functi onssuch as cel l
non-carbohydratecompounds (e.g. acetic acid, growth, adhesionand fertilization.

C2HaO2;lacticacid, C3H6O3)which alsoappear 4. They are structuralcomponentsof many

as hydrates of carbon. Further, some of the organi smsT. hesei ncl udethe fi ber (cel l ul oseo) f
genuine carbohydrates (e.g. rhamnohexose, plants,exoskeletonof some insectsand the cell

C6H12O5ideoxyriboseC, 5H16Oad) o not satisfy w al l of mi croorgani sms.

the generalformula.Hencecarbohydratecsannot 5. Carbohydratesalso serve as the storage

be alwaysconsideredas hydratesof carbon. form of energy(glycogent)o meetthe immediate

Carbohydrates may be defined as energydemandsof the body.

polyhydroxyaldehydes or ketones or compounds
which produce them on hydrolysis. The term CLASSIFICATION

'sugar' is applied to carbohydratessoluble in OF GARBOHYDRATES

water and sweet to taste. Carbohydrates are often referred to as

#-ur*c;tEerEsof earbohydrates saccharides (Greek: sakcharon-sugar).They
are broadly classifiedinto three major groups-

Carbohydratesparticipatein a wide rangeof monosaccharides, oligosaccharides and

functions polysaccharidesT. his categorizationis basedon

t0 BIOCHEMISTRY

Monosaccharides(empirical formula) AIdose Ketose

Triose(sCgHoOg) Glyceraldehyde Dihydroxyacetone
Telrose(Cs +HoO+) Erythrose Erythrulose
Pentos(eCssHroOs) Ribose Ribulose
Hexose(Cs oHrzOo) Glucose Fructose
Heptose(Cs zHr+Oz) Glucoheptose Sedoheptulose

the number of sugar units. Mono- and oligo- liberatedon hydrolysis.Basedon the numberof
saccharidesare sweet to taste, crystalline in monosaccharide units present, the oligo-
characterand soluble in water, hence thev are saccharides are further subdivided to
commonly known as sugars. disaccharides,trisaccharidesetc.

FJtonosaccharides Polysace harides

Monosaccharide(sGreek: mono-one)are the Polysacchari6ls(Creek:poly-many)are poly-
simplestgroup of carbohydratesand are often mers of mondficcharide units with high mole-
referredto as simple sugars.They have the cul ar w ei ght (up to a mi l l i on).They are usual l y
generalformula Cn(H20)n,and they cannot be tasteless(non-sugars)and form colloids with
further hydrolysed. The monosaccharidesare water. The polysaccharidesare of two types-
divided into different categories,based on the homopolysaccharidesand heteropolysaccharides.
functionalgroupand the numberof carbonatoms

Aldoses : When the functional group in

IH \

monosaccharidesis an aldehydel-C:oi, ,h"u

are known as aldoses e.g. glyceraldehyde,

g lu c o s e . Stereoisomerismis an importantcharacterof

Ketoses: When the functionalgroup is a keto monosaccharides. Stereoisomers are the
compounds that have the same structural
\
lt group, they are referred to as ketoses formulaebut differ in their spatialconfiguration.
\-C:O.l

e.g. dihydroxyacetonef,ructose. A carbon is said to be asymmetric when it is

Basedon the number of carbon atoms,the attached to four different atoms or groups. Ihe
monosaccharidesare regarded as trioses (3C), number of asymmetric carbon atoms (n)
tetroses (4C), pentoses (5C), hexoses (6C) and determines the possible isomers of a given
heptoses(7C).Thesetermsalong with functional compound w hi ch i s equal to 2n. C l ucose
groupsare usedwhile namingmonosaccharides. contains4 asymmetriccarbons,and thus has 16

For instance, glucose is an aldohexose while tsomers.

fructose is a ketohexose(Table 2,1). Glyeeraldehyde

T he c om mo nm o n o s a c c h a ri d easn d d i sa ccha- -tfu e ref erqlrt*e cff rb$hyd$'er'&€3
rides of biological importanceare given in the

Table 2.2. Clyceraldehyde(triose)is the simplestmono-

SSlgosaccharides saccharidewith one asymmetriccarbon atom. lt
existsas two stereoisomerasnd hasbeen chosen

Oligosaccharides(Creek: oligo-few) contain as the referencecarbohydrateio representthe

2- 1O m ono s a c c h a ri d em o l e c u l e s w h i ch are structureof all other carbohvdrates.

Ghapter 2 : CARBOHYDRATES 11

Trioses i Glyceraldeh3y-dpehosphiastaenintermediate
Glyceraldehyde Foundincellsasphosphate i inglycolysis

DihydroxyacetoneFoundincellsasphosphate i ttst -pnosphaistaenintermediiantgelycolysis
Tetroses
----t ------..-.. -. --...---
D-Erythrose i Widespread

Pentoses

D-Ribose I Widespreaasdaconstitueonf t ForthestructuorefRNAandnucleotide
I RNAandnucleotides coenzym(eAsTPN, AD+N,ADP+)
ForthestructuorelDNA
D-Deoxyribose i AsaconstitueonfDt NA Itisanimportamntetaboliintehexose
monophospshhautent
D-Ribulose : Producdeudrinmg etabolism Involveindthefunctioonfglycoproteins

D-Xylose i Asaconstitueonfgt lycoproteins Excreteindurineinessenlipael ntosuria
i anogums Asaconstitueonltlvxollavoinfhearmt uscle

L-Xylulose i ls anintermediiantueroniaccidpathway The'sugafur elo' flifee; xcreteindurinien
diabeteSs.tructuruanl itofcellulosienplants
D-Lyxose i Hearmt uscle
Hexoses Convertteodglucosfea,ilurleeadtso
i --. --. -- --.. ---.. -.. -. --. galactosemia
Forthestructuorefpolysaccharides
D-Glucose Asaconstitueonltpolysaccharides
(starcghl,ycogecne,llulosaen)d Itsphosphataerseintermediaotefgslycolysis
disacchari(dmesaltoslea,ctose,
Its7-phosphaitseanintermediaitnehexose
sucroseA)l.sofoundinfruits monophosphashteunta, ndinphotosynthesis

D-Galactose Asaconstitueonfltactose
(milskugar)

D-Mannose Founidnplanpt olysaccharides
andanimagllycoproteins

D-Fructose Fruitasndhoneya,saconstituent
ofsucrosaendinulin

Heptoses

D-SedoheptuloseFoundinolants

Disaccharides Occurrence Biochemical importance

Sucrose Asaconstitueonftcanesugaar nd Mosctommonulsyedtablseugasrupplying
Lactose b e est u g apr ,i n e a p p l e calories
Milksugar
Maltose Exclusicvaerbohydrsaoteurcteobreasfet d
Produocftstarchydrolysis, infantsL.actasdeeficien(clayctosinetolerance)
occurisngerminatsinegeds leadtsodianheaandflatulence

Animportainnttermedinaltheedigestiofn
starch

12 E}IOCHEMISTFIY

H-C:O H-C:O to compoundsthat respectivelyrotatethe plane
I HO-C-H of polarizedlight to the right or to the left.

H-C-OH

cH2oH cH2oH An optical isomer may be designatedas

D-Glyceraldehyde L-Glyceraldehyde D(+), D(-), L(+)and L(-) basedon its structural
relation with glyceraldehyde.lt may be noted

H-C:O H-C:O that the D- and L-configurationsof sugarsare
I I primarily based on the structure of
glyceraldehyde,the optical activities however,
H-C-OH HO-C-H may be different.
I H-C-OH
I Racemic mixture : lf D- and L-isomersare
HO-C-H presentin equal concentration,it is known as
I HO-C-H racemi cmi xtureor D L mi xture.R acemi cmi xture
HO-C-H
H-C-OH
I cH2oH

H-Q-OH

I

cHzoH

D-Glucose L-Glucose does not exhibit any optical activity, since the

Fig.2.1 : DandL- forms of glucosecompared with dextro- and levorotatorv activities cancel each
D and L- glyceraldehydes (the reference carbohydrate). other.

Configuration of D-aldoses

D" and L-isomers The configuration of possible D-aldoses

The D and L isomersare mirror imagesof starting from D-glyceraldehydeis depicted in
each other. The spatialorientationof -H and Fig.2.2. This is a representation of Killiani-
-OH groups on the carbon atom (Cs for Fischersynthesisb, y increasingthe chain length
glucose)that is adjacentto the terminal primary of an aldose,by one carbon at a time. Thus,
startingwith an aldotriose(3C),aldotetrose(s4C),
alcohol carbon determineswhether the sugar is aldopentoses(5C) and aldohexoses(6C) are
D- or L-isomer.lf the -OH group is on the right formed. Of the 8 aldohexosesg, lucose,mannose
side, the sugar is of D-series,and if on the left and galactoseare the most familiar. Among
these, D-glucose is the only aldose mono-
side, it belongs to L-series.The structuresof saccharidethat predominantlyoccurs in nature.
D- and L-glucosebasedon the referencemono-
saccharide, D- and L-glyceraldehyde (glycerose)

are depictedin Fig.2.1. Gonfiguration of D-ketoses

It may be noted that the naturallyoccurring Startingfrom dihydroxyacetone(triose),there
monosaccharideisn the mammaliantissuesare are five keto-sugarswhich are physiologicallr
mostlyof D-configurationT.he enzymemachinery important.Their structuresare given in Fig,2.3
of cells is specific to metaboliseD-seriesof

monosaccharides. Epimers

fn the medical practice, the term dextroseis ff two monosaccharides differ from eac-
used for glucosein solution. This is becauseof other in their configuration around a singk
the dextrorotatorynature of glucose. specificcarbon (otherthan anomeric)atom. L*ei

Optlcal activity of sugars are referred to as epimersto each orher '.Fig,21
For instance, glucoseand galactose are efilwl

Optical activity is a characteristicfeature of with regard to carbon 4 (Ca-epimer-s -^:i 's
compounds with asymmetric carbon atom. they differ in the arrangementof -OH g.'ELcr
When a beam of polarized light is passed Ca. Clucose and mannose are epi-'e--' q drl
througha solutionof an optical isomer,it will be
regardto carbon 2 (C2-epimers).

rotated either to the right or left. The term The interconversionof epimers e I r::r'e
dextrorotatory (+) and levorotatory (-) are used to galactose and vice versai s i-- -^,-' a*

Ghapter 2 : CABBOHYDFATES 13
Aldotriose
cHo
(3c)
I
HCOH

I
cH2oH

Aldotetroses
(4c)

D-Erythrose D.Threooe

cHo cHo cHo cHo toses
)
HCOH HOCH I I
I HCOH HOCH
HCOH
I HCOH I I Aldo
HOCH HOCH
HCOH HCOH
cH2oH cH2oH I I
D-Ribose HCOH
D-Arabinose HCOH
cH2oH I
D-Xylose cH2oH

I D-Lyxoee

/\ cHo J/ \T cHo *//+\\ cHo cHo cHo

JT cHo HOCH cHo
HOCH
cHo HOCH HCr lOH I II
HCOH HCOH HOCH HCOH HOCH
tl HCOH II I Aldo-
HCOH HCOH HOCH I HCOH HOCH HOCH
HCOH tl cH2oH HCOH I he(x6ocse)s
HCOH HCOH HOCH HoCH noCH
tl tl I tl
HCOH HCOH HCOH HOCH HCOH HCOH HCOH
ll
tl cH2oH cH2oH I I tt
HCOH HCOH cH2oH cHzoH cHzoH

ll cH2oH
cH2oH

D-Allose D-Altrose D-Glucose D-Mannose D-Gulose D-ldose D-Galactose D-Talose

Fig.2.2 : ThestructuralrelationshipbetweenD-aldosesshownin Fischerprojection.
(TheconfigurationaroundC2(ed) distinguishesthemembersof eachpair).

epimerization, and a group of enzymes- The term diastereomersis used to represent
namely-epimerases catalysethis reaction. the sfereoisomers that are not mirror imagesof
one another.
Enantiomers

Enantiomers are a special type of For a better understanding of glucose
stereoisomers that are mirror images of structure, let us consider the formation of
each other. The two membersare designatedas hemiacetals and hemiketals, respectively
D- and L-sugars.Enantiomersof glucose are producedwhen an aldehydeor a ketone reacts
depicted in Fig.2.5. with alcohol.

Majority of the sugarsin the higher animals
(includingman) are of D-type (Fig.2.5'1.

14 E}IOCHEMISTRY

?H2oH cH2oH cH20H cH2oH cH2oH

C:O I I I I
I
cH2oH C:O C:O C:O C:O
I I
HOCH HCOH HOCH HOCH
I I
HCOH HCOH HCOH
HCOH I
I I HCOH
I I
cH2oH cH2oH HCOH
HCOH I
Dlhydroxyacetone D-Xylulose D-Ribulose I
cH2oH cH2oH

D-Fructose D-Sedoheptulose

Fig.2.3 : Structuresof ketosesof physiologicalimportance.

