2. Dr, lJ, Satyanarayana
M.Sc,.Ph.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.
BCDCDT(SAn|D ALLTED lPf Ltd.
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3. Eiochemistrg
First Published : March 1999
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RevisedReprint: August 2000
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Second Revised Edition : June 2002
Reprinted: 2003
RevisedReprint: 2004
RevisedReprint: 2005
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4. Prefaceto the Third Edition
Theresponseto the first andthe secondeditionsof my book'Biochemistry'(reprintedseveraltimesin
just 6 years)from the studentsandteachersis simplyoverwhelming.I wasfloodedwith highlyappreciative
lettersfrom all cornersof Indiaandabroad!Thisgivesme immensesatisfactionandencouragemLntin this
academicventure.
I havecorrespondedwith manybiochernistryteachers,invitingtheir commentsandopinionsfor further
improvingthe book.Mostof them havebeenkind enoughto offerconstructivesuggestions.I alsovisited
severalcollegesandhadpersonalinteractionwith facultymembersandstudents.Theseexercises,spreadover
the past 6 years,have helpedme to get direct feedbackon my book, besidesrealisingthe additional
requirementsof students.
I havegreatpleasurein presentingthe third editionof my bookwith severalunique/novelfeatures,some
high-lightsof which are listedbelow.
. A thoroughrevisionandupdatingof eachchapterwith latestadvances-
. Multicolouredillustrationsfor a betterunderstandingof chemicalstructuresandbiochemicalreactions.
. Increasein the font sizeof the text for morepleasantandcomfortablereading.
o Incorporationof a newSectionon MolecularBiologyandBiotechnology.
. Additionof ten new chapters-humangenomeproject,genetherapy,bioinformatics,free radicalsand
antioxidants,tissueproteinsandbodyfluids,environmentalbiochemistry,genetics,immunologyetc.
. An improvedorientationand treatmentof humanbiochemistryin healthanddisease.
. Additionof practicalbiochemistryandclinicalbiochemistrylaboratoryin the appendix.
It is true that I representa selectedgroupof individualsauthoringbooks,havingsometime at disposal,
besideshardwork,determinationanddedication.I considermyselfan eternallearneranda regularstudent
of biochemistry.However,it is beyondmy capabilityto keeptrackof theevergrowingadvancesin biochemistry
dueto the exponentialSrowthof the subject.And this makesme nervous,wheneverI think of revisingthe
book.I honestlyadmitthat I haveto dependon maturereadersfor subsequenteditionsof this book.
AN INVITATION TO READERS
It is not all the time possiblefor me to meetthe readersindividuallyandgettheir feedback,despitemy
ferventwish.Of course,I dowrite to somepeoplepersonaliyseekingtheir opinions.However,I wishto have
the commentsandsuggestionsof eachoneof the readersof my book.I sincerelyinvitethe readersto feelfree
andwrite to me expressingtheir frank opinions,criticalcommentsandconstructivesuggestions.
DT.U. SATYANARAYANA
trl
5. I owea deepdebtof gratitudeto my parents,the lateSri U.VenkataSubbaiah,andSmt. Vajramma,for
cultivatingin me the habitof earlyrising.Thewriting of this bookwouldneverhavebeenpossiblewithout
this healthyhabit.I am gratefulto Dr. B. S.NarasingaRao(formerDirector,NationalInstituteof Nutrition,
Hyderabad)for discipliningmy professionallife, andto my eldestbrother Dr. U. Gudaru(formerProfessorof
PowerSystems,WalchandCollegeof Engineering,Sangli)for discipliningmy personallife.
My elder son, U. Chakrapani(MBBS)deservesa specialplacein this book. He madea significant
contributionat everystageof its preparation-writing, verification,proof-readingandwhat not. I hadthe rare
privilegeof teachingmy sonashehappenedto bea studentof our college.Anda majorpartof this bookwas
writtenwhilehewaslearningbiochemistry.Thus,hewasthe firstpersonto learnthesubjectof biochemistry
from my handwrittenmanuscript.Thestudent-teacherrelation(ratherthan the father-son)hashelpedme in
receivinSlconstantfeedbackfrom him and restructurethe book in a way an undergraduatestudentwould
expecta biochemistrytextbookto be.
Next,I thankDr. G.PitcheswaraRao(formerProfessorof Anatomy,SMC,Vijayawada)for his constructive
criticism and advice,and Dr. B. Sivakumar(Director,NationalInstituteof Nutrition, Hyderabad)for his
helpful sugi5lestionson the microfigures.I am gratefulto 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 encouragementthis book could never have beenwritten. The manuscriptwas carefully
nurturedlike a newborn babyandthe bookhasnow becomea full-pledgedmemberof our family.
ACKNOWLEDGEMENTSTO THE THIRD EDITION
I amindebtedto a largenumberof friends,pen-friendsandstudentswhohelpedmeto reviseandimprove
the qualityof this book.I haveindividuallyandpersonallythankedall of them (whonumbera fewhundreds!).
I onceagainexpressmy gratitudeto them.
I thank my friend and colleague,Mr. M.S.T.JaganMohan,who has helpedme with his frequent
interactionsto improvethe book,andmakeit morestudent-friendly.I wouldlike to placeon recordmy deep
senseof appreciationto my post-graduate(M.D.)students,Dr. (Mrs.)U.B.VijayaLakshmiandDr. (Mrs.)Vidya
DesaiSripad,whoseperiodicalacademicinteractionandfeedbackhavecontributedto the improvementof the
biomedicaVclinicalaspectsin somechapters.I acknowledgethe helpof my friend,Dr. P.Ramanujam(Reader
in English,AndhraLoyolaCollege,Vijayawada)for his helpandencouragementin revisingthe book.
I expressmy gratitudeto Mr. ArunabhaSen, Director,Books & Allied (P) Ltd. Kolkata,for his
wholeheartedsupportand constantencouragementin revisingthe bookin multicolour,and takingall the
painsto bring it out to my satisfaction.I 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
encouragement.I am grateful to UppalaAuthor-PublisherInterlinks, Vijayawada,for sponsoringand
supportingme to bring out this edition.
Iiii]
DT.U. SAIYANARAYANA
6. Biochemistry
The term Biochemistrywas introducedby Carl Neubergin 1903.Biochemistrybroadlydealswith the
chemistrvof life and living processes.Thereis 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 medicine.Thisbecomesevidentfromthe factthat over
the years,the major shareof NobelPrizesearmarkedfor Medicineand Physiologyhasgoneto researchers
engagedir: biochemistry.
The disciplineof biochemistryservesas a torch light to trace the intricate complexicitiesof biology,
besidesunravellingthe chemicalmysteriesof life.Biochemicalresearchhasamplydemonstratedthat all living
thingsarecloselyrelatedat 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
diseases.the products(insulin,interferon,€rowthhormoneetc.)obtainedfrom geneticengiineering,andthe
possibleuseof genetherapyin the nearfuture.
0rganizationof the Book
This texthook,comprising43 chapters,is orgianizedinto serrensecl:ionsin the heirarchicalorder of
learninSbiochemistry.
. SectionI dealswith the chemicalconstituentsof life-carbohydrates,lipids,proteinsandaminoacids,
nucleicacidsandenzymes.
. SectionII physiologicalchemistryincludesdigestionandahsorption,plasmaproteins,hemoglobinand
prophyrins,andbiologicaloxidation.
. SectionIII incorporatesall the metabolisms(carbohydrates,lipids,aminoacids,nucleotides,minerals)
. Section[V covershormones,organfunctiontests,water,electrolyteandacid-basebalance,tissueproteins
andtrodi'fluids,andnutrition.