,H Rr- Anomers-nrutarotation
nt-C.1^ + R2-oH l- Hemiacetal
The a and p cyclic forms of D-glucose are
LJ known as anomers.Thev differ from each other
in the configurationonly around C1 known as
Aldefry<b Alcohol

The hydroxyl group of monosaccharidecsan anomericcarbon (hemiacetacl arbon).In caseof

reactwith its own aldehydeor keto functional o anomer,the -OH group held by anomeric

group to form hemiacetaland hemiketal.Thus, carbon is on the opposite side of the group

the aldehydegroup of glucoseat C1 reactswith -CH2OH of sugar ring. The reverseis true for

alcohol group at C5 to form two types of cyclic B-anomerT. he anomersdiffer in certainphysical

hemiacetalsnamely a and B, as depicted in and chemical properties.

Fig.2.6. The configuration of glucose is Mutarotation : The a and p anomers of
conveniently represented either by Fischer glucose have different optical rotations. The
formulaeor by Haworth projectionformulae. specific optical rotation of a freshly prepared

Fyranose and furanose structures glucose(c anomer)solution in water is +112.2o
which gradually changes and attains an

Haworth projectionformulaeare depictedby equi l i bri umw i th a constantval ue of + 52.7" . l n

a six-memberedring pyranose(basedon pyran) the presenceof alkali, the decreasein optical

or a five-memberedring furanose (based on rotation is rapid. The optical rotation of

furan).The cyclic formsof glucoseare known as p-glucose is +18.7o. Mutarotation is defined as

a-D-glucopyranose and c-D-glucofuranose the change in the specific optical rotation

(Fig.2.V. representing the interconversion of u and p

H-C:O H-C:O H-C=O H H
I
I II O=C C:O
H-C-OH H-C-OH HO-C-H
I II HO_C- H I
HO-C-H HO-C-H HO-C-H I f{
c-oH
H-C- Cl-i I
I H .CI -O H I I HC-C-H
HO - C - H H-C-OH
HO-C-H
II I i-l- c-oH
H-C-OH H-C-OH H-C-OH
I II HO-C- Fl
cH2oH CHzOH cH2oH I H-C-OH
I
D-Galactose D-Glucose D-Mannose H-C- ii
t"1-c-H
I
Fig.2,4: Structuresof epimers (glucoseand galactose HO
are Co-epimerswhileglucoseand mannoseare OH D-Glucose
C2-epimers).
L-Glucose

H9.2.5 : Enantiomers(mirrorimages)of glucose.

Ghapter 2 : CARB 1 t5

I H-C:O iHron

cH20H I ftD-Glucose
(+18.7-)
o'D'Glucose H-C-OH
(+ 112.2"\ I HOH
FD-GlucoPYranose
cH20H HO-C-H
I
fil
H-C-OH
(B)
tc
H6\?H
H-C-OH
o-D-GlucoPYranose I
cH2oH

D-Glucose
(aldehYdfeorm)

l/A

H6\?H

HOH
D-Glucose
(aldehydfeorm,acYclic)

forms of D'glucose to an equilihrium mixture' 1 7 oo 'pi ,e nchain form. ln a q u e o u ss olu ti on't he p
Mutarotationdepictedin Fig'2'6, is summartzeo forrn more predom inant due to its s table

below. conformation.The cr and p forms of glucoseare
interconvertiblewhich occurs through a linear
cx-D-Clucos#e Equilibriummixture# B-D-Clucose form. The latter, as such, is present in a"
+ 18.7" i nsi gni fi canqt uanti tY .
+ 112.2" + 52.7"

(Specificoptical rotation tctl2p0) Mutarotation of fructose z Frur'

The equilibrium mixture contains 63o/" exhibits mutarotation.ln case or
p-anomer and 36"/ocl-anomer of glucose with pyranose ring (six-memberqd'

f u r a n o s e( fi v e - m e m b e r e d ) ' o '

is attained.And fruqt'

rotation of -92)2.

Ihe conv'
to levor

':ut" :;r'

cH20H
t-
H-C-OFi

OH on\
is kn,
HOH HOH anome'
cr-D-GlucoPYranose cr-D-Glucuorfanose in alkalir

Fig.2.7: Structurcofglucose-pyranose When gt.
andfuranosetorms' s e v e r a lh o u r s ,

chapter 2 : CAFIBoHYDFATES 1 15

I H-C=C) cH2oH
cH2oH I H

cr-D-Glucose H-C-OH
(+ 112.2")
I

HO-C-H

I

H-C-OH
l5

H-C-OH

cH2oH

D-Glucose
(aldehydfeorm)

20H

HOH HOH pD-Glucopyranose
o-D-Glucopyranose D-Glucose
(aldehydeform,acyclic)

Fig. 2.6 : Mutarotation of glucose representing a and p anomers (A) Fischer projections (B) Haworth projections.

forms of D-glucose to an equilibrium mixture. 17oopen chain form. In aqueoussolution,the p

Mutarotationdepictedin Fi9.2.6, is summarized form is more predominant due to its stable

below. conformation.The s and p forms of glucoseare

s-D-Clucose# Equilibriummirctur#e p-D-Glucose interconvertiblewhich occurs through a linear
form. The latter, as such, is present in an
+ 112.2" + 52.7" + 18.7o i nsi gni fi canqt uanti ty.

(Specificoptical rotation talf;) Mutarotation of fructose : Fructose also

The equilibrium mixture contains 63"/" exhibits mutarotation.ln case of fructose,the
p-anomer and 36h cl-anomerof glucose with pyranose ring (six-membered)is converted to

furanose(five-memberedr)ing,till an equilibrium

is attained.And fructosehas a specific optical

rotationof -92" at equilibrium.

The conversion of dextrorotatory(+) sucrose
to levorotatory fructose is explained under
inversionof sucrose(seelater in this chapter).

REACTIONS OF MONOSACCHARIDES

Tautomerization or enolization

cr-D-Glucopyranose c-D-Glucofuranose The processof shiftinga hydrogenatom from
one carbon atom to anotherto produce enediols
Fig.2.7 : Structureof glucose-pyranose is known as tautomerization. Sugarspossessing
and furanoseforms. anomericcarbon atom undergotautomerization
i n al kal i nesol uti ons.

When glucoseis kept in alkalinesolutionfor
severalhours,it undergoesisomerizationto form

16 BIOCHEMISTFIY

H-C:O H
H- I-OH
n-C-ot

(

HO-(

2H2O+ CueO{- t

2Cu(OH)

HO-( It may be noted that the reducing property of
sugarscannothelp for a specificidentificationof
any one sugar,since it is a generalreaction.

R 0xida*iern
Enediol Depending on the oxidizing agent used, the
(common)

Fig.2.8 : Formationof a commonenediolfrom terminal aldehyde (or keto) or the terminal
glucose,fructoseandmannose alcohol or both the groupsmay be oxidized.For

{fr,f,o,F|F|lP:f!floi .,tf|:tPfrI:Is?Iboncolnmonsttanrf:?l,l instance,considerglucose:

1. Oxidation of aldehydegroup (CHO ------>

COOH) resultsin the formationof gluconicacid.
D-fructose and D-mannose. This reaction-

known as the Lobry de Bruyn-von Ekenstein 2. Oxidation of terminal alcohol group

transformatiorr-results in the formation of a (CH2OH ------+COOH) leadsto the production of

common intermediate-namely enediol--$or all gl ucuroni caci d.

the three sugars,as depicted in Fig.2.8.

Reduetion

The enediolsare highly reactive,hencesugars When treatedwith reducing agentssuch as
sodium amalgam,the aldehydeor keto group of
in alkaline solution are powerful reducing
agents. monosaccharideis reduced to corresponding

f t + r , . l u l eF r ' . l gr ! s . lF e F t l s F alcohol, as indicatedby the generalformula :

The sugarsare classifiedas reducingor non- H-C:O H
reducing.The reducingproperty is attributedto I H-C-Ol-t
the free aldehyde or keto group of anomeric RR

carbon. The important monosaccharidesand their
correspondingalcoholsare given below.
ln the laboratory, many testsare employed to
identify the reducing action of sugars.These D-Glucose

incfude Benedict's test, Fehling's test, Barfoed's D-Galactose------+ D-Dulcitol
tesf etc. The reduction is much more efficient D-Mannose ------+D-Mannitol
in the alkaline medium than in the acid D-Fructose --) D-Mannitol+ D-Sorbitol
medium. D-Ribose -+ D-Ribitol

The enediolforms(explainedabove)or sugars Sorbitol and dulcitol when accumulate in

reduce cupric ions (Cu2+)of copper sulphate tissuesin large amounts cause strong osmotic

to cuprous ions (Cu+), which form a yellow effectsfeadingto swelling of cells,and certain

precipitate of cuprous hydroxide or a pathologicalconditions.e.g. cataract,peripheral

red precipitate of cuprous oxide as shown neuropathy,nephropathy.Mannitol is usefulto

next. reduce intracranialtension bv forced diuresis.

Ghapter 2 : CAFIBOHYDRATES 17

H-C--O H-C:O Formation of esters
I I
The alcoholic groups of monosaccharides
H-C-OH cH20H may be esterified by non-enzymatic or
I Hydrorymethfyul rfural enzymatic reactions. Esterificationof carbo-
hydrate with phosphoric acid is a common
HO-C-H reaction in metabolism.Glucose 6-phosphate
I and glucose 1-phosphateare good examples.
ATP donates the phosphate moiety in ester
H-C-OH formation.
I
lClycoside bond formation (seebelow) and
H-C-OH mutarotation (discussedalready) may also be
referred to, as these are also the characteristic
I propertiesof monosaccharides.l

cH2oH GLYCOSIDES

D-Glucose

H-C:O H-C:O
I
I
H-C-OH C----r
tlll
H-Q L
H-C-OH ConcH. eSoo
IU
I rH
H-C I
H-C-OH
H-d---l
CHrou 3H'12o
Furfural
D-Ribose

Fig.2.9 : Dehydration of monosaccharides Glycosidesare formed when the hemiacetal

with concentrated H o. or hemiketal hydroxyl group (of anomeric
"SO carbon)of a carbohydratereactswith a hydroxyl

Dehydration group of another carbohydrate or a non-
carbohydrate (e.g. methyl alcohol, phenol,

When treatedwith concentratedsulfuricacid, glycerol). The bond so formed is known as

monosaccharidesundergo dehydrationwith an glycosidic bond and the non-carbohydrate

eliminationof 3 water molecules.Thus hexoses moiety (when present)is referredto as aglycone.

give hydroxymethylfurfuralwhile pentosesgive The monosaccharidesare held together by
furfural on dehydration (Fi9.2.9).These furfurals glycosidic bonds to result in di-, oligo- or
can condense with phenolic compounds polysaccharides(seelater for structures).
(a-naphthol)to form coloured products.This is

the chemical basisof the popular Molisch test.

In case of oligo- and polysaccharidest,hey are H-C=O
firsthydrolysedto monosaccharidebsy acid, and I _ + HrN-NH-CuHu
this is followed by dehydration.
H-C-OH

Osazone formation R Phenylhydrazine
Glucose

Phenylhydrazinein acetic acid, when boiled H-C:N-NH-CoHs
with reducing sugars, forms osazones in a
reactionsummarizedin Fig,2,10. I

As is evident from the reaction, the first two H-C-OH
carbons (Cr and C2) are involved in osazone I
formation. The sugars that differ in their R
configurationon these two carbons give the
same type of osazones,since the differenceis Glucohydrazone
m as k edby b i n d i n gw i th p h e n y l h y d ra zi neT.hus
glucose,fructoseand mannosegive the same Il7-H2N-NH-C6H'
type (needle-shapedo)sazones.
H-C:N-NH-CoHs
Reducingdisaccharidesalso give osazones-
maltose sunflower-shaped,and lactose powder- I
puff shaped.
C:N-NH-CoHs

I

R
Glucosazone

Fig. 2.10 : A summatyof osazonefomation
(RrcprcsentsCrto Crof glucose).

t8 BIOCHEMISTRY

Naming of glycosidic bond : The The amino groups of amino sugars are

nomenclatureof glycosidic bonds is based on sometimes acetylated e.g. N-acetyl D-gluco-

the Iinkagesbetweenthe carbon atoms and the samrne.

status of the anomeric carbon (o or p). For N-Acetylneuraminicacid (NANA) is a

instance,lactose-which is formed by a bond derivativeof N-acetylmannoseand pyruvic acid.
between C1 of p-galactoseand Ca of glucose- It is an important constituentof glycoproteins
and glycolipids.The term sialicacid is used to
is namedas 0(.1-+ 4) glycosidicbond. The other include NANA and its other derivatives.
glycosidic bonds are describedin the structure

of di- and polysaccharides.

Physiologieally important glycosides Certain antibiotics contain amino sugars
which may be involvedin the antibioticactivity

1. G luc o v a n i l l i n (v a n i l l i n -D -g l u c o si dei)s a e.g. erythromycin.

naturalsubstancethat impartsvanilla flavour. 5. Deoxysugars: These are the sugarsthat

2. Cardiac glycosides(steroidalglycosides:) contain one oxygen lessthan that presentin the
Digoxin and digitoxin contain the aglycone parent molecule. The groups -CHOH and
steroidand they stimulatemuscle contraction. -C H 2OH become-C H 2 and -C H 3 due to the

3. Streptomycin,an antibiotic used in the absenceof oxygen.D-2-Deoxyriboseis the most
treatmentof tuberculosisis a glycoside. important deoxysugarsince it is a structural
constituentof DNA (in contrastto D-ribose in
4. Ouabain inhibits Na+- K+ ATPase and RNA).
blocksthe active transportof Na+.

6. L-Ascorbicacid (vitamin C) : This is a
DERIVATIVESOF MONOSACCHARIDESwater-solublevitamin, the structure of which

There are severalderivativesof monosaccha- closely resemblesthat of a monosaccharide.

r ides , s ome o f w h i c h a re p h y s i o l ogi cal l y The structuresof selected monosaccharide
important
derivativesare depicted in Fig.2.l1.

1. Sugar acids : Oxidation of aldehyde or

primaryalcoholgroup in monosaccharideresults

in sugaracids.Cluconic acid is producedfrom

glucose by oxidation of aldehyde (C1 group)

whereasglucuronicacid is formedwhen primary

alcoholgroup (C6)is oxidized. A mong the ol i gosacchari desd,i sacchari de s
are the most common (Fig.2,l2).As is evident
2. Sugar alcohols (polyols) : They are from the name, a disaccharideconsistsof two
producedby reductionof aldosesor ketosesF. or monosacchari dueni ts(si mi l aror di ssi mi l arh) el d
instance,sorbitol is formed from glucose and together by a glycosidic hond. They are
mannitolfrom mannose. crystalline,water-solubleand sweetto taste.The
disaccharidesare of two types
3. Alditols : The monosaccharides,on
reduction,yield polyhydroxyalcohols,known as '1. Reducingdisaccharideswith free aldehyde
alditols. Ribitol is a constituent of flavin or keto group e.g. maltose, lactose.
coenzymes; glycerol and myo-inositol are

componentsof lipids.Xylitol is a sweetenerused 2. Non-reducingdisaccharideswith no free

in s ugarlesgs u m s a n d c a n d i e s . aldehyde or keto group e.g. sucrose,trehalose.

4. Amino sugars : When one or more
hydroxyl groups of the monosaccharidesare Maltose

replaced by amino groups, the products Maltose is composed of two a-D-glucose

formed are amino sugarse.g. D-glucosamine, unitsheld togetherby cl (1 -+ 4) glycosidicbond.

D-galactosamine.They are present as consti- The freealdehydegroup presenton C1 of second

tuentsof heteropolysaccharides. glucoseanswersthe reducingreactions,besides

Ghapter & : CAFIBOHYDRATES 19

H-C:O cH2oH HOH
I myo-lnositol
I H3C-C--HN
H-C-OH
I H-C-OH

HO-C-H I
I
cH2oH
H-C-OH
Glycerol
I

H-C-OH
I
COOH

D-Glucuroniaccid

OHH H NHz HOH
D-2-Deoxyribose D-Glucosamine N-Acetylneuraminaic id

Fiq.2.11 : Structuresol monosaccharidederivatives(selectedexamples).

the osazone formations (sunflower-shaped). and keto) are held togetherand protectedfrom
Maltosecan be hydrolysedby dilute acid or the oxidative attacks.

enzyme maltaseto liberate two moleculesof Sucroseis an important source of dietary
cr-D-glucose. carbohydrate.lt is sweeter than most other
c o m m o n s u g a r s( e x c e p tf r u c t o s e )n a m e l yg l u c o s e ,
ln isomaltose,the glucose units are held lactoseand maltose.Sucroseis employed as a
togetherby o (1 --+6) glycosidiclinkage. sweeteningagentin food industry.The intestinal
enzyme-sucrase-hydrolysessucroseto glucose
Cellobioseis another disaccharide,identical and fructosewhich are absorbed.
in structurewith maltose,exceptthat the former
has p (1 -r 4 ) g l y c o s i d i cl i n k a g e .C e l l o bi osei s

f or m edduri n g th e h y d ro l y s i so f c e l l u l o se. F-aetsse

Suoroee Lactoseis more commonlv known as milk

Sucrose(canesugar)is the sugarof commerce, sugarsi nce i t i s the di sacchari defound i n mi l k.
mostlyproducedby sugarcane and sugarbeets. Lactoseis composed ol p-D-galactoseand B-D-
Sucrose is made up of a-D-glucose and p- glucoseheld togetherby 0 (1 -r a) glycosidic

D-fructose.The two monosaccharideasre held bond.The anomericcarbonof C1 glucoseis free,

togetherby a glycosidicbond (a1 -+ B2),between hence lactoseexhibits reducing propertiesand
Cj of c-glucose and C2 of B-fructose.The formsosazones(powder-pufof r hedgehogshape).

reducing groups of glucose and fructose are Lactose of milk is the most important

inv olv e din g l y c o s i d i cb o n d , h e n c es u crosei s a carbohydratein the nutritionof young mammals.

non-reducing sugar,and it cannot form osazones. It is hydrolysedby the intestinalenzyme lactase

Sucroseis the major carbohydrateproduced to glucoseand galactose.

in photosynthesis. lt is transported into the
storageorgans of plants (such as roots, tubers lnversion ef suerose

and s ee ds )S. u c ro s ei s th e mo s ta b u n d a n tamong Sucrose,as such is dextrorotatory(+66.5o).
the naturally occurring sugars.lt has distinct But, r,r,hen hydrolysed, sucrose becomes
advantagesover other sugarsas a storageand levorotatory(-28.2"). The processof change in
transoortform. This is due to the fact that in optical rotation from dextrorotatory (+) to
sucrose,both the functional groups (aldehyde levorotatory(-) is referredto as inversion.The

BIOCHEMISTF|Y

HOH Polysaccharide(sor simply glycansc) onsistof
repeat units of monosaccharidesor their
Glucose Fructose derivatives,held together by glycosidic bonds.
Sucrose They are primarilyconcernedwith two important
functions-structuraal,nd storageof energy.
(a-D-glucosy(1l --+2) p-D-fructose)
Polysaccharides are linear as well as
branched polymers. This is in contrast to
structureof proteinsand nucleicacidswhich are
only linear polymers. The occurrence of
branchesin polysaccharidesis due to the fact
that glycosidic linkagescan be formed at any
one of the hydroxylBroupsof a monosaccharide.

Polysaccharidesare of two types

1. Homopolysaccharidewshich on hydrolysis
yield only a singletype of monosaccharideT.hey
are named based on the nature of the
monosaccharideunit. Thus,glucans are polymers
of glucose whereas fructosans are polymers of
fructose.

2. Heteropofysaccharideson hydrolysisyield
a mixture of a few monosaccharidesor their
derivatives.

Galactose
Lactose

(p-D-galactos(y1l-+ a)p-D-glucose)

Fig. 2.12 : Structures of disaccharides $tarch
-maltose, sucrose and lactose.
Starch is the carbohydrate reserveof plants
which is the most importantdietary sourcefor

hi gherani mal s,i ncl udi ngman. H i gh contentof

hydrolysed mixture of sucrose, containing starchis found in cereals,roots,tubers,vegetables
gfucoseand fructose, is known as invert sugar. etc. Starch is a homopolymer composed of
The processof inversionis explainedbelow. D-glucoseunits held by a-glycosidicbonds. lt is

Hydrolysis of sucroseby the enzyme sucrase known as glucosan or glucan.

(invertasdor dilute acid liberatesone molecure Starch consists of two polysaccharide

each of glucoseand fructose.ft is postulatedthat components-water soluble amylose (15-20o/ol

sucrose (dextro) is first split into a-D- and a water insoluble amylopectin (80-85%).
glucopyranose(+52.5") and p-D-fructofuranose, Chemically, amylose is a long unbranched

both being dextrorotatory. However, p-D- chain with 200-1,00OD-glucoseunits held by c

fructofuranoseis lessstableand immediatelygets (1 + 4) gl ycosi di cl i nkagesA. myl opecti no, n the

converted to p-D-fructopyranose which is other hand, is a branchedchain with a (1 --r 6t

stronglylevorotatory(-92"). The overall effect is gl ycosi di cbondsat the branchi ngpoi ntsand c

that dextro sucrose (+66.5") on inversion is (1 -; 4) linkageseverywhereelse (Fig.2.13).

converted to levo form (28.2'\. Amylopectin molecule containing a few

ChapteF 2 : CARBOHYDFATES 21

D-Glucose D-Glucose

o-Amylose

Mainchain Lg +- (1-* 6) Branch

6nu

vt t2

Amylopectin

thousandglucose units looks like a branched saccharideeasily soluble in water. Inulin is not

tree (20-30 glucoseunits per branch). utilized by the body. lt is used for assessing

Starches are hydrolysed by amylase kidney function through measurement of
(pancreaticor salivary)to liberatedextrins,and glomerular filtration rate (GFR).

finally maltoseand glucoseunits.Amylaseacts Glycogen
specificallyon a (1 -+ 4) glycosidicbonds.

Clycogen is the carbohydrate reserve in

Dextrins animals,hence often referredro as animal starch.

Dextrins are the breakdown products of It is present in high concentration in liver,
starchby the enzyme amylaseor dilute acids. followed by muscle,brainetc. Clycogenis also
Starch is sequentially hydrolysed through found in plantsthat do not possesschlorophyll
differentdextrins and, finally, to maltoseand ( e . 9 . y e a s t ,f u n g i ) .

glucose.The variousintermediates(identifiedby The structureof glycogenis similarto that of

iodine colouration) are soluble starch (blue), amylopectin with more number of branches.

amylodextrin (violet), erythrodextrin (red) and Glucoseis the repeatingunit in glycogenjoined

achrodextrin (no colour). togetherby u (1 + 4) glycosidicbonds, and a

Inulin (1+ 6) glycosidicbonds at branchingpoints
(Fi9.2.1Q.The molecularweight (up to 1 x 108)

fnulin is a polymerof fructosei.e., fructosan. and the number of glucoseunits (up to 25,000)

I t oc c ursi n d a h l i a b u l b s ,g a rl i c ,o n i o n etc. l t i s vary in glycogendependingon the sourcefrom

a low molecularweight (around5,000) poly- which glycogenis obtained.

22 BIOCHEMISTRY

decreasing the absorption of glucose and
cholesteroflrom the intestine,besidesincreasing
the bulk of feces. (For details,Chapter 23)

Ghitin

Chitin is composed of N-acetyl D-
glucosamineunits held togetherby F (1 -+ a)
gl ycosi di cbonds.l t i s a structurapl ol ysaccha r i de
found in the exoskeletonof some invertebrates
e.g. insects,crustaceans.

T

N

T
Ot

(B) When the polysaccharidesare composedof
differenttypesof sugarsor their derivativest,hey
9H2OH uqt, CH2oH are referred to as heteropolvsaccharidesor
heteroglycans.
o--J (y+'-O., i) ---' \ r,+F--o. ,L^_K,r.4-Or X^_
- \, , . MUCOPOLYSACCHARIDES
/'o L-/ " \--l
\J

Fiq.2.14: Structureofglycogen(A) Generasl tructure Mucopolysaccharideasre heteroglycanms ade
(B) Enlargedat a branchpoint. up of repeatingunitsof sugarderivativesn, amely
amino sugarsand uronic acids.Theseare more

commonly known as glycosaminoglycans

Cellulose (GAG).Acetylatedamino groups,besidessulfate
and carboxyl groups are generally present in
Ce llul o s eo c c u rse x c l u s i v e l yi n p l a n tsand i t i s CAC structure.The presenceof sulfate and
the most abundant organic substancein plant carboxyl groups contributesto acidity of the
kingdom. lt is a predominantconstituentof molecules, making them acid mucopoly,-
plant cell wall. Celluloseis totally absentin saccharides.
animal body.
Some of the mucopolysaccharideasre found
Cellulose is composed of p-D-glucose units in combination with proteins to forrn
linked by 9 0 -+ 4) glycosidic bonds(Fi9.2.1fl. mucoproteins or mucoids or proteoglycans
Cellulosecannot be digestedby mammals- (Fig.2.l6l.Mucoproteinsmay contain up to 95o,
includingman-due to lack of the enzymethat carbohydrate and 5o/"protein.
cleavesB-glycosidicbonds (a amylasebreakscr

bonds only). Certain ruminantsand herbivorous

anim a lsc o n ta i nm i c ro o rg a n i s misn th e gut w hi ch

produce enzymesthat can cleave p-glycosidic

bonds. Hydrolysis of cellulose yields a

disaccharide cellobiose, followed by P-D-
glucose.