. SectionV is exclusivelydevotedto molecularbiologyandbiotechnology(DNA-replication,recombination,
ar"lnrepair,transcriptionandtranslation,regulationof geneexpression,recombinantDNAandbiotechnology)
. SectionVI givesrelevantinformation on current topics such a^shuman genomeproject,genetherapy,
bioirrtormatics,prostaglandins,diabetes,cancer,AIDSetc.
. Section VII dealswith the basic aspectsfor learning and understandingbiochemistry (bioorganic
chenristry',hiophysicalchemistrytoolsof biochemistry,genetics,immunology).
Each chapterin this book is carefully craftedwith colour illustrations, headingsand subheadingsto
facilitatequickunderstanding.Theimportantapplicationsof biochemistryto humanhealthanddiseaseareput
togetherasbiomedical/clinicalconcepts.Iconsare usedat appropriateplacesto serveas 'landmarks'.
The origins of biochemicalwords, confusablesin biochemistry,practicalbiochemistryand clinical
biochemistrylaboratory,givenin the appendixare novelfeatures.
Thebriokis so organizedasto equipthe readerswith a comprehensiveknowledgeof biochemistry.
Iiu]
9. BflomnoXeeutrssaildthsCelll
-l- hu living matter is composedof mainly six
I elements-carbon, hydrogen, oxygenl
nitrogen, phosphorus and sulfur. Theseelements
togetherconstituteabout 90% of the dry weight
of the human body. Severalother functionally
importantelementsare also found in the cells.
Theseinclude Ca, K, Na, Cl, Mg, Fe,Cu, Co, l,
Zn, F, Mo and Se.
earbon-a unique element of life
Carbonis the most predominantand versatile
elementof life. lt possessesa unique propertyto
form infinite number of compounds. This is
attributedto the ability of carbon to form stable
covalentbonds and C-C chains of unlimited
length. lt is estimated that about 90% of
compounds found in living system invariably
contain carbon.
Ghemical molecules of li#e
Life is composed of lifeless chemical
molecules. A single cell of the bacterium,
Escherichiacoli containsabout 6.000 different
organiccompounds.lt is believedthat man may
contain about 100,000 different types of
moleculesalthough only a few of them have
been characterized.
Sornpiex *riomoleeules
The organiccompoundssuchasamino acids,
nucleotidesand monosaccharidesserve as the
monomeric unitsor building blocksof complex
biomolecules-proteins,nucleicacids(DNA and
RNA) and polysaccharides,respectively.The
important biomolecules(macromolecules)with
their respective building blocks and major
functions are given in Table 1.1. As regards
lipids, it may be noted that they are not
biopolymers in a strict sense,but majority of
them contain fatty acids.
Structural heirarehy off asn organisnl
The macromolecules(proteins,Iipids,nucleic
acidsand polysaccharides)form supramolecular
assemblies(e.g. membranes)which in turn
organize into organelles,cells, tissues,organs
and finally the whole organism.
3
10. BIOCHEMISTFIY
Biomolecule Buildingblock
(repeatingunit)
Major functions
1. Protein Aminoacids
2. Deoxyribonucleicacid(DNA) Deoxyribonucleotides
Ribonucleotides3. Ribonucleicacid(RNA)
4. Polysaccharide(glycogen)Monosaccharides(glucose)
Fundamentalbasisofstructureand
functionofcell(staticanddynamicfunctions).
fl_eq_o_sitoryo.l.!9199iFryi{9l1llgt
Essentiallyrequiredlorproteinbiosynthesis.
Storageformofenergytomeetshortterm
demands.
5. Lipid Fattyacids,glycerol Storagetormofenergytomeetlongterm
demands;structuralcomponentsofmembranes.
Chem*ca! composition of man
The chemicalcompositionof a normal man,
weighing 65 kg, is given in Table 1.2.Water is
the solventof life and contributesto more than
60"h of the weight. This is followed by protein
(mostlyin muscle)and lipid (mostlyin adipose
tissue).The carbohydratecontent is rather low
which is in the form of glycogen.
The cell is the structuraland functional unit
of life. ft may be also regardedas the basic unit
of hiological activity.
The concept of cell originated from the
contributionsof Schleidenand Schwann(1838).
However, it was only after 1940, the
complexitiesof cell structurewere exposed.
Constituent Percent(7") Weight (kg)
Prokaryotic and eukaryotic cells
The cells of the living kingdom may be
divided into two categories
1. Prokaryotes(Creek: pro - before;karyon-
nucleus)lacka well definednucleusand possess
relatively simple structure.These include the
variousbacteria.
2. Eukaryotes(Greek: eu-true; karyon-
nucleus)possessa well definednucleusand are
more complex in their structureand function.
The higher organisms(animalsand plants)are
composedof eukaryoticcells.
A comparisonof the characteristicsbetween
prokaryotesand eukaryotesis listedin Table 1.3.
The human body is composedof about 1014
cells.There are about 250 typesof specialized
cel{s in- the human body'G.g. erythrocytes,
nerve-cells, muscle cells, B cells of pancreas.
An eukaryoticcell is generally10 to 100 pm
in diameter. A diagrammatic representation
of a typical rat liver cell is depicted in
Fig.I.t.
The plant cell differsfrom an animalcell by
possessinga rigid cell wall (mostlycomposedof
cellulose)and chloroplasts.The latter are the
sitesof photosynthesis.
Water
Protein
Lipid
Carbohydrate
Minerals
61.6
17.0
13.8
6.1
40
11
I
'|
4
11. Chapter 1 : BIOMOLECULESAND THE CELL
Characteristic Prokaryotic cell Eukaryoticcell
1. Size Small(generally1-10pm) Large(generally10-100pm)
2. Cellmembrane Cellisenvelopedbyaflexibleplasmamembrane
Distinctorganellesarefound
(e.9.mitochondria,nucleus,lysosomes)
3. Sub-cellular
organelles
4, Nucleus Notwelldefined;DNAisfound
asnucleoid,histonesareabsent
Nucleusiswelldefined,surroundedbya
membrane:DNAisassociatedwithhistones
5. Energymetabolism Mitochondriaabsent,enzymesof
energymetabolismboundto
Enzymesolenergymetabolismarelocated
inmitochondria
membrane
6. Celldivision
7. Cytoplasm
Usuallyfissionandnomitosis Mitosis
0rganellesandcytoskeleton
absent
Containsorganellesandcytoskeleton
(anetworkoftubulesandfilaments)
The cell consistsof well definedsubcellular
organelles,envelopedby a plasma membrane.
By differential centrifugation of tissue
homogenate, it is possible to isolate each
cellular organelle in a relatively pure form
(Refer Chapter 41). The distribution of major
enzymes and metabolic pathways in different
cellular organelles is given in the chapter
on enzymes (Refer Fig.6.6). The subcellular
organellesare briefly describedin the following
pages.
Nucleus
Nucleus is the largest cellular organelle,
surroundedbv a double membrane nuclear
envelope.The outer membraneis continuous
with the membranesof endoplasmicreticulum.
At certainintervals,the two nuclearmembranes
have nuclearporeswith a diameterof about 90
nm. Theseporespermit the free passageof the
products synthesizedin the nucleus into the
surrounding cytoplasm.
Roughendoplasmicreticulum
Golgiapparatus
Lysosome
Mitochondrion
Plasmamembrane
Vacuole
Ribosomes
Peroxisome
Cytoskeleton
Cytosol
Coatedpits
Ftg. 1.1: Diagrammaticrepresentationof a nt liverell.