Cellulose, though not digested, has great S-D-Glucose
importancein human nutrition. lt is a major
constituentol fiber, the non-digestablecarbo- Fig. 2.15 : Structureof cellulose(The repeat:r; -- '
hydrate.The functionsof dietary fiber include may be several thousands).

CARBOHYDRATES 23

Hyaluronicacid M u c o p o l y s a c c h a r i d easr e e s s e n t i acl o m p o n e n t s
of tissue structure.The extracellularspacesof
s:\ -;-- ti ssue (parti cul arl yconnecti ve ti ssue-carti l age,
skin, blood vesselst,endons)consistof collagen
s\' t'/ : - -sS and elastinfibersembeddedin a matrixor ground
substanceT.he groundsubstanceis predominantly
-\'\- composedof CAC.

Fig. 2.16 : Diagrammaticrepresentationof a The i mportantmucopol ysacchari dei ns cl uoe
prateoglycan complex. hyal uroni caci d, chondroi ti n4-sul fate,hepari n,
dermatansulfateand keratansulfate(Fig.Z.'[1.

j 'i ' ,i r :r :, | '.i . :,\{,ti i i El 'l

H yal uroni caci d i s an i mportantGA C found
i n the groundsubstancoef synovi afl l ui d of j oi nts
and vitreoushumor of eyes.it is also presentas
a ground substancei n connecti veti ssues,and
forms a gel around the ovum. H yal uroni caci d
servesas a l ubri cant and shock absorbanti n
joints.

BToMEDtCAt/ CLtft|ICALCO$CEpTS

rlr Glucose is the most important energy sourceol carbohgdratesto the mammals (except
ruminants).The bulk of dietary carbohydrote(starch)is dlgestedond finally obsorbedas
glucose into the body.

Ea Dextrose (glucosein solution in dextrorotatory form) is frequently used in medical
practice.

Rq'- Fructose is obundantly found in the semen which is utilized by the spermsfor energy.
CF Seueral diseoses are associated with carbohydrate.se.g., diabetes mellitus, glycogen

storage diseoses, galactosemia.

trs Accumulation of sorbitol and dulcitol in the fissuesmoy cause certoin pathological
conditionse.g. cotaract, nephropothy.

t-s' Inulin, a polymer of t'ructose,is used fo ossessrenal function by meosuringglomerular
filtration rate (GFR).

ue The non-digestiblecarbohydratecellulose plays a signilicant role in human nutriticsn.
These include decreasing the intestinal absorption ol glucose and cholesterol, qnd
increasingbulk of feces to ouoid eonstipation.

rt The mucopolysaccharidehyaluronic acid seruesas a lubricant and shock absorbantin
ioints.

The enzgmehgaluronidaseof semendegradesthe gel (contains hyaluronic acid) around
the ouum. This qllows eft'ectiuepenetration of sperm into the ouum.

!3:. The mucopolysaccharideheparin is an onticoagulant(preuentsblood clotting).

[j- The suruiual of Antarctic lish below -2"C is attributed to the antit'reeze glycoproteins.

IF streptomycin is a glycosideemployed in the treatment oJ tuberculosis.

24 BIOCHEMISTFIY

Hyaluronic acid is composedof alternate D-Glucuronicacid N-Acetylglucosamine
Hyaluronic acid
units of D-glucuronic acid and N-acetyl

D-glucosamine.These two molecules form
dis ac c ha rideu n i ts h e l d to g e th e rb y 0 (t -+ S )
glycosidic bond (Fi9,2,15).Hyaluronic acid

c ont ainsa bo u t 2 5 0 -2 5 ,0 0 0 d i s a c c h a ri d euni ts
(heldby p 1 -+ 4 bonds)with a molecularweight

uo to 4 million.

Hyaluronidase is an enzyme that breaks H NH-CO-CH3
( B1 - + 4 link a g e s )h y a l u ro n i c a c i d a n d other
CAC. This enzyme is present in high D-Glucuronaiccid N-Acetylgalactosamine
c onc ent r atio ni n te s te s ,s e mi n a lfl u i d , a nd i n 4-sulfate
certainsnakeand insectvenoms. Hyaluronidase
of semen is assignedan important role in Chondroitin 4-sulfate
fertilization as this enzyme clears the gel
( hy alur on ica c i d ) a ro u n d th e o v u m a l l o w i ng a -o-so3
better penetration of sperm into the ovum.
Hy alur on ida s eo f b a c te ri ah e l p s th e i r i n vasi on o-
into the animaltissues.
H O-SO; H NH-SOa
Ghondroitin sulfates
D-Glucuronate-2-sulfaNte-Sulfoglucosamine
Chondroitin 4-sulfate (Greek: chondros- 6-sulfate
cartilage) is a major constituent of various
mammalian tissues(bone, cartilage,tendons, Heparin
heart,valves,skin, corneaetc.).Structurallyi,t is
c om par a blew i th h y a l u ro n i ca c i d . C h o n droi ti n t -o'r
4-sulfateconsistsof repeatingdisaccharideunits
composedof D-glucuronicacid and N-acetyl H NH_CO_CH.
D - g a l a c t o s a m i n e4 - s u l f a t e( F i g . 2 . lV . N-Acetylgalactosamine
4-sulfate
Chondroitin 5-sulfateis also presentin many Dermatansulfate
tissues.As evident from the name, the sulfate
group is found on C6 insteadof Ca. qH2oH

Heparin o

Heparin is an anticoagulant(preventsblood
c lot t ing)t h ato c c u rsi n b l o o d ,l u n g ,l i v e r ,ki dney,
spleenetc. Heparin helps in the releaseof the
enzyme lipoprotein lipase which helps in
c lear ingt h e tu rb i d i tyo f l i p e m i c p l a s ma .

Heparin is composed of alternatingunits of H NH_CO :-
N-sulfoD-glucosamine6-sulfateand glucuronate N-Acetylglucosamine
2 - s u l f a t e( F i 9 . 2 . 1 7 ) . 6-sulfate
Keratansulfate
Dermatan sulfate
Fiq.2.17 : Structuresof common glycosaminogi',-;-: -
The name dermatansulfateis derived from the disaccharidesas repeatingunits.
the fact that this compoundmostlyoccursin the
skin. lt is structurallyrelated to chondroitin

Ghapter 2 : CAFIBOHYDHATES 25

Glycosaminoglycan Composition Tissuedistribution Function(s)

Hyaluronaicid D-Glucuroancicd, Connectitvisesues,ynoviaflul id, Serveassalubricanatn.d
Chondroistiunlfate
Heparin N-acetylglucosaminevitrouhsumor shocakbsorbePrr.omotes
Dermatasnulfate
Keratasnulfate wounhdealing

D-Glucuroancicd, Cartilagbeo,nes,kinb, loovdessel Helptsomaintatinhestructure

N-acetylgalactosaminwealls andshapeosftissues

4-sulfate

D-Glucuro2n-astuelfate,Bloodlu, ngli,verk,idnesyp, leen Actsasananticoagulant
N-sulfoglucosamine

6-sulfate

L-lduronaiccidN, -acetyl- Bloovdessevlalvesh,earvtalves, Maintaitnhseshapeosftissues
galactosam4i-nseulfate skin

D-GalactosNe-,acetyl- Cartilagceo,rneac,onnective Keepcsornetaransparent
glucosam6in-seulfate tissues

4-sulfate.The only differenceis that there is an and receptors.A selected list of glycoproteins

inversion in the configuration around C5 of and their major functionsis given in Table2.4.

D-glucuronic acid to form L-iduronic acid The carbohydratesfound in glycoproteins

(Fi9.2.1V. include mannose, galactose, N-acetyl-

Keratan sulfate glucosamine, N-acetylgalactosamine,xylose,
L-fucoseand N-acetylneuraminicacid (NANA).

It is a heterogeneousCAG with a variable NANA is an importantsialic acid (SeeFig.2,l1).

sulfate content, besides small amounts of Antifreeze glycoproteins : The Antarctic fish
mannose, fructose, sialic acid etc. Keratan live below -2oC, a temperatureat which the
sulfateessentiallyconsistsof alternatingunitsof

D-galactosamine and N-acetylglucosamine

6 - s ul f a t e .

A summary of the glycosaminoglycanws ith Glycoprotein(s) Major function(s)
regardto composition,distributionand functions
is given in Table 2.3. Collagen Structure
Hydrolasperso, teases, Enzymes
glycosidases

Several proteins are covalently bound to Ceruloplasmin Transport
carbohydrateswhich are referredto as glyco- lmmunoglobulins Defensaegainsint fection
proteins. The carbohydrate content of Synovigallycoproteins Lubrication
glycoproteinvariesfrom 1o/oto 90o/oby weight, Thyrotropeinry, lhropoietin Hormones
Sometimes the term mucoprotein is used for Bloogdroupsubstances Antigens
glycoprotein with carbohydrateconcentration Fibronecltainm,inin Cell-cerlel cognitioand
more than 4"/o. Clycoproteins are very widely adhesion
distributedin the cells and perform variety of Intrinsfiacctor Absorptionfvitami8n,,
f unc t ions .T h e s ei n c l u d eth e i r ro l e a s enzymes, Fibrinogen Bloocdlotting
h o r m o n e s ,t r a n s p o r tp r o t e i n s ,s t r u c t u r a lp r o t e i n s

26 ElIOCHEMISTF|Y

blood would {reeze.lt is now known that ihese ri#i .f*iCA# '? r,.4F!".r.Ii $:F"1r .*fi { "'.3i4 t: * :il
fish contain antifreezeglycogtrateinwhich lower
The blood group antigens (of erythrocyte

the freezingpoint of water and interferewith tne membrane) contain carbohydratesas glyco-

crystalformationof ice. Antifreezegiycoproteins protei nsor gl ycol i pi ds.N -,A .cetyl gai actosa m i ne,

c ons istof 5 0 re p e a ti n gu n i ts o f th e tri pepti de, gal actosef,ucose,si al i c aci d etc. are found in

alanine-alawine-threonine. Each threonine the blood group substancesT. he carbohydrate

r es idu e is b o u n d to B-g a l a c to s y l(1 + 3) o( contental so pl ays a determi nantrol e i n bl oo d

N-acetygl alactosamine. Eroup!n8.

X. Carbohydrs,tesare the polyhydroxyaldehydesor ketones,or campounds whichproduce
them on hydrolysis. The term sugor is applied to carbohydratessoluble in water and
stDeetto taste. Carbahgdratesqre the major dietary energy sources,besidestheir
inualuementin cell structure and uariousother t'unctions.

2. Carbohydrqtesare broadly c/ossiJiedinta 3 groups-ffionasqccharides,oligosoccharides
and ytoiysaccharidesT. he monosacchsridesare further diuided into dit't'erentcategories
bqsed an the presenceaf t'wnctionalgroups {oldosesar ketoses)and the number of
carbon atoms (trioses, tetroses,pentases, hexosesand heptcses).

3. Glyceraldehyde{triose) is the simplest carbohydrateand is chosen as a reJerenceto
write the cont'iguratian of all other rnonasaccharides(D- anc L- forms). It' two
rnonosaccharidedsiffer in their structure around o single carbon atom, they ore known
as eplmers.Glucoseand galactoseare C4-epimers.

4. D'Glucose is the most predominant naturally occurring aldosdmonosaccharide.
Giucoseexisfs cs a and p anemerswith dit'Jerentoptical rotations. The interconuersion
of a and B anomericforms with changein the opticalrotatianis knoun as mutsratation.

5. Manosaccharidespariicipate in seuercl recctions"These include oxidation, reduction.
dehydration, asazone formetion etc. Formatian ol esters and glycosides by
manosacchqridesis af special significanceln biochemical reactions.

6. Among the oligosacchqridesd, isoccharidesare the most common. These include the
reducing disaccharidesnamely lactose(rnilk sugar)and maltase(malt sugar)and the
n o n - r e d u c i n gs u c r o s e( c a n e s u g a r ) .