12. BIOCHEMISTF|Y
Nucleus contains DNA, the repository of
genetic information. Eukaryotic DNA is
associatedwith basic protein (histones)in the
ratio of 1 : 1, to form nucleosomes.An assembly
of nucleosomesconstituteschromatin fibres of
chromosomes(Creek'.chroma - colour; soma-
body). Thus, a single human chromosomeis
comoosedof abouta million nucleosomes.The
number of chromosomes is a characteristic
feature of the species. Humans have 46
chromosomes,compactlypackedin the nucleus.
The nucleusof the eukaryoticcell containsa
dense bodv known as nucleolus.lt is rich in
RNA, particularlythe ribosomal RNA which
entersthe cytosolthrough nuclearpores.
The ground materialof the nucleus is often
referredto as nucleoplasm.lt is rich in enzymes
such as DNA polymerases and RNA
polymerases.To the surpriseof biochemists,the
enzymes of glycolysis,citric acid cycle and
hexose monophosphateshunt have also been
detectedin the nucleoplasm.
Mitochondria
The mitochondria (Creek'. mitos- thread;
chondros- granule) are the centres for the
cellularrespirationand energymetabolism.They
are regarded as the power housesof the cell
with variablesize and shape.Mitochondriaare
rod-like or filamentousbodies, usuallv with
dimensions of 1.0 x 3 pm. About 2,0O0
mitochondria,occupyingabout 1/5thof the total
cell volume,are presentin a typicalcell.
The mitochondriaare comoosedof a double
membrane system. The outer membrane is
smooth and completelyenvelopsthe organelle.
The inner membrane is folded to form cristae
(Latin- crests)which occupy a larger surface
area. The internal chamber of mitochondriais
referred to as matrix or mitosol.
The componentsof electron transportchain
and oxidative phosphorylation (flavoprotein,
cytochromesb, c1, C, a and a3 and coupling
factors)are buried in the inner mitochondrial
membrane.The matrixcontainsseveralenzvmes
concerned with the energy metabolism of
carbohydrates,lipidsandaminoacids(e.g.,citric
acid cycle, p-oxidation).The matrix enzymes
also parlicipate in the synthesisof heme and
urea. Mitochondria are the principal producers
of ATP in the aerobic cells. ATP, the energy
currency,generatedin mitochondriais exported
to all partsof the cell to provideenergyfor the
cellularwork.
The mitochondrialmatrixcontainsa circular
double stranded DNA (mtDNA), RNA and
ribosomes.Thus,the mitochondriaareequipped
with an independent protein synthesizing
machinery.It is estimatedthat about 10% of the
mitochondrial oroteins are produced in the
mitochondria.
The structureand functionsof mitochondria
closely resemble prokaryotic cells. lt is
hypothesizedthat mitochondria have evolved
from aerobicbacteria.Further,it is believedthat
duringevolution,the aerobicbacteriadeveloped
a symbiotic relationship with primordial
anaerobiceukaryoticcellsthat ultimatelyled to
the arrival of aerobiceukaryotes.
Endoplasmic reticulum
The network of membraneenclosedspaces
that extends throughout the cytoplasm
constitutesendoplasmicreticulum(ER).Someof
these thread-like structuresextend from the
nuclearporesto the plasmamembrane.
A large portion of the ER is studded with
ribosomesto give a granularappearancewhich
is referred ro as rough endoplasmic reticulum.
Ribosomes are the factories of protein
biosynthesis. During the process of cell
fractionation,roughERisdisruptedto form small
vesiclesknown as microsomes.It may be noted
that microsomesas such do not occur in the
cell.
The smoothendoplasmicreticulumdoes not
containribosomes.lt is involvedin the synthesis
of lipids (triacylglycerols,phospholipids,sterols)
and metabolismof drugs,besidessupplyingCa'?.
for the cellularfunctions.
Golgi apparats,r$
Eukaryoticcells contain a unique clusterof
membrane vesicles known as dictyosomes
13. Chapter 1 : BIOMOLECULESAND THE CELL
which, in turn, constituteColgi apparatus(or
Colgi complex).The newly synthesizedproteins
are handed over to the Colgi apparatuswhich
catalysethe addition of carbohydrates,lipids or
sulfatemoietiesto the proteins.Thesechemical
modificationsare necessaryfor the transportof
proteinsacrossthe plasmamembrane.
Certainproteinsand enzymesareenclosedin
membrane vesicles of Colgi apparatusand
secreted from the cell after the appropriate
signals.The digestiveenzymesof pancreasare
oroducedin this fashion.
Colgi apparatusare also involved in the
membrane synthesis, particularly for the
formation of intracellular organelles (e.g.
peroxisomes,lysosomes).
Lysosornes
Lysosomesare sphericalvesiclesenveloped
by a singlemembrane.Lysosomesare regarded
as the digestivetract of the cell, sincethey are
actively involved in digestion of cellular
substances-namely proteins, lipids, carbo-
hydratesand nucleic acids.Lysosomalenzymes
are categorizedas hydrolases.Theseinclude the
following enzymes(with substratein brackets)
a-C lucosidase(glycogen)
Cathepsins(proteins)
Lipases(lipids)
Ribonucleases(RNA)
The pH of the lysosomalmatrixis moreacidic
(pH< 5) than the cytosol (pH-7) and this
facilitatesthe degradationof differentcompounds.
The lysosomal enzymes are responsiblefor
maintaining the cellular compounds in a dynamic
stafe, by their degradationand recycling.The
degradedproductsleavethe lysosomes,usually
by diffusion, for reutilization by the cell.
Sometimes,however,certain residualproducts,
rich in lipidsand proteins,collectivelyknown as
Iipofuscinaccumulatein the cell. Lipofuscinis
the agepigmentor wear and tearpigmentwhich
has been implicatedin ageingprocess.
The digestiveenzymesof cellularcompounds
are confinedto the lvsosomesin the bestinterest
of the cell. Escapeof theseenzymesinto cytosol
will destroythe functionalmacromoleculesof tne
cell and result in many complications.The
occurrence of several diseases(e.g. arthritis,
musclediseases,allergicdisorders)hasbeenpartly
attributedto the releaseof lysosomalenzymes.
Feroxisomes
Peroxisomes,also known as microbodies, are
single membranecellularorganelles.They are
spherical or oval in shape and contain the
enzyme catalase.Catalaseprotectsthe cell from
the toxic effectsof HrO, by converting it to HrO
and Or. Peroxisomesare also involved in tne
oxidation of long chain fatty acids (> C,s),and
synthesisof plasmalogensand glycolipids.Plants
contain glyoxysomes, a specialized type of
BTOMED|eAL/ CLINICAL COIUCEPTS
A liuing cell is a true representotiueof life with its own organizotionand 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.
14. E}IOCHEMISTF|Y
peroxisomes, which are involved in the
glyoxylate pathway.
Peroxisome biogenesisdisorders (PBDs), are
a Broup of rare diseasesinvolving the enzyme
activities of peroxisomes. The biochemical
abnormalitiesassociatedwith PBDs incluoe
increasedlevelsof very long chain fatty acids
(C2aand C26)and decreasedconcentrationsof
plasmalogens.The most severeform of PBDsis
Zellweger syndrome, a condition characterized
by the absenceof functional peroxisomes.The
victimsof this diseasemav die within one vear
after birth.
{iytosol and cytoskeleton
The cellular matrix is collectively referredto
as cytosol. Cytosol is basicallya compartment
containing several enzymes/ metabolites and
saltsin an aqueousgel like medium.More recent
studies however, indicate that the cytoplasm
actuallycontainsa complex network of protein
filaments, spread throughout, that constitutes
cytoskeleton.The cytoplasmicfilamentsare of
three types- microtubules, actin filaments and
intermediatefilaments.The filamentswhich are
polymers of proteins are responsiblefor the
structure,shapeand organizationof the cell.
INTEGRATIOI{ OF
CELLULAR FUNCTIONS
The eukaryoticcells performa wide rangeof
complex reactionsfunctionsto maintaintissues,
and for the ultimatewell-beingof the whole
organism. For this purpose, the various
intracellularprocessesand biochemicalreactions
are tightly controlledand integrated.Divisionof
a cell intotwo daughtercellsis goodexampleof
the orderlyoccurrenceof an integratedseriesof
cellularreactions.
Apoptosisis the programmedcell death or
cell suicide. This occurs when the cell has
fulfilled its biologicalfunctions.Apoptosismay
be regardedas a natural cell deathand it differs
from the cell death caused by injury due to
radiation,anoxiaetc. Programmedcell death is
a highly regulatedprocess.
1.
2.
3.
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,monosaccharidesand fotty acids,respectluely.
The cell is the structuroland functional unit of life. The eukoryoticcell consisfsof well
det'inedsubcellulororganelles,enuelopedin a plasma membrane.
The nucleus contoinsDNA, the repositoryol genetic int'ormation.DNA, in association
with proteins (histones),forms nucleosomeswhich,in turn, make up the chromosomes.
The mitochondria qre the centresfor energymetobolism. Theyare the principalproducers
of ATP which is exported to all parts of the cell to ptouide energylor cellular work.
Endoplosmic reticulum (ER) ts the network of membrane enclosed spocesthat 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.
Lysosomesare the digestiue bodiesol the cell, actiuely involued in the degradotion of
cellular compounds. Peroxisomescontoln the enzymecatalosethat 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.
Theeukaryoticcellsperform a widerangeof complex lunctionsin a well coordinatedand
integrated fashion. Apoptosis is the processol programmed cell death or cell suicide.
5.
6.
7.
15. 1^ arbohydratesare the most abundantorganic
- molecules in nature. They are primarily
composedof the elementscarbon, hydrogen and
oxygen.The name carbohydrateliterallymeans
'hydratesof carbon'.Someof the carbohydrates
possessthe empiricalformula (C.H2O)nwhere
n 3 3, satisfyingthat thesecarbohydratesare in
fact carbonhydrates.However,thereare several
non-carbohydratecompounds(e.g. acetic acid,
C2HaO2;lacticacid,C3H6O3)which alsoappear
as hydratesof carbon. Further, some of the
genuine carbohydrates (e.g. rhamnohexose,
C6H12O5ideoxyribose,C5H16Oa)do not satisfy
the generalformula.Hencecarbohydratescannot
be alwaysconsideredas hydratesof carbon.
Carbohydrates may be defined as
polyhydroxyaldehydes or ketones or compounds
which produce them on hydrolysis. The term
'sugar' is applied to carbohydratessoluble in
water and sweet to taste.
#-ur*c;tEerEsof earbohydrates
Carbohydratesparticipatein a wide rangeof
functions
1. Theyarethe mostabundantdietarysource
of energy (a Cal/S)for all organisms.
2. Carbohydratesare precursorsfor many
organic compounds(fats,amino acids).
3. Carbohydrates(asglycoproteinsand glyco-
lipids) participate in the structure of cell
membraneand cellular functionssuch as cell
growth, adhesionand fertilization.
4. They are structuralcomponentsof many
organisms.Theseincludethe fiber (cellulose)of
plants,exoskeletonof some insectsand the cell
wall of microorganisms.
5. Carbohydratesalso serve as the storage
form of energy(glycogen)to meetthe immediate
energydemandsof the body.
CLASSIFICATION
OF GARBOHYDRATES
Carbohydrates are often referred to as
saccharides (Greek: sakcharon-sugar).They
are broadlyclassifiedinto three major groups-
monosaccharides, oligosaccharides and
polysaccharides.This categorizationis basedon
16. t0 BIOCHEMISTRY
Monosaccharides(empiricalformula) AIdose Ketose
Trioses(CgHoOg)
Telroses(C+HoO+)
Pentoses(CsHroOs)
Hexoses(CoHrzOo)
Heptoses(CzHr+Oz)
Glyceraldehyde
Erythrose
Ribose
Glucose
Glucoheptose
Dihydroxyacetone
Erythrulose
Ribulose
Fructose
Sedoheptulose
the number of sugar units. Mono- and oligo-
saccharidesare sweet to taste, crystalline in
characterand soluble in water, hence thev are
commonly known as sugars.
FJtonosaccharides
Monosaccharides(Greek: mono-one)are the
simplestgroup of carbohydratesand are often
referred to as simple sugars.They have the
generalformula Cn(H20)n,and they cannot be
further hydrolysed.The monosaccharidesare
divided into differentcategories,based on the
functionalgroupandthe numberof carbonatoms
Aldoses : When the functional group in
IH
monosaccharidesis an aldehydel-C:oi, ,h"u
are known as aldoses e.g. glyceraldehyde,
glucose.
Ketoses: When the functionalgroup is a keto
lt
-C:O.l group, they are referredto as ketoses
e.g. dihydroxyacetone,fructose.
Basedon the number of carbon atoms,the
monosaccharidesare regarded as trioses (3C),
tetroses (4C), pentoses (5C), hexoses (6C) and
heptoses(7C).Thesetermsalongwith functional
groupsare usedwhile namingmonosaccharides.
For instance, glucose is an aldohexose while
fructose is a ketohexose(Table 2,1).
Thecommonmonosaccharidesand disaccha-
rides of biological importanceare given in the
Table 2.2.
SSlgosaccharides
Oligosaccharides(Creek: oligo-few) contain
2-1O monosaccharidemolecules which are
liberatedon hydrolysis.Basedon the numberof
monosaccharide units present, the oligo-
saccharides are further subdivided to
disaccharides,trisaccharidesetc.
Polysace harides
Polysacchari6ls(Creek:poly-many)are poly-
mers of mondficcharide units with high mole-
cular weight (up to a million).They are usually
tasteless(non-sugars)and form colloids with
water. The polysaccharidesare of two types-
homopolysaccharidesand heteropolysaccharides.
Stereoisomerismis an importantcharacterof
monosaccharides. Stereoisomers are the
compounds that have the same structural
formulaebut differ in their spatialconfiguration.
A carbon is said to be asymmetric when it is
attached to four different atoms or groups. Ihe
number of asymmetric carbon atoms (n)
determines the possible isomers of a given
compound which is equal to 2n. Clucose
contains4 asymmetriccarbons,and thus has 16
tsomers.
Glyeeraldehyde
-tfu e ref erqlrt*e cff rb$hyd$'er'&€3
Clyceraldehyde(triose)is the simplestmono-
saccharidewith one asymmetriccarbonatom. lt
existsastwo stereoisomersand hasbeenchosen
as the referencecarbohydrateio representthe
structureof all other carbohvdrates.
17. Ghapter 2 : CARBOHYDRATES 11
Trioses
Glyceraldehyde
Dihydroxyacetone
Tetroses
D-Erythrose
Foundincellsasphosphate
Foundincellsasphosphate
i Widespread
I Widespreadasaconstituentof
I RNAandnucleotides
i AsaconstituentofDNA
: Producedduringmetabolism
i Asaconstituentofglycoproteins
i anogums
i ls anintermediateinuronicacidpathway
i Heartmuscle
i --. --. -- --.. ---.. -.. -. --.