7. Palysacclwridesare the poiymers ot' monosaccharideosr their deriuatiues,held together
by glycosidic bonds.Homopalysaccharidessre compased ot' a single manosaccharicle
(e.g., starch, glycogen, cellulose, inulin). Heteropolysaccharidescontain a mixture af
Jew monasaceharidesor thetr derluatiues(e.g., rnucapolysacaharides).

8. Slorch and glgcogensre the carbohydratereseruesot' plants and animals respectiuelg.
Cellulose,exclusiuelyt'ound in plants, is the structural constituent.Inulin is utilized to
ossesskidney tunction bg measuringglomerular t'iltration rate (GFR).

9. Mucopoiysaccharides(glycosominoglycans)are the essential companents o/ tlssue
structure. They prouide the mstrix or grownd substanceof extracellular tissue spacesin
whtchcollagenand elastinfibers are embedded.Hyaluranic ocid,chondroitin 4'sult'ote,
heporin, are amang the important glycosaminaglgcdns.

70. Glycoproteins are a group of biochernically important compaunds with a uariable
composition of carbohyd.rate(7-900/o)c,aualently bound to protein. Seueralenzyrnes,
hormanes, structura! proteins and cellular receptorsare in fact glycoproteins.

Ghapter 2 : CAFIBOHYDHATES 27

I. Essayquestions
1. Define and classifycarbohydratews ith suitableexamplesA. dd a note on the functionsof
carbohydrates.
2. Describethe structureand functionsof mucopolysaccharides.
3. Cive an accountof the structuracl onfigurationof monosaccharidews,ith specialreferenceto
glucose.
4. Discussthe structureand functionsof 3 biochemicallyimportantdisaccharides.
5. Definepolysaccharideasnd describethe structureof 3 homopolysaccharides.

II. Short notes
( a ) E p i m e r s(,b ) M u t a r o t a t i o n(,c ) O s a z o n ef o r m a t i o n ,( d ) C l y c o s i d i cb o n d ,( e )S u g a rd e r i v a t i v e s(,f l
Anomers,(g) Enediol,(h) Amino su8ars(,i) Inversionof sucrose(,j) Deoxysugars.

III. Fill in the blanks
1. Namea n o n -re d u c i ndgi s a c c h a ri de
2. The carbohydratethat is taken as a referencefor writing the configurationof others

3. lf two monosaccharidedsiffer in configurationarounda singlecarbonatom,they are known
as

4. The s and B cyclic formsof D-glucoseare referredto as
5. The non-carbohydratmeoietyfound in glycosidesis known as
6. Cive an exampleof a glycosideantibiotic
7. The glycosidicbondsat the branchingpointsin the structureof starchare
B . The polysaccharideemployedfor the assessmenot f kidneyfunction
9. The glycosaminoglycathnat servesas a lubricantand shockabsorbanot f joints
10. Namethe sialicacid, mostlyfound in the structureof glycoproteinasnd glycolipids

IV. Multiple choice questions
11. Riboseand deoxyribosediffer in structurearounda singlecarbon,namely
(a)Cr (b)Cz (c)C: (d)Cq.
12. O n e o f th e fo l l o w i n gi s n o t a n a l dose
(a)Clucose(b) Calactose(c) Mannose(d) Fructose.
13. The glycosaminoglycathnat servesas an anticoagulant
(a) Heparin(b) Hyaluronicacid (c) Chondroitinsulfate(d) Dermatansulfate.
14. The followingpolysaccharidies composedof B-glycosidibc onds
(a) Starch(b) Clycogen(c) Dextrin(d) Cellulose.
15. The carbonatomsinvolvedin the osazoneformation
(a)'l and 2 (b)2 and 3 (c)3 and 4 (d)5 and 6.

Lirpirdls

R--flc-o1?H"'-o-fr i The Jat speaks :
CH2-H R3 "\ffith uater, I say, 'Touch me not':

T'otlte tongue,I am tasteful;
IY'ithin limits, I am datiful;
fn excessI, am dangerous!"

I ipids (Creek: lipos-fat) are of Breat 1. Simple lipids : Estersof fatty acids with

L im por t anc e to th e b o d y a s th e chi ef al cohol s.Theseare mai nl y of tw o types

concentratedstorageform of energy, besides (a) Fatsand oils (triacylglycerols: )Theseare
t heir r ole in c e l l u l a rs tru c tu rea n d v a ri o u sother estersof fatty acids with glycerol. The
biochemicalfunctions.As such. lioids are a difference between fat and oil is only
heterogeneous group of compounds ano, physical.Thus,oil is a liquid while fat is
therefore,it is rather difficult to define them a solid at room temperature.
preciselv.

Lipids may be regarded as organic substances (b) W axes: E stersof fatty aci ds(usual l yl ong
relatively insoluble in water, soluble in organic chai n)w i th al cohol sother than gl ycerol .
solvents (alcohol, ether etc.), actually or These alcohols may be aliphatic or
potentially related to fatty acidsand utilized by al i cycl i c.C etylal coholi s mostcommonl y
the living cells. found in waxes.

Unlik e the p o l y s a c c h a ri d e sp, ro te i n s and 2. C ompl ex(or compound)l i pi ds: Theseare

nuc leic acids , l i p i d s a re n o t p o l y me rs .F u rther, estersof fatty aci ds w i th al cohol s contai ni ng

lipidsare mostlysmallmolecules. additional groups such as phosphate,

ni trogenousbase, carbohydrate,protei n etc

They are furtherdivided as follows

Lipids a re b ro a d l y c l a s s i fi e d(mo d i fi e dfrom (a) P hosphol i pi dsT: hey contai n phosphor,c
Bloor) into simple, complex, derived and aci d and frequentl ya ni trogenousbase
m is c ellaneoul isp i d s ,w h i c h a refu rth e rs u b d i vi ded This is in additionto alcohol and fai:.
into differentgroups acids.

28

Chapter 3 : LIPIDS 29

(i) Glycerophospholipid: Ts hesephospho- 5. Lipidsprotectthe internalorgans,serveas

lipids contain glycerolas the alcohol insulatingmaterialsand give shapeand smooth

e .9 .,l e c i th i n ,c e p h a l i n . appearanceto the body.

(ii) Sphingophospholipid:sSphingosineis
the alcohol in this group of phospho-
lipidse.g.,sphingomyelin.

(b) Glycolipids: Theselipids contain a fatty Fatty acids are carboxylic acids with

acid, carbohydrateand nitrogenousbase.
T h e a l c o h o l i s s p h i n g o s i n eh, ence they hydrocarbonside chain. They are the simplest
are also called as glycosphingolipids.form of lipids.

Clycerol and phosphateare absente.g., Occurrence
cerebrosidesg, angliosides.

(c) Lipoproteins: Macromolecularcomplexes Fattyacidsmainly occur in the esterifiedform
as major constituentsof variouslipids. They are
of lipidswith proteins.

(d) Other complexlipids: Sulfolipids,amino- also present as free (unesterified)fatty acids.
li p i d sa n d l i p o p o l y s a c c h a ri d easre among Fattyaci dsof ani mal orgi n are much si mpl eri n
the othercomplexlipids. structure in contrast to those of plant origin
which often containgroupssuch as epoxy, keto,

3. Derived lipids: Theseare the derivatives hydroxy and cyclopentanerings.

obtainedon the hydrolysisof group 1 and group

2lip ids w h i c h p o s s e s sth e c h a ra cteri sti csof Even and odd carbon fatty acids

lipids .T h e s ei n c l u d eg l y c e ro la n d o th eral cohol s, Most of the fatty acids that occur in natural
fatty acids,mono- and diacylglycerolsl,ipid (fat) lipids are of even carbons(usually14C-2OC).
soluble vitamins, steroid hormones, hydro- This is due to the fact that biosynthesisof fatty
carbonsand ketone bodies. acidsmainly occurswith the sequentialaddition

4. M i s c e l l a n e o u sl i p i d s : T h e se i ncl ude a of 2 carbon units. Palmitic acid (l6C) and

large number of compounds possessingthe stearic acid (l$C) are the most common. Among

characteristics of lipids €.g., carotenoids, the odd chain fatty acids, propionic acid (3C)

squalene,hydrocarbonssuch as pentacosane(in and val eri caci d (5C )are w el l know n.

beeswax), terpenesetc.

NEUTRAT LIPIDS: The lipids which are Saturated and unsaturated
unchargedare referredto as neutrallipids.These fatty acids

are mono-, di-, and triacylglycerolsc, holesterol Saturatedfatty acids do not contain double
and cholesterylesters.
bonds,while unsaturatedfatty acidscontainone

Functions of lipids or more double bonds. Both saturated and
Lipids perform severalimportantfunctions unsaturatedfatty acids almost equally occur in
the natural lipids. Fattyacids with one double

1. They are the concentratedfuel reserveof bond are monounsaturateda,nd thosewith 2 or
more double bonds are collectivelvknown as
the body (triacylglycerols).

polyunsaturated fafty acids (PIJFA).
2. Lipids are the constituentsof membrane

structure and regulate the membrane Nomenclature of fatty acids
per m ea b i l i ty(p h o s p h o l i p i das n d c h o l esterol ).

3. They serve as a source of fat soluble The naming of a fatty acid (systematicname)
v it am in s(4 , D , E a n d K ). is based on the hydrocarbonfrom which it is
derived. The saturatedfatty acids end with a

4. Li p i d sa re i mp o rta n ta s c e l l u l a rmetabol i c suffix -anoic (e.g., octanoic acid) while the
regulators(steroidhormonesand prostaglandins). unsaturatedfatty acids end with a suffix -enoic

30 BIOCHEMISTF|Y

(e.9., octadecanoic acid). In addition to Length of hydrocarbon

systematicnames/ fatty acids have common cha:n of fatty acids

nameswhich are more widely used (Iable J. l). Dependingon the length of carbon chains,

Numbering of carbon atoms : lt startsfrom fatty acids are categorizedinto 3 groups-short

the carboxylcarbonwhich is takenas number 1. chain with less than 6 carbons; medium chain

The carbonsadjacentto this (carboxylC) are 2, with 8 to 14 carbons and long cfiain with 16 to

3, 4 and so on or alternatelya, F, T and so on. 24 carbons.

The terminalcarbon containingmethyl group is

known omega (or) carbon. Starting from the Shorthand representation

methylend,the carbonatomsin a fattyacid are of latty aclds

numberedas omega 1, 2, 3 etc. The numbering lnstead of writing the full structures,
of carbon atoms in two different ways is given biochemists employ shorthand notations (by
below numbers)to representfatty acids. The general

7654321 rule is that the total numberof carbonatomsare

cH3 - cH2 - cH2- cH2-cH2 - cH2 - COOH written first,followed by the nunrberof double

01 a2 o)3 ()4 ol5 (t)6 bonds and finally the (firstcarbon) position of

Common Name Systematicname Abbreviationx Structure

l.Saturatefdattyaclds Ethanoiaccid 2:0 CHsCO0H
n-Propanoaic id 3:0 CHgCHzCOOH
Aceticacid n-Butanoaiccid 4:0 CHs(CHz)z0O0H
Propionaiccid n-Pentanoaic id CHo(CHz)gCOOH
Butyricacid n-Hexanoaiccid F.n CHs(CHe)+COOH
Valeriac cid n-Octanoiaccid CHe(CHz)oCOOH
Caproiaccid n-Decanoaiccid 6:0 CHs(CHz)eC0OH
Capryliaccid n-Dodecanoaiccid 8:0 CHs(CHz)roCOOH
Capricacid n-Tetradecanaocicid 10:0 CHs(CHzhzCOOH
Lauricacid n-Hexadecanaocicid 12:0 CHg(CHz)t+CO0H
Myristiaccid n-Octadecanaocicid 14:0 CHs(CHz)roC0OH
Palmitiaccid n-Eicosanoaicid 16:0 CHg(CHz)reCOOH
Steariaccid n-Docosanoaiccid 1 8 :0 CHs(CHz)zo00OH
Arachidiaccid n-Tetracosanaocicid 2 0 :0 CH3(CHz)zzCOOH
Behenaiccid 22:0
Lignoceraiccid 24:0

ll. Unsaturatfeadttyacids

Palmitoleaiccid cr1s9-Hexadeceancoidic 1 6 :1 ; 9 CHg(CHz)sC= HCH(CHz)zCOOH

Oleicacid cls-9-Octadecenaocicid 1 8 :1 ; 9 CHs(CHz)zC=HCH(CHz)zCOOH
Linoleaiccid** cls,cls-9,12-Octadeca- 1 8: 2 ; 91, 2 CHg(CHz)+C=HCHCHzCH= CH(CHz)zCOOH

Linolenaiccid*x dienoiaccid 1 8: 3 ;9 ,1 2',1 5 CHoCHzC=HCHCHzC=HCHCHzCH
Allce9,12,15-0cta- =CH(CHz)zCO0H

decatrienoaicid

Arachidonaic id Allcls-5,8,11,14- 2 0 : 4 ; 5 , 8 , 1 1 , 1 4 CHg(CHz)+=CCHHCHzC=HCHCHzCH

Elc0:a!tr3e!o!1ci1___ __=9H9'tcl=_cl9F!)49oli

* Totanl unbeor fcarbonatonsf,ollowebdythenumbeortdoublebondasndthefirctcarbopnosrtioontthedoublbeond(s).