Asaconstituentolpolysaccharides
(starch,glycogen,cellulose)and
disaccharides(maltose,lactose,
sucrose).Alsofoundinfruits
Asaconstituentoflactose
(milksugar)
Foundinplantpolysaccharides
andanimalglycoproteins
Fruitsandhoney,asaconstituent
ofsucroseandinulin
Foundinolants
i Glyceraldehyde3-phosphateisanintermediate
i inglycolysis
i ttst-pnosphateisanintermediateinglycolysis
----t------..-..-.--...---
ForthestructureofRNAandnucleotide
coenzymes(ATP,NAD+,NADP+)
ForthestructureolDNA
Itisanimportantmetaboliteinhexose
monophosphateshunt
Involvedinthefunctionofglycoproteins
Excretedinurineinessenlialpentosuria
Asaconstituentollvxollavinofheartmuscle
The'sugarfuel'oflife;excretedinurinein
diabetes.Structuralunitofcelluloseinplants
Convertedtoglucose,failureleadsto
galactosemia
Forthestructureofpolysaccharides
Itsphosphatesareintermediatesofglycolysis
Its7-phosphateisanintermediateinhexose
monophosphateshunt,andinphotosynthesis
Pentoses
D-Ribose
D-Deoxyribose
D-Ribulose
D-Xylose
L-Xylulose
D-Lyxose
Hexoses
D-Glucose
D-Galactose
D-Mannose
D-Fructose
Heptoses
D-Sedoheptulose
Disaccharides Occurrence Biochemical importance
Sucrose
Lactose
Asaconstituentofcanesugarand
beetsugar,pineapple
Milksugar
Productofstarchhydrolysis,
occursingerminatingseeds
Mostcommonlyusedtablesugarsupplying
calories
Exclusivecarbohydratesourcetobreastfed
infants.Lactasedeficiency(lactoseintolerance)
leadstodianheaandflatulence
Animportantintermediateinlhedigestionof
starch
Maltose
18. 12 E}IOCHEMISTFIY
H-C:O
I
H-C-OH
cH2oH
D-Glyceraldehyde
H-C:O
HO-C-H
cH2oH
L-Glyceraldehyde
H-C:O
I
HO-C-H
H-C-OH
I
HO-C-H
HO-C-H
cH2oH
L-Glucose
Fig.2.1 : DandL- formsof glucosecomparedwith
D and L- glyceraldehydes (the reference carbohydrate).
D" and L-isomers
The D and L isomersare mirror imagesof
each other. The spatialorientationof -H and
-OH groups on the carbon atom (Cs for
glucose)that is adjacentto the terminal primary
alcohol carbon determineswhetherthe sugaris
D- or L-isomer.lf the -OH group is on the right
side,the sugaris of D-series,and if on the left
side, it belongs to L-series.The structuresof
D- and L-glucosebasedon the referencemono-
saccharide, D- and L-glyceraldehyde (glycerose)
are depicted in Fig.2.1.
It may be noted that the naturallyoccurring
monosaccharidesin the mammaliantissuesare
mostlyof D-configuration.Theenzymemachinery
of cells is specific to metaboliseD-seriesof
monosaccharides.
fn the medical practice, the term dextroseis
used for glucosein solution. This is becauseof
the dextrorotatorynature of glucose.
Optlcal activity of sugars
Optical activity is a characteristicfeature of
compounds with asymmetric carbon atom.
When a beam of polarized light is passed
througha solutionof an optical isomer,it will be
rotated either to the right or left. The term
dextrorotatory (+) and levorotatory (-) are used
to compoundsthat respectivelyrotatethe plane
of polarizedlight to the right or to the left.
An optical isomer may be designatedas
D(+), D(-), L(+)and L(-) basedon its structural
relation with glyceraldehyde.lt may be noted
that the D- and L-configurationsof sugarsare
primarily based on the structure of
glyceraldehyde,the optical activitieshowever,
may be different.
Racemicmixture : lf D- and L-isomersare
presentin equal concentration,it is known as
racemicmixtureor DL mixture.Racemicmixture
does not exhibit any optical activity, since the
dextro- and levorotatorv activities cancel each
other.
Configuration of D-aldoses
The configuration of possible D-aldoses
startingfrom D-glyceraldehydeis depicted in
Fig.2.2. This is a representation of Killiani-
Fischersynthesis,by increasingthe chain length
of an aldose,by one carbon at a time. Thus,
startingwith an aldotriose(3C),aldotetroses(4C),
aldopentoses(5C) and aldohexoses(6C) are
formed. Of the 8 aldohexoses,glucose,mannose
and galactoseare the most familiar. Among
these, D-glucose is the only aldose mono-
saccharidethat predominantlyoccurs in nature.
Gonfiguration of D-ketoses
Startingfrom dihydroxyacetone(triose),there
are five keto-sugarswhich are physiologicallr
important.Their structuresare given in Fig,2.3
Epimers
ff two monosaccharides differ from eac-
other in their configuration around a singk
specificcarbon (otherthan anomeric)atom. L*ei
are referred to as epimersto each orher '.Fig,21
For instance, glucoseand galactose are efilwl
with regardto carbon 4 (Ca-epimers- -^:i 's
they differ in the arrangementof -OH g.'ELcr
Ca. Clucose and mannose are epi-'e--' q drl
regardto carbon 2 (C2-epimers).
The interconversionof epimers e
-
I r::r'e
to galactose and vice versai s i - -^,'- a*
H-C:O
I
H-C-OH
I
HO-C-H
I
H-C-OH
I
H-Q-OH
I
cHzoH
D-Glucose
19. Ghapter 2 : CABBOHYDFATES 13
Aldotriose
(3c)
Aldotetroses
(4c)
cHo
I
HOCH
I Aldo toses
HOCH )
I
HCOH
I
cH2oH
D-Lyxoee
cHo
HOCH
I I Aldo-
HOCH HOCH hexoses
HoCH noCH (6c)
tl
HCOH HCOH
tt
cHzoH cHzoH
D-Galactose D-Talose
'l
t-
cHo
HOCH
I
HCOH
HCOH
cH2oH
D-Arabinose
/
JT
cHo cHo
HCOH HOCH
rl
HOCH HOCH
tl
HCOH HCOH
tl
HCOH HCOH
ll
cH2oH cH2oH
D-Glucose D-Mannose
HCOH
I
HCOH
cH2oH
D-Ribose
/
JT
cHo cHo
tl
HCOH HOCH
HCOH HCOH
tl
HCOH HCOH
tl
HCOH HCOH
ll
cH2oH cH2oH
D-Allose D-Altrose
cHo
HCOH
cHo
I
HCOH
I
HCOH
I
HOCH
I
HCOH
cH2oH
D-Gulose
cHo
I
HCOH
cHo
I
HOCH
I
HCOH
I
HOCH
HCOH
I
cH2oH
D-ldose
cHo
I
HCOH
I
cH2oH
D-Erythrose D.Threooe
cHo
I
HCOH
I
HOCH
I
HCOH
cH2oH
D-Xylose
I
/
/
*+
Fig.2.2 : ThestructuralrelationshipbetweenD-aldosesshownin Fischerprojection.
(TheconfigurationaroundC2(ed) distinguishesthemembersof eachpair).
epimerization, and a group of enzymes-
namely-epimerases catalysethis reaction.
Enantiomers
Enantiomers are a special type of
stereoisomers that are mirror images of
eachother. The two membersare designatedas
D- and L-sugars.Enantiomersof glucose are
depicted in Fig.2.5.
Majority of the sugarsin the higher animals
(includingman) are of D-type (Fig.2.5'1.
The term diastereomersis used to represent
the sfereoisomers that are not mirror imagesof
one another.
For a better understanding
structure, let us consider the
hemiacetals and hemiketals,
producedwhen an aldehydeor a
with alcohol.
of glucose
formation of
respectively
ketone reacts
20. 14 E}IOCHEMISTRY
?H2oH
C:O
I
cH2oH
Dlhydroxyacetone
cH2oH
I
C:O
I
HOCH
HCOH
I
cH2oH
D-Xylulose
cH20H
I
C:O
HCOH
HCOH
I
cH2oH
D-Ribulose
cH2oH
I
C:O
HOCH
I
HCOH
I
HCOH
I
cH2oH
D-Fructose
cH2oH
I
C:O
I
HOCH
I
HCOH
I
HCOH
I
HCOH
I
cH2oH
D-Sedoheptulose
Fig.2.3 : Structuresof ketosesof physiologicalimportance.
,H
nt-C.1^ + R2-oH l- Rr-
LJ
Aldefry<b Alcohol Hemiacetal
The hydroxyl group of monosaccharidescan
react with its own aldehydeor keto functional
group to form hemiacetaland hemiketal.Thus,
the aldehydegroup of glucoseat C1 reactswith
alcohol group at C5 to form two typesof cyclic
hemiacetalsnamely a and B, as depicted in
Fig.2.6. The configuration of glucose is
conveniently represented either by Fischer
formulaeor by Haworth projectionformulae.
Fyranose and furanose structures
Haworth projectionformulaeare depictedby
a six-memberedring pyranose(basedon pyran)
or a five-memberedring furanose (based on
furan).The cyclic formsof glucoseare known as
a-D-glucopyranose and c-D-glucofuranose
(Fig.2.V.
Anomers-nrutarotation
The a and p cyclic forms of D-glucose are
known as anomers.Thev differ from each other
in the configurationonly around C1 known as
anomericcarbon(hemiacetalcarbon).In caseof
o anomer,the -OH group held by anomeric
carbon is on the opposite side of the group
-CH2OH of sugarring. The reverseis true for
B-anomer.The anomersdiffer in certainphysical
and chemical properties.
Mutarotation : The a and p anomers of
glucose have different optical rotations. The
specific optical rotation of a freshly prepared
glucose(c anomer)solutionin water is +112.2o
which gradually changes and attains an
equilibriumwith a constantvalue of +52.7". ln
the presenceof alkali, the decreasein optical
rotation is rapid. The optical rotation of
p-glucose is +18.7o. Mutarotation is defined as
the change in the specific optical rotation
representing the interconversion of u and p
H-C:O
I
H-C-OH
I
HO-C-H
I
HO- C -H
H-C-OH
I
cH2oH
D-Galactose
H-C:O
I
H-C-OH
I
HO-C-H
I
H .C-OH
I
H-C-OH
I
CHzOH
D-Glucose
H-C=O
I
HO-C-H
I
HO-C-H
I
H-C-OH
I
H-C-OH
I
cH2oH
D-Mannose
H
I
C:O
I
f{ c-oH
I
HC-C-H
i-l- c-oH
H-C-OH
I
t"1-c-H
HO
H
O=C
HO_C- H
I
H-C- Cl-i
I
HO-C-H
I
HO-C- Fl
I
H-C- ii
I
OH
Fig.2,4: Structuresof epimers(glucoseand galactose
are Co-epimerswhileglucoseand mannoseare
C2-epimers).
L-Glucose D-Glucose
H9.2.5 : Enantiomers(mirrorimages)ofglucose.
21. t5
Ghapter 2 : CARB
I
cH20H
o'D'Glucose
(+ 112.2"
fil
H6?H
o-D-GlucoPYranose
1
H-C:O
I
H-C-OH
I
HO-C-H
I
H-C-OH
tc
H-C-OH
I
cH2oH
D-Glucose
(aldehYdeform)
l/A
H6?H
HOH
D-Glucose
(aldehydeform,acYclic)
iHron
ftD-Glucose
(+18.7-)
(B)
HOH
FD-GlucoPYranose
cH20H
forms of D'glucose to an equilihrium mixture'
Mutarotationdepictedin Fig'2'6, is summartzeo
below.
cx-D-Clucose# Equilibriummixture# B-D-Clucose
+ 112.2" + 52.7" + 18.7"
(Specificoptical rotation tctl2p0)
The equilibrium mixture contains 63o/"
p-anomer and 36"/ocl-anomer of glucose with
Fig.2.7: Structurcofglucose-pyranose
andfuranosetorms'
HOH
cr-D-GlucoPYranose
cH20H
t-
H-C-OFi
OH
HOH
cr-D-Glucofuranose
17oopen chainform. ln aqueoussolution'the p
forrn
'i,
more predominant due to its stable
conformation.The cr and p formsof glucoseare
interconvertiblewhich occurs through a linear
form. The latter, as such, is present in a"
insignificantquantitY.
Mutarotation of fructose z Frur'
exhibits mutarotation.ln case or
pyranose ring (six-memberqd'
furanose(five-membered)'o'
is attained.And fruqt'
rotation of -92)2.
Ihe conv'
to levor
':ut" :;r'
on
is kn,
anome'
in alkalir
When gt.
severalhours,
22. chapter 2 : CAFIBoHYDFATES 15
I
cH2oH
cr-D-Glucose
(+ 112.2")
1
H-C=C)
I
H-C-OH
I
HO-C-H
I
H-C-OH
l5
H-C-OH
cH2oH
D-Glucose
(aldehydeform)
HOH
D-Glucose
(aldehydeform,acyclic)
forms of D-glucose to an equilibrium mixture.
Mutarotationdepictedin Fi9.2.6, is summarized
below.
s-D-Clucose# Equilibriummircture# p-D-Glucose
+ 112.2" + 52.7" + 18.7o
(Specificoptical rotation talf;)
The equilibrium mixture contains 63"/"
p-anomer and 36h cl-anomerof glucosewith
cr-D-Glucopyranose
17oopen chain form. In aqueoussolution,the p
form is more predominant due to its stable
conformation.The s and p formsof glucoseare
interconvertiblewhich occurs through a linear
form. The latter, as such, is present in an
insignificantquantity.
Mutarotation of fructose : Fructose also
exhibits mutarotation.ln case of fructose,the
pyranose ring (six-membered)is converted to
furanose(five-membered)ring,till an equilibrium
is attained.And fructosehas a specificoptical
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
The processof shiftinga hydrogenatom from
one carbon atom to anotherto produce enediols
is known as tautomerization. Sugarspossessing
anomericcarbon atom undergotautomerization
in alkalinesolutions.
When glucoseis kept in alkalinesolutionfor
severalhours,it undergoesisomerizationto form
HOH
o-D-Glucopyranose pD-Glucopyranose
Fig. 2.6 : Mutarotation of glucose representing a and p anomers (A) Fischer projections (B) Haworth projections.
Fig.2.7 : Structureof glucose-pyranose
and furanoseforms.