** Essentfiawl acids.

Ghapten 3 : LIPIDS 31

double bonds, startingfrom the carboxyl end. H..ar(CHz)zCOOH

Thus,saturatedfattyacid, palmiticacid is written

as.l6:0, oleic acid as 18:1;9, arachidonic H'c'1cHr;rcu,
acid as 20 : 4; 5, 8, 11, 14.

There are other conventionsof representing Oleic acid Elaldic acid
t he double b o n d s .Ae i n d i c a te sth a t th e doubl e (cls form) (fransform)
bond is between9 and 10 of the fatty acid. o 9
representsthe double bond position(9 and 10) Fig. 3.1 : Cis-trans isomerism in
unsaturated fattv acids.

from the <oend. Naturallyoccurringunsaturated

fatty acids belong to ro 9, ol 6 and o 3 series. on the posteriorand lateralpartsof limbs,on the

a 3 s eri e s L i n o l e n i ca c i d (18 : 3 ;9 , 12, 15) back and buttocks,lossof hair and poor wound

a 6 series Linoleic acid ('l8 : 2; 9, 12) and heal i ng.
arachidonicacid (20 : 4; 5, 8,

11,14) lsomerism in

ro9 series Oleic acid(18 : 1; 9) unsaturated fatiy aeids

The biochemically important saturatedand Unsaturated fatty acids exhibit geometric
unsaturated fatty acids are given in the isomerismdependingon the orientationof the
Table 3.1. groupsaroundthe double bond axis.

lf the atomsor acyl groupsare presenton the

same side of the double bond, it is a cis

configuration. On the other hand, if the groups

occur on the opposite side, it is a trans

The fatty acidsthat cannot be synthesizedby configuration. Thus oleic acid is a cis isomer

the body and, therefore, should be supplied in while elaidic acid is a transisomer,as depicted

the diet are known as essentiaflatty acids(EFA). in Fig.3.1. Cis isomersare lessstable than frans

Chemically, they are polyunsaturated fatty isomers. Most of the naturally occurring

acids, namely linoleic acid (18 : 2; 9, 12) and unsaturatedfatty acids exist as crs isomers.

Iinolenic acid (18 : 3; 9, 12, 15). Arachidonic In the cis isomericform, there is a molecular
ac id ( 20 :4 ;5 ,8 , 1 1 ,1 4 ) b e c o m e se ssenti ali,f binding at the double bond. Thus,oleic acid
its precursorlinoleic acid is not provided in the exi sts i n an L-shapew hi l e el ai di c aci d i s a
diet in sufficientamounts.The structuresof EFA strai ghtchai n. Increasei n the numberof doubl e
are given in the Table 3.1. bonds will cause more bends (kinks) and

Biochemical basis for essentiality: Linoleic arachi doni caci d w i th 4 doubl e bondsw i l l have

ac id an d l i n o l e n i c a c i d a re e s s e n ti alsi nce a U-shape.lt is believed that cis isomersof fatty

humans lack the enzymesthat can introduce acids with their characteristic bonds will

double bonds beyond carbons9 to 10. compactly pack the membranestructure.

Functions of EFA: Essentialfatty acids are Hydroxy fatty acids: Some of the fatty acids

required for the membrane structure and are hydroxylated.p-Hydroxybutyricacid, one of

function, transportof cholesterol,formation of the ketonebodiesproducedin metabolism,is a

lipoproteins,preventionof fatty liver etc. They simple example of hydroxy fatty acids.

are also needed for the synthesisof another Cerebronic acid and recinoleic acid are long

important group of compounds, namely chain hydroxyfatty acids.

eicosanoids(Chapter 32. Cyclic fatty acids: Fatty acids with cyclic

Deficiency of EFA: The deficiency of EFA structuresare rather rare e.g./ chaulmoogric acid

results in phrynoderma or toad skin, found in chaulmoogra oil (used in leprosy

characterizedby the presenceof horny eruptions treatment)containscyclopentenylring.

32 BIOCHEMISTFIY

U o
o cH2-o-c-R,
A CH2-O-C Fl, R z - C- o - c H
ltl
R2-C-O-CH I
O
ttl cH2oH
cH2-o-c-R3 1,2-Diacylglycerol

Triacylglycerol O CH,_OH
ill
o R-C-O-CH

ctH- 2-o-c -B I
HO_CH cH2oH
2-Monoacylglycerol
I

cH20H
1-Monoacylglycerol

Fig. 3.2 : General structures of acylglycerols
(For palmitoyl R = CtsHati for stearoyl R = C.rzHssiFor linoleoyl R = qtHsi

Eicosanoids:Thesecompoundsare relatedro of glycerol, are known (Fi5.3.2).Among these,

eicosapolyenoicfatty acids and include prosta- triacylglycerols are the most important

glandins ,pro s ta c y c l i n sl e, u k o tri e n eas n d throm- bi ochemi cal l y.

boxanes.Theyarediscussedtogether(Chapter 32). Simpletriacylglycerolscontainthe sametype

of fatty acid residueat all the threecarbonse.g.,

tristearoylglycerol or tristearin.

Mixed triacylglycerols are more common.

Triacylglycerols (formerly triglycerides) are They contain2 or 3 different typesof fatty acid

the estersof glycerol with fatty acids. The fats residues.In general,fatty acid attachedto C1 is

and oils that are widely distributedin both plants saturated,that attached to C2 is unsaturated

and animals are chemically triacylglycerols. while that on C3 can be either.Triacylglycerols

They are insolublein water and non-polarin are named according to placement of acyl
characterand commonly known as neutral fats. radi calon gl ycerole.9.,' l,3-pal mi toy2l -l i nol eoyl

Fats as stored fuel : Triacylglycerolsare the gl ycerol .

most abundantgroup of lipids that primarily Triacylglycerols of plants, in general, have

function as fuel reservesof animals. The fat higher content of unsaturated fatty acids

reserveof normal humans (men 2Oo/ow, omen compared to that of animals.

25% by weigh$ is sufficientto meet the body's

caloric requirementsfor 2-3 months. $tereospecific numbering

Fats primarily occur in adipose tissue: of gl ycerol

Adipocytes of adipose tissue-predominantly The structureof glycerol givesan impression

found in the subcutaneouslayer and in the thatcarbons1 and 3 are identical.This is not true

abdominalcavity-are specializedfor storageof in a 3-dimensionasl tructureI.n orderto represent

triacylglycerolsT. he fat is stored in the form of the carbonatomsof glycerolin an unambiguous

globulesdispersedin the entire cytoplasm.And manner, biochemists adopt a stereospecific

surprisinglyt,riacylglycerolsare not the structural numbering(sn)and prefix glycerolwith sn.

componentsof biological membranes. 6n,on
no-C'.-H
Structures of acylglycerols: Monoacyl-
glycerols, diacylglycerolsand triacylglycerols,

respectivelyconsistingof one, two and three 6tr,ot

moleculesof fatty acids esterifiedto a molecule sn-GfcJrol

C*rapter'3: LIPIDS 33

It should be noted that C1 and C3 are a. tipid peroxidationin vivo: In the living

different. Cells possess enzymes that can cells, lipids undergo oxidation to produce

dis t inguis h th e s e tw o c a rb o n s. Thus peroxidesand free radicalswhich can damage
glycerokinasephosphorylatesn-3(and not sn-l) the tissue.The free radicalsare believedto cause

glycerol to give sn-glycerol3-phosphate. inflammatory diseases, ageing, cancer/

atherosclerosiestc. lt is fortunatethat the cells

PROPERTIESOF TRIACYLGTYCEROLSpossessantioxidantssuchas vitamin E,urateand

A few importantpropertiesof triacylglycerols, superoxidedismutaseto prevent in vivo lipid
which have biochemical relevance, are peroxidation (Chapter 34).

d i s c u s s e db e l o w Tests to check purity

1. Hydro l y s i s: T ri a c y l g l y c e ro l s u n dergo of fats and oils

stepwiseenzymatichydrolysisto finally liberate Adulterationof fats and oils is increasingday

free fatty acids and glycerol. The processof
hydrolysis,catalysedby lipasesis importantfor by day. Several tests are employed in the
digestionof fat in the gastrointestinatlract and laboratoryto check the purity of fats and oils.
Some of them are discussedhereunder
fat mobilizationfrom the adiposetissues.

2. Saponification: The hydrolysisof triacyl- lodine number: lt is defined as the grams

glycerolsby alkalito produceglyceroland soaps (number) of iodine absorbedby 100 g of fat or
oil. lodine number is usefulto know the relative
is known as saoonification.
unsaturationof fats,and is directly proportional

Triacylglycerol+ 3 NaOH ---------+ to the content of unsaturatedfatty acids. Thus

Clycerol+ 3 R-COONa(soaps) lower is the iodine number,lessis the degreeof

3. Rancidity: Rancidity is the term used to unsaturati onT. he i odi ne numbersof common
represent the deterioration of fats and oils oils/fatsare given below.

resultingin an unpleasant aste.Fatscontaining FaUoil lodine number
unsaturatedfatty acids are more susceptibleto

ranciditv. Coconouitl 7- 10
Butter 25- 28
Rancidity occurs when fats and oils are Palmoil
exposed to air, moisture, light, bacteria etc. Oliveoil 4C- 55
Hydrolytic rancidity occurs due to partial Groundnouitl
hydrolysis of triacylglycerols by bacterial Cottonseoeidl 80- 85
enzymes.Oxidativerancidityis due to oxidation Sunflowoeirl 85- 100
of unsaturatedfatty acids. This results in the Linseeodil 100- 110
formation of unpleasant products such as 125- 135
175-200

dicarboxylic acids, aldehydes, ketones etc. D etermi nati onof i odi ne numberw i l l hel p to

Rancid fats and oils are unsuitablefor human know the degreeof adulterationof a given oil.

consumotion. Saponificationnumber : lt is defined as the

Antioxidants : The substanceswhich can mg (number) of KOH required to hydrolyse

preventthe occurrenceof oxidativerancidityare (saponify)one gram of fat or oiL Saponification

known as antioxidants. Trace amounts of number is a measureof the averagemolecular

ant iox ida ntss u c h a s to c o p h e ro l s(v i ta mi n E ), sizeof the fattyacidspresentT. he value is higher

hy dr oqu inon e ,g a l l i c a c i d a n d c ,-n a p h tholare for fats containing short chain fatty acids. The

addedto the commercialpreparationsof fatsand saponificationnumbersof a few fatsand oils are

oils to preventrancidity.Propylgallate,butylated given below

hydroxyanisole(BHA) and butylated hydroxy- H umanfat : 195-200

toluene (BHT) are the antioxidantsused in food Butter :230-240

preservation. Coconutoil : 250-260

34 ElIOCHEMISTRY

Reichert-Meiss(lRM) number: lt is definedas There are two classesof phospholipids
the number of ml 0.1 N KOH required to
completelyneutralizethe soluble volatile fatty 1. C l ycerophosphol i pi ds(or phosphogl yce-
acidsdistilledfrom 5 g fat. RM number is useful rides)that contain glycerol as the alcohol.
in testingthe purity of butter since it containsa
good concentrationof volatilefattyacids(butyric 2. S phi ngophosphol i pi d(osr sphi ngomyel i ns)
that contai nsphi ngosi neas the al cohol .

acid, caproicacid and caprylicacid).This is in 1.i t\ .;:i r,. : . ,,,.i., i-l,
contrastto other fats and oils which have a ".t

negligibleamount of volatile fatty acids. Butter C l ycerophosphol i pi dsare the maj or l i pi ds
hasa RM numb e ri n th e ra n g e2 5 -3 0 ,w h i l e i t i s that occur in biologicalmembranesT. hey consist
les st han I f o r mo s t o th e r e d i b l e o i l s . T h u s a ny of glycerol 3-phosphateesterifiedat its C1 and

adulteration of hutter can be easily tested by C2 with fatty acids. Usually, C1 contains a
saturated fatty acid while C2 contains an
this s ens it iv eRM n u m b e r.

unsaturatedfatty acid.