20H cH2oH
H
c-D-Glucofuranose
23. 16 BIOCHEMISTFIY
H
n-C-ot
H-C:O (
I
H- -OH HO-(
HO-(
R
Enediol
(common)
Fig.2.8 : Formationof a commonenediolfrom
glucose,fructoseandmannose
{fr,f,o,F|F|lPffi:!lo.t|tfr,ft:PI:Is?Iboncolnmonstnftar:?l,l
D-fructose and D-mannose. This reaction-
known as the Lobry de Bruyn-von Ekenstein
transformatiorr-results in the formation of a
common intermediate-namely enediol--$or all
the three sugars,as depicted in Fig.2.8.
Theenediolsare highlyreactive,hencesugars
in alkaline solution are powerful reducing
agents.
ft+r,.luleFr'.lgr!s.lFeFtlsF
The sugarsare classifiedas reducingor non-
reducing.The reducingpropertyis attributedto
the free aldehyde or keto group of anomeric
carbon.
ln the laboratory, many testsare employed to
identify the reducing action of sugars.These
incfude Benedict's test, Fehling's test, Barfoed's
tesf etc. The reduction is much more efficient
in the alkaline medium than in the acid
medium.
The enediolforms(explainedabove)or sugars
reduce cupric ions (Cu2+)of copper sulphate
to cuprous ions (Cu+), which form a yellow
precipitate of cuprous hydroxide or a
red precipitate of cuprous oxide as shown
next.
t2H2O+ CueO{- 2Cu(OH)
It may be noted that the reducing property of
sugarscannothelp for a specificidentificationof
any one sugar,since it is a generalreaction.
0xida*iern
Depending on the oxidizing agent used, the
terminal aldehyde (or keto) or the terminal
alcoholor both the groupsmay be oxidized.For
instance,considerglucose:
1. Oxidation of aldehydegroup (CHO ------>
COOH) resultsin the formationof gluconicacid.
2. Oxidation of terminal alcohol group
(CH2OH------+COOH) leadsto the production of
glucuronicacid.
Reduetion
When treatedwith reducing agentssuch as
sodiumamalgam,the aldehydeor keto groupof
monosaccharideis reduced to corresponding
alcohol, as indicatedby the generalformula :
H
H-C:O H-C-Ol-t
I
RR
The important monosaccharidesand their
correspondingalcoholsare given below.
D-Glucose
D-Galactose------+D-Dulcitol
D-Mannose ------+D-Mannitol
D-Fructose --) D-Mannitol+ D-Sorbitol
D-Ribose -+ D-Ribitol
Sorbitol and dulcitol when accumulate in
tissuesin large amounts cause strong osmotic
effectsfeadingto swelling of cells,and certain
pathologicalconditions.e.g.cataract,peripheral
neuropathy,nephropathy.Mannitol is usefulto
reduce intracranialtensionbv forced diuresis.
24. Ghapter 2 : CAFIBOHYDRATES 17
H-C--O
I
H-C-OH
I
HO-C-H
I
H-C-OH
I
H-C-OH
I
cH2oH
D-Glucose
H-C:O
I
H-C:O
I
cH20H
Hydrorymethylfurfural
H-C:O
I
Formation of esters
The alcoholic groups of monosaccharides
may be esterified by non-enzymatic or
enzymatic reactions. Esterificationof carbo-
hydrate with phosphoric acid is a common
reaction in metabolism.Glucose 6-phosphate
and glucose 1-phosphateare good examples.
ATP donates the phosphate moiety in ester
formation.
lClycoside bond formation (see below) and
mutarotation(discussedalready) may also be
referred to, as these are also the characteristic
propertiesof monosaccharides.l
GLYCOSIDES
Glycosidesare formed when the hemiacetal
or hemiketal hydroxyl group (of anomeric
carbon)of a carbohydratereactswith a hydroxyl
group of another carbohydrate or a non-
carbohydrate (e.g. methyl alcohol, phenol,
glycerol). The bond so formed is known as
glycosidic bond and the non-carbohydrate
moiety (when present)is referredto as aglycone.
The monosaccharidesare held together by
glycosidic bonds to result in di-, oligo- or
polysaccharides(seelaterfor structures).
H-C=O
I _ + HrN-NH-CuHu
H-C-OH
R
Glucose Phenylhydrazine
H-C:N-NH-CoHs
I
H-C-OH
I
R
Glucohydrazone
l7-H2N-NH-C6H'
I
H-C:N-NH-CoHs
I
C:N-NH-CoHs
I
R
Glucosazone
Fig.2.10: A summatyof osazonefomation
H-C-OH C----r
tlll
H-C-OH Conc.HeSoo H-Q L
I rH I U
H-C-OH
'1
H-C I
CHrou 3H2o H-d---l
D-Ribose Furfural
Fig.2.9 : Dehydration of monosaccharides
with concentrated H
"SO
o.
Dehydration
When treatedwith concentratedsulfuricacid,
monosaccharidesundergodehydrationwith an
eliminationof 3 water molecules.Thus hexoses
give hydroxymethylfurfuralwhile pentosesgive
furfural on dehydration (Fi9.2.9).Thesefurfurals
can condense with phenolic compounds
(a-naphthol)to form coloured products.This is
the chemical basisof the popular Molisch test.
In case of oligo- and polysaccharides,they are
firsthydrolysedto monosaccharidesby acid,and
this is followed by dehydration.
Osazone formation
Phenylhydrazinein acetic acid, when boiled
with reducing sugars, forms osazones in a
reactionsummarizedin Fig,2,10.
As is evident from the reaction, the first two
carbons (Cr and C2) are involved in osazone
formation. The sugars that differ in their
configuration on these two carbons give the
same type of osazones,since the differenceis
maskedby bindingwith phenylhydrazine.Thus
glucose,fructoseand mannosegive the same
type (needle-shaped)osazones.
Reducingdisaccharidesalso give osazones-
maltose sunflower-shaped,and lactose powder-
puff shaped.
(RrcprcsentsCrto Crofglucose).
25. t8 BIOCHEMISTRY
Naming of glycosidic bond : The
nomenclatureof glycosidic bonds is based on
the Iinkagesbetweenthe carbon atomsand the
status of the anomeric carbon (o or p). For
instance,lactose-which is formed by a bond
between C1 of p-galactoseand Ca of glucose-
is namedas 0(.1-+ 4) glycosidicbond. The other
glycosidicbonds are describedin the structure
of di- and polysaccharides.
Physiologieally important glycosides
1. Glucovanillin (vanillin-D-glucoside)is a
naturalsubstancethat impartsvanilla flavour.
2. Cardiac glycosides(steroidalglycosides):
Digoxin and digitoxin contain the aglycone
steroidand they stimulatemusclecontraction.
3. Streptomycin, an antibiotic used in the
treatmentof tuberculosisis a glycoside.
4. Ouabain inhibits Na+- K+ ATPase and
blocksthe activetransportof Na+.
DERIVATIVESOF MONOSACCHARIDES
Thereare severalderivativesof monosaccha-
rides, some of which are physiologically
important
1. Sugar acids : Oxidation of aldehyde or
primaryalcoholgroupin monosaccharideresults
in sugaracids.Cluconic acid is producedfrom
glucose by oxidation of aldehyde (C1 group)
whereasglucuronicacid is formedwhen primary
alcoholgroup (C6)is oxidized.
2. Sugar alcohols (polyols) : They are
producedby reductionof aldosesor ketoses.For
instance,sorbitol is formed from glucose and
mannitol from mannose.
3. Alditols : The monosaccharides, on
reduction,yield polyhydroxyalcohols,known as
alditols. Ribitol is a constituent of flavin
coenzymes; glycerol and myo-inositol are
componentsof lipids.Xylitol is a sweetenerused
in sugarlessgumsand candies.