Acid number: lt is definedas the numberof 1. Phosphatidicacid : This is the simplest
mg of KOH requiredto completely neutralize phospholipid. lt does not occur in good
free fatty acids presentin one gram fat or oil. In concentration in the tissues. Basically,

normalcircumstancesr,efinedoils shouldbe free phosphati di caci d i s an i ntermedi atei n the
from any free fatty acids. Oils, on synthesi sof tri acyl gl ycerolasnd phosphol i pi ds.

decomoosition-due to chemical or bacterial
contamination-yield free fatty acids.Therefore, The other gl ycerophosphol i pi dcsontai ni ng
oils with increasedacid number are unsafefor differentnitrogenousbasesor other groupsmay
be regardedas the derivativesof phosphatidic
humanconsumption.

acid.

2. Lecithins (phosphatidylcholine)zThese are *

the mostabundantgroupof phosphol i pi dsi n the
cel l membranes.C hemi cal l y,l eci thi n (C reek :
These are complex or compound lipids lecithos-egg yolk) is a phosphatidicacid with
c o n t a i n i npgh o s p h o r iacc i d ,i n a d d i t i o nt o f a t t y choline as the base. Phosphatidylcholines
acids,nitrogenoubs aseand alcohol (Fig.3.3). represent the storage form of hody's choline.

BtoMEDtCAL/ CLtNtCAt CONCEpTS

os Lipids are important to the body as constituentsof membranes,source ol fat soluble
(A, D, E and K) uitaminsqnd metabolic regulators(steroid hormonesand prostaglandlns),

e Triacylglycerols (fots) primarily stored in the adipose tissue ore concentrated t'uel
reseruesof the body. Fatst'ound in the subcutoneoustissue and around certaln orgons
serueos thermal insulators,

se The unsaturatedfatty acids-linoleic and linolenic acid-<re essentiolto humans, the
deficiency of which cousesphrynodermo or toad skin.

s The cyclicfatty acid, namelychoulmoogricocid,isemployedin the treatmentof leprosy.

og Fqts and oils on exposureto ah; moisture, bacteria etc. undergo rancidity (deterioration).
Thts can be preuented by the addition ol certain antioxidants (uitamin E, hgdroquinone,
gallic acid).

w In food preseruation,antioxidants-namely propyl gallote, butylated hydroxyanisole
and butylated hydroxytoluene--arecommonly used.

Chapter 3 : LIPIDS 35

o ,11
ll ill
g
cH2-o-c-R1 i tz)Leclthln(phosphatidylcholine)

ill .:1

R-I-l C-O-CH

CH2-i-'-r'- i't

(1) Phosphatldicacid

o

tl

I CH2-O-C-R1
ill
rf
R2-C-O-qH

C H 2 - C -- - l - C H 2 - C H 2 - N H 2 myalnositol
(4)Phosphatidyllnosltol
C- Ethanolamine
(3)Cephalln(phosphatidylethanolamine)

o
o -t l c
2?-c cH2 - {l - R l A QH2-O-CF{=CH-Rl
-?H ltl
.:1
R - o R2-C-O-CH

CH2-r-:- = C-CHz-CH-COO- CH2-t', -i' i----CHz-CH2-NH2
C,t_-_ Ethanolamine
o .),f,l
(5)Phosphatldylserlne (6) Plasmalogen(phosphatidalelhanolamine)

? cH2 - o- c - R 1cH2-, r, n -HccH- o2- c?- R 3
-C-O-CH I H?-OH tr.
R2
?
CH2-.i ,: r-.'-CHe R4-C-O-CH2
^ +

l- ehospnatioytgty."rIo,

(7)Cardlollpin(diphosphatidylglycerol)

lCeramid" _

(/t'soninoosrne$)>

CH3-(CH2)12-CH:CH-CH-?H-NH-C-R

',
r_-CHz-CHz-*.NCHf9g,Tt

Choline \'n3

(8)Sphlngomyelln

Fig. 3.3 : Sttucturesof phospholipids.

36 BIOCHEMISTF|Y

(a) Dipalmitoyl lecithin is an important by an amide linkageto a fatty acid to produce

ph os p h a ti d y l c h o l i nfeo u n d i n l u n g s,l t i s a ceramide.The alcohol group of sphingosineis

surface active agent and prevents the bound to phosphoryl chol i nei n sphi ngomyel i n

adherence of inner surface of the structure(Fig.3.3)S. phingomyelinas re important

lungs due to surfacetension. Respiratory constituentsof myelin and are found in good

distresssyndromein infantsis a disorder quantityin brain and nervoustissues.

characterizedby the absenceof dipalmitoyl Action of phospholipases

lecithin.

(b) Lysolecithinis formed by removalof the Phospholipaseasre a group of enzymesthat
fatty acid either at C, or C, of lecithin. hydrolysephospholipids.There are four distinct
phosphol i pase(sA r, 42, C and D ), each one of
3. Cephafins (phosphatidylethanolamine):
Ethanolamineis the nitrogenousbasepresentin them specificallyacts on a particularbond. For
c ephalinsT, h u s ,l e c i th i na n d c e p h a l i nd i ffe rw i th details,refer lipid metabolism(Chapter l4).

regardto the base. Functions of phospholipids

4. Phosphatidylinosito:l The steroisomer P hosphol i pi dsconsti tutean i mportantgroup
myo-inositolis attachedto phosphatidicacid to of compound lipids that performa wide variety
giv ephos ph a ti d y l i n o s i to l (PTlh)i.si s a n i mp ortant of functions

c om Done nto f c e l l me mb ra n e s .T h e a cti on of 1. In associ ati own i th protei nsp, hosphol i pi ds
certain hormones(e.9.oxytocin, vasopressini)s form the structural componentsof membranes
m ediat edthro u g hPl . and regulatemembranepermeability.

5. Phosphatidylserine:The amino acid 2. Phospholipids (lecithin, cephalin and

s er ineis pre s e n ti n th i s g ro u p o f g l y c e rophos- cardi ol i pi n)i n the mi tochondri aare responsi bl e
pholipids. Phosphatidylthreoninisealso found in for maintaining the conformation of electron
c e r t a i nt i s s u e s .
transportchai n components,and thus cel l ul ar

6. Plasmalogen:s When a fatty acid is respiration.

attachedby an ether linkageat C1 of glycerol in 3. Phospholipidsparticipatein the absorption

the glycerophospholipids, the resultant of fat from the intestine.

compound is plasmalogen. Phosphatidal- 4. Phospholipids are essential for the
et hanola min ei s the most imoort antwhich is
similarin structu r et o phosphatidy lethanolamine synthesi sof different lipoproteins,and thus
but for the ether link age(in place of ester).An participate in the transport of lipids.

unsaturatedfatty acid occurs at C1. Choline, 5. Accumulationof fat in liver (fattyliver)can

inositoland serinemay substituteethanolamine be preventedby phospholipids,hence they are
regarded as lipotropic factors.
to give otherplasmalogens.

Z. Cardiolipin: lt is so named as it was first 6. Arachidonicacid, an unsaturatedfattyacid

isolated from heart muscle. Structurally, a liberated from phospholipids, serves as a
cardiolipin consists of two molecules of precursorfor the synthesisof eicosanoid(sprosta-

phos pha tidi ca c i d h e l d b y a n a d d i ti o n a lg l ycerol glandins,prostacyclinst,hromboxanesetc.).

through phosphategroups. lt is an important 7. P hosphol i pi dsparti ci patei n the reverse

c om pone nto f i n n e r mi to c h o n d ri a lm e mbrane. chol esterol transport and thus hel p i n the

Car diolip in i s th e o n l y p h o s p h o g l y ce ri dethat removalof cholesterolfrom the body.

possessesantigenic properties. 8. Phospholipidsact as surfactants(agenL.

Sphingomyelins lowering surface tension). For instance

Sphingosineis an amino alcohol presentin di pal mi toylphosphati dyl chol i nies an i mportar:
s phingomy e l i n(ss p h i n g o p h o s p h o l i p i dTsh).ey do fung surfactant. Respiratory distresssyndrome ^
infantsis associatedwith insufficientproductio^

not containglycerolat all. Sphingosineis attached of this surfactant.

Chapter 3 r LIPIDS 37

Sphingosine

loHlo

YIY]

o-cH2

Fig. 3.4 : Structure ot galactosylceramide (R = H). Fot sulfagalactosylceramideR is a sulfatide (R = SOi-).

9. Cephalins,an importantgroupof phospho- Ceramide
lipids partic i p a tei n b l o o d c l o tti n g .
I
10. P h os p h o l i p i d s(p h o s p h a ti d y l i n o si toal )re
inv olv edin s i g n atl ra n s mi s s i oanc ro s sme mbranes. Glucose

f Galactos
tl

N-Acetyl- N-Acetyl-
galactosamine neuraminiaccid

C lyco Iipids (glycosphingol ipids) are important

c ons t it uentso f c e l l me mb ra n e a n d n ervous

tissues(particularlythe brain). Cerebrosidesare

t he s im p les tfo rm o f g l y c o l i p i d sT. h e y co ntai na Lipoproteinsare molecular complexes of
ceramide (sphingosineattachgdto a fatty acid) lipids with proteins. They are the transport
and one or more sugars.Galactocerebroside vehi cl esfor l i pi ds i n the ci rcul ati on.There are
(galactosylceramidea)nd glucocerebrosideare five types of lipoproteins,namely chylomicrons,
the most importantglycolipids.The structureof very low density lipoproteins (VLDL), low
galactosylceramidies given in Fig3.a. lt contains density lipoproteins (LDL), high density

the fatty acid cerebronic acid. Iipoproteins (HDL) and free fatty acid-albumin
Sulfagalactosylceramideis the sulfatide derived complexes. Their structure, separation,
from galactosylceramide. metabolismand diseasesare discussedtogether

Gangliosides: Theseare predominantlyfound (Chapter l4).

in ganglionsand are the most complex form of

glycosphingolipidsT. hey are the derivativesof

cerebrosidesand containone or more molecules

of N- ac ety l n e u ra m i n iac c i d (N A N A ), th e most

im oor t ants i a l i c a c i d . T h e s tru c tu reo f N A N A i s Steroids are the compounds containing a

given in carbohydratechemistry(ReferFig.2.l1\. cycl i c steroi d nucl eus (or ri ng) namel y

The most important gangliosidespresent in cyclopentanoperhydrophenanthrene (CPPP). lr
t he br a in a re C M1 , C M2 , C D , a n d C T, consistsof a phenanthrenenucleus (ringsA, B
(G representsganglioside while M, D and T and C ) to w hi ch a cycl opentaneri ng (D ) i s

indicaternono-, di- or tri- sialic acid residues, attached.

and the number denotes the carbohydrate The structureand numbering of CPPP are

sequence attached to the ceramide). The shown in Fi9.3.5.The steroidnucleusrepresents

gangliosideC, M2 that accumulatesin Tay-Sachs saturatedcarbons,unlessspecificallyshown as
diseaseis reoresentednext (outlinestructure). doubl e bonds.The methyl si de chai ns(19 and

38 BIOCHEMISTF|Y

Structure and occurrence

The structure of cholesterol (C27Ha6O)is
depictedin Fig.3.5.lt hasone hydroxylgroup at
C3 and a double bond between C5 and C6.
An 8 carbon aliphatic side chain is attachedto
C17. Cholesterolcontains a total of 5 methyl
Sroups.

Due to the presence of an -OH group,
chol esteroils w eakl yamphi phi l i cA. s a structural
component of plasma membranes,cholesterol
is an important determinant of membrane
permeabilityrp, roperties. The occurrence of
cholesterolis much higher in the membranesof
sub-ceIlular organeles.

Cholesterolis founi in associationwith fany
acids to- form cholestervlesters(esterification
occursat the OH group of C3).

Properties and reactions: Cholesterol is an
yel l ow i sh crystal l i nesol i d. The crystal s,under
the microscope, show a notched (E)
appearance.Cholesterolis insoluble in water
and soluble in organic solvents such as
chloroform,benzene,ether etc.

Fig. 3.5 : Sttucturcs of steroids (A, B, C-Perhydro- Several reactionsgiven by cholesterolare
phenanthrene; D-Cyclopentane). useful for its qualitative identification and
quantitativeestimationT. heseincludeSalkowski's

test, Liebermann-Burchardreaction and Zak's

18) attachedto carbons10 and 13 are shown as test.

s ingle bond s . At c a rb o n 1 7 , s te ro i d susual l y Functions of cholesterol: Cholesterol is a

containa sidechain. poor conductor of heat and electricity,since it

There are severalsteroidsin the biological has a high dielectric constant.lt is presentin

s y s t em .The s e i n c l u d e c h o l e s te ro l ,b i l e aci ds, abundance in nervous tissues.lt appearsthat
vitamin D, sex hormones, adrenocortical cholesterolfunctions as an insulatingcover for

hor m ones ,s i to s te ro l s ,c a rd i a c g l y c o s i d esand the transmi ssi onof el ectri cal i mpul sesi n the
alk aloids . lf th e s te ro i d ,c o n ta i n so n e o r more nervous tissue. Cholesterol performs several

hy dr ox yl g ro u p s i t i s c o m m o n l y k n o w n as other bi ochemi calfuncti onsw hi ch i ncl ude i ts

sterol (meanssolid alcohol). role in membranestructureand function, in the
synthesisof bile acids, hormones (sex and

CI{OLESTEROL cortical) and vitamin D (for details, Refer

Chofesterol, exclusively found in animals, is Chapters 7 and l9).

the most abundantanimal sterol.lt is widely

distributedin all cells and is a major component ERGOSTEROL

of cell membranesand lipoproteins.Cholesterol Ergosterool ccurs in plants.lt is also found as

(Creek: chole-bile)was first isolatedfrom bile. a structuralconstituentof membranesin yeast

Cholesterolliterally means 'solid alcohol from and fungi. Ergosterol(Fig.3.5)is an important

bile.' precursorfor vitamin D. When exposedto light,

Ti 39
Cfrraptee3 : LIPIDS

li
I the ring B of ergosterool pensand it is converted

cH3(cHdn-coo- to ergocalciferol, a compound containing

Hydrophobic Hydrophiic vitamin D activity.
The other sterolspresentin plant cells include
hydrocarbonchain carboxylgroup

(tail) (head)

(A) Fatty acid stigmasterol and ftsitosterol.

o

tl

Hydrophobic HYdroPhilic A s per defi ni ti on,l i pi ds are i nsol ubl e(hydr o-
tail head phobic) in water.This is primarilydue to the
predominant presenceof hydrocarbongroups.
(B) Phospholipid
However, some of the lipids possesspolar or
hydrophi l i cgroupsw hi ch tend to be sol ubl ei n
water. Molecules which contain both
hydrophobicand hydrophilicgroupsare known
as amphipathic (Creek : amphi-both, pathos-
passion).

(C)Amphipathic lipid Examplesof amphipathic lipids: Among the
l i pi ds,fatty aci ds,phosphol i pi dss, phi ngol i pi ds,
bile salts and cholesterol(to some extent)are
amphi pathi ci n nature.

Aqueous Phospholipidshavea hydrophilichead (phos-
pnase phate group attachedto choline, ethanolamine,
inositoletc.) and a long hydrophobictail. The
generalstructureof an amphipathicIipid may be
representedas a polar or hydrophilic head with
a non-polaror hydrophobictail (Fig.3.6).

(D)Micelle Fattyacidscontain a hydrocarbonchain with
a carboxyl (COO-) group at physiologicalpH.
The carboxyl group is polar in nature with
affinityto water (hydrophilic)while hydrocarbon
chain of fatty acid is hydrophobic.

OtOtOtOt lC Orientation amphipathic lipids: When the
ltttl
Nonpolarphase Aqueousphaasmephipathiclipids are mixed in water (aqueous

oo phase), the polar groups (heads) orient
Aqueousphase themsel vestow ards aqueous phase w hi l e t he
(E)Lipidbilayer non-polar (tails) orient towards the opposite

directions.This leadsto the formation of mr'celles
(Fi9.3.6).Micelle formation, facilitated by bile

salts is very important for lipid digestionand
absorption (Chapter 8).

Fig. 3.6 : Summary of amphipathic lipids in the Me*nhrane bilayers
formation of micelle and lipid bilayer.
In caseof biologicalmembranes,a bilayerof
l i pi ds i s formedori enti ngthe pol ar headsto t he

40 BIOCHEMISTRY
BIOMEDICAL/ CLIITIICALCOilCEPTS

The phospholipid4ipalmitoyl lecithin-preuents the adherence of inner surface of the
lungs, the absenceof which is ossociofed with respiratory disfress syndrome in infants.

!ei- Cepholinsparticipate in blood clotting.

€ The action of certain hormones is mediated through phosphatidylinositol.

g Phospholipids are important for the synthesisand transport of lipoproteins ond reuerse
tronsport ol cholesterol.

Cholesterolis essentialfor the synfhesisol bile ocids, hormones(sexand cortical)and
uitaminD.

Lipoproteinsoccur in the membronestructure, besidesseruingos o meansol transport
uehiclesfor lipids.
Lipids are associatedwith certain disorders----obesityond atherosclerosis.

out er aq u e o u s p h a s e o n e i th e r s i d e and the combi nati onw i th ti ssue speci fi canti B ens,ar e

nonpolar tails into the interior (Fig.3.6\.fhe used as carriersof drugs to tarBettissues.

formation of a lipid bilayer is the basis of Emulsions: These are produced when non-
membranestructure. pol ar l i pi ds (e.g. tri acyl gl ycerol s)are mi xed

Liposomes: They are producedwhen amphi- with water. The particlesare larger in size and

pat hic lip i d s i n a q u e o u sm e d i u m a re subj ected stabi l i zed by emul si fyi ng agents (usual l r

to sonification. They have intermittent aqueous amphipathic lipids), such as bile salts and

phases in the lipid bilayer. Liposomes,in phospholipids.

LIPIDS

saucbtusatallnycoerspreoltaetniutieallylyinrseola,ltuebdletoin water, soluble in organic
totti-o"iJl'ord or" utilized

2 Lipids are cr ass ifie dinto simp re(fat s
deriued (fatty aci ds, steriodn"r _"n"rl , aanndd. ,
omi ri ssc)c,e,lol omn p r e x( p h o s p h o r i p i d sg, rg colip i d s) ,
eous(carotenoids).
3' iaFi[aci:ti"tdy0sa"aicr:m'i:d;so:a:sir:eeti qthueamrsaaoioncrdccutoirninnost"tietttun"e*ion,i',t1osidifo.ou1a,1"rii,othruise'[.r'siip"':siduesr,s."'artirtaiuiroirraiattvteedoda,niafda,uttnhtysaaattcnuierdoest(deptdfora'FtbtAye)

4' ;Pni;Tn:hurIia:oam,dsc:ipJbyphlieog:o'sRrl:ty,e:icpM;t'eii,dsnr:ssoiu;aul,srm(e:esia:ibn;mc,de:opoprml)yrta:pifmrlaeeetoxmsr)ialpriapryrein,od*tsdgnhretecthiindcoeetrnts.snotteaei",nrinisco7'i,nftiuo-.g"gurlliyrd.prJc"eheoror'r"r"os/"-lpi'iw"hooioijt"ti"rh-oii'""clfiaiia'^nrcto"ttitdyiirnn..pe,Gaupscleyrihdicutoseeyo.Trsrfoahpprefhhoyteoatasssatrtpresn(ahfidooovdulroiipninniidredoss,iitot,
5'

6' tsJpih:;ifnig,!o:,pthtvocspehroolpiphidos(sspph.hointig;oidrnsyii.eh-l-inossc)pohntoarinipsidph;"ini;;,g;;'o;;"s;in:,eas

the alcohol in place of

constituenotsf ptasma

7 oncneeerreuoborruosmstiiodsrseeusmea.roeGteathcnuegl.resisoimosfipdrNeess-aatrfceoerptmryelndoeofumrgainrmoyicnnotircrgi.p"ri;od,;uswnifhdi;i:ci;nhr;:to,i)"cic."utgroinrsttihoensm. Temhebyrcaonnetsaoinf
8. 2achiSmho.dtor"eomlpr?euoosbo;indtl:eeres:rtc:sbaa:o-orA:ninn'sittdnacs:thi(.tncecee"5rtr:hom1"u,i)Tdeodr"sci"nentoa#agn.lnbctu[dauy:tic:nan:t;ldnio:"nq:pig";inei,',gton":alr,tx".nta"i:ino,mc_o,ro"tapi*,Jr:eorsaar'rtihe.oroyr.,nads,arr,hir,d.o,,yIretr,pd,"rhcr,-ohecrixonoiiyiraninrJ'nti,ngat,t"irhi'",noi"rr;"snueio;o.pn;na.seeTJi's.I":hbk,:yn,#do,cr,rwpon:.nx:eytaogrscsrih/os!iobts^uelpeiir,ros("earteJ.at rloccflzr-i)i,ds,,isZ,,,
9'

r0 'kT;:nh;roe:w#l:ni:p.ai''dasstahtmhicapthoipoar:t:hr:riipci.dbrshooethnsehvii^ndp,roio"ipaoh"i obioicr(icn,ooonn,s,;-t;pi.t;ou:r;e;a;nrht);asii;nn,d:;t:hh, yedbroiipahesierpirhcs(ionpfsoortahrirep)ibdgisaroonruodpqsaibcriaetel

42 BIOCHEMISTRY

I . Essayquestions
1. Writean accountof classificatioonf lipidswith suitableexamples.
2. Describethe structureandfunctionsof phospholipids.
3. Discussthesaturatedandunsaturatefdattyacidsof biologicalimportancea, longwith theirstructures.
4. Describethe structureof steroidsA. dd a noteon the functionsof cholesterol.
5. Discussthe biologicailmportanceof amphipathiclipids.

II. Short notes
(a) Structureof triacylglycerols(b, ) Clycolipids,(c) Essentiafal tty acids,(d) Cis-transisomerism,
(e) Rancidity(,0 lodinenumber,(g) Phosphatidylinosito(hl,) Sphingomyelins(i,) Steroidnucleus,
(j)Micelles.

III. Fillin the blanks
1 . Thelipidsthatfunctionasfuel reservein animals
2 . The isomerismassociatewd ith unsaturatefdatWacids
3 . Thecyclic fattyacidemployedin the treatmenot f leprosy
4. The lipidsthatare not the structuracl omponentsof biologicalmembranes
5. Theprefixsn usedto represengt lycerol,snstandsfor
6. Thenumberof mg of KOH requiredto hydrolyse1 g fator oil is knownas
The phospholipidthat preventsthe adherenceof innersurfacesof lungs
B . Thephospholipitdhatproduces econdmessengerisn hormonaal ction
9. NametheglycolipidscontainingN-acetylneuraminaic id

10 . Thesteroidcsontaina cyclicringknownas
IV. Multiple choice questions

11. Thenitrogenoubsasepresenitn lecithin
(a)Choline(b)Ethanolamin(ec)Inosito(l d)Serine.

12. Thenumberof doublebondspresenitn arachidonicacid
(a)1 (b)2 (c)3 @)a.

13. On eo f th efo l l o w i n gi sa n a mp h i pathilci pi d
(a)Phospholipid(sb)Fattyacid(c)Bilesalts(d)All of the above.

14. Esterificatioonf cholesterool ccursat carbonposition
(a)1 (b)2 (c)3 (d)4.

15. Namethe testemployedto checkthe purityof butterthroughthe estimationof volatilefattyacids
(a)lodinenumber(b)Reichert-Meisnsul mber(c)Saponificationnumber(d)Acid number.

rl trii,

p rotuint are the most ahundant organic Structuralfunctions: Certainproteinsperform
I molecules of the living system. They occur
in every part of the cell and constituteabout brick and mortar roles and are primarily
50'h of the cellular dry weight. Proteinsform responsiblefor structureand strengthof body.
the fundamentalbasisof structureand function Theseinclude collagen and elastinfound in bone
of life. matrix, vascularsystemand other organs and
a-keratin presentin epidermaltissues.

Origin of the wotrd 'protein' Dynamic functions : The dynamic functions
of proteinsare more diversifiedin nature.These

The term protein is derived from a Creek include proteins acting as enzymeq hormones,

word proteiog meaning holding the first place. blood clotting factors, immunoglobulins,

Berzelius(Swedishchemist)suggestedthe name membrane receptors,storageproteins, besides

proteinsto the group of organiccompoundsthat thei r functi on i n geneti c control , mus cl e

are utmost important to life. Mulder (Dutch contraction,respirationetc. Proteinsperforming

chemist)in 1838 used the term proteinsfor the dynamic functionsare appropriatelyregardedas

high mo l e c u l a rw e i g h t n i tro g e n -ri chand most the working horsesof cell.

abun dan t s u b s ta n c e sp re s e n t i n a n i mal s and Elermental cornposition clf Broteins
olants.

Proteinsare predominantlyconstitutedbv five

Functions of proteins major elementsin the following proportion.

Proteinsperforma greatvarietyof specialized Carbon 50 - 55%
Hydrogen 6 - 7.3%
and e s s e n ti aflu n c ti o n si n th e l i v i n g ce l l s.These Oxygen
functions may be broadly grouped as static Nitrogen 19 - 24%
(structural) and dynamic. Sulfur 13 - 19%

0 - 4o/"

43