4. Amino sugars : When one or more
hydroxyl groups of the monosaccharidesare
replaced by amino groups, the products
formed are amino sugarse.g. D-glucosamine,
D-galactosamine.They are present as consti-
tuentsof heteropolysaccharides.
The amino groups of amino sugars are
sometimes acetylated e.g. N-acetyl D-gluco-
samrne.
N-Acetylneuraminic acid (NANA) is a
derivativeof N-acetylmannoseand pyruvicacid.
It is an important constituentof glycoproteins
and glycolipids.The term sialicacid is usedto
include NANA and its other derivatives.
Certain antibiotics contain amino sugars
which may be involvedin the antibioticactivity
e.g. erythromycin.
5. Deoxysugars: These are the sugarsthat
contain one oxygen lessthan that presentin the
parent molecule. The groups -CHOH and
-CH2OH become-CH2 and -CH3 due to the
absenceof oxygen.D-2-Deoxyriboseis the most
important deoxysugarsince it is a structural
constituentof DNA (in contrastto D-ribose in
RNA).
6. L-Ascorbic acid (vitamin C) : This is a
water-solublevitamin, the structureof which
closelyresemblesthat of a monosaccharide.
The structuresof selected monosaccharide
derivativesare depictedin Fig.2.l1.
Among the oligosaccharides,disaccharides
are the most common (Fig.2,l2).As is evident
from the name, a disaccharideconsistsof two
monosaccharideunits(similaror dissimilar)held
together by a glycosidic hond. They are
crystalline,water-solubleand sweetto taste.The
disaccharidesare of two types
'1. Reducingdisaccharideswith free aldehyde
or keto group e.g. maltose, lactose.
2. Non-reducingdisaccharideswith no free
aldehyde or keto group e.g. sucrose,trehalose.
Maltose
Maltose is composed of two a-D-glucose
unitsheldtogetherby cl (1 -+ 4) glycosidicbond.
Thefreealdehydegrouppresenton C1of second
glucoseanswersthe reducingreactions,besides
26. Ghapter & : CAFIBOHYDRATES 19
H-C:O
I
H-C-OH
I
HO-C-H
I
H-C-OH
I
H-C-OH
I
COOH
D-Glucuronicacid
OHH
D-2-Deoxyribose
cH2oH
I
H-C-OH
I
cH2oH
Glycerol
H NHz
D-Glucosamine
HOH
myo-lnositol
H3C-C--HN
HOH
N-Acetylneuraminicacid
Fiq.2.11 : Structuresol monosaccharidederivatives(selectedexamples).
the osazone formations (sunflower-shaped).
Maltosecan be hydrolysedby dilute acid or the
enzyme maltaseto liberate two moleculesof
cr-D-glucose.
ln isomaltose,the glucose units are held
togetherby o (1 --+6) glycosidiclinkage.
Cellobioseis another disaccharide,identical
in structurewith maltose,exceptthat the former
has p (1 -r 4) glycosidiclinkage.Cellobioseis
formedduringthe hydrolysisof cellulose.
Suoroee
Sucrose(canesugar)isthe sugarof commerce,
mostlyproducedby sugarcane and sugarbeets.
Sucrose is made up of a-D-glucose and p-
D-fructose.The two monosaccharidesare held
togetherby a glycosidicbond (a1-+ B2),between
Cj of c-glucose and C2 of B-fructose.The
reducing groups of glucose and fructose are
involvedin glycosidicbond, hencesucroseis a
non-reducing sugar,and it cannot form osazones.
Sucroseis the major carbohydrateproduced
in photosynthesis. lt is transported into the
storageorgansof plants (such as roots, tubers
and seeds).Sucroseis the mostabundantamong
the naturallyoccurring sugars.lt has distinct
advantagesover other sugarsas a storageand
transoortform. This is due to the fact that in
sucrose,both the functional groups (aldehyde
and keto)are held togetherand protectedfrom
oxidativeattacks.
Sucrose is an important source of dietary
carbohydrate.lt is sweeter than most other
commonsugars(exceptfructose)namelyglucose,
lactoseand maltose.Sucroseis employed as a
sweeteningagentin food industry.The intestinal
enzyme-sucrase-hydrolysessucroseto glucose
and fructosewhich are absorbed.
F-aetsse
Lactoseis more commonlv known as milk
sugarsinceit is the disaccharidefound in milk.
Lactoseis composed ol p-D-galactoseand B-D-
glucoseheld togetherby 0 (1 -r a) glycosidic
bond.The anomericcarbonof C1glucoseis free,
hence lactoseexhibits reducing propertiesand
formsosazones(powder-puffor hedgehogshape).
Lactose of milk is the most important
carbohydratein the nutritionof young mammals.
It is hydrolysedby the intestinalenzyme lactase
to glucoseand galactose.
lnversion ef suerose
Sucrose,as such is dextrorotatory(+66.5o).
But, r,r,hen hydrolysed, sucrose becomes
levorotatory(-28.2"). The processof change in
optical rotation from dextrorotatory (+) to
levorotatory(-) is referredto as inversion.The
27. BIOCHEMISTF|Y
HOH
Glucose Fructose
Sucrose
(a-D-glucosyl(1 --+2)p-D-fructose)
Galactose
Lactose
(p-D-galactosyl(1 -+ a)p-D-glucose)
Fig. 2.12 : Structures of disaccharides
-maltose, sucrose and lactose.
hydrolysed mixture of sucrose, containing
gfucoseand fructose, is known as invert sugar.
The processof inversionis explainedbelow.
Hydrolysisof sucroseby the enzyme sucrase
(invertasdor dilute acid liberatesone molecure
each of glucoseand fructose.ft is postulatedthat
sucrose (dextro) is first split into a-D-
glucopyranose(+52.5") and p-D-fructofuranose,
both being dextrorotatory. However, p-D-
fructofuranoseis lessstableand immediatelygets
converted to p-D-fructopyranose which is
stronglylevorotatory(-92"). The overalleffectis
that dextro sucrose (+66.5") on inversion is
converted to levo form (28.2'.
Polysaccharides(or simply glycans)consistof
repeat units of monosaccharides or their
derivatives,held togetherby glycosidicbonds.
Theyareprimarilyconcernedwith two important
functions-structural,and storageof energy.
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. Homopolysaccharideswhich on hydrolysis
yield only a singletype of monosaccharide.They
are named based on the nature of the
monosaccharideunit. Thus,glucans arepolymers
of glucose whereas fructosans are polymers of
fructose.
2. Heteropofysaccharideson hydrolysisyield
a mixture of a few monosaccharidesor their
derivatives.
$tarch
Starch is the carbohydrate reserveof plants
which is the most importantdietary sourcefor
higheranimals,includingman. High contentof
starchis found in cereals,roots,tubers,vegetables
etc. Starch is a homopolymer composed of
D-glucoseunits held by a-glycosidicbonds. lt is
known as glucosan or glucan.
Starch consists of two polysaccharide
components-water soluble amylose (15-20o/ol
and a water insoluble amylopectin (80-85%).
Chemically, amylose is a long unbranched
chain with 200-1,00OD-glucoseunitsheld by c
(1 + 4) glycosidiclinkages.Amylopectin,on the
other hand, is a branchedchain with a (1 --r 6t
glycosidicbondsat the branchingpointsand c
(1 -; 4) linkages everywhere else (Fig.2.13).
Amylopectin molecule containing a few
28. ChapteF 2 : CARBOHYDFATES 21
D-Glucose D-Glucose
Amylopectin
o-Amylose
+- (1-* 6) Branch
MainchainLg
6nu
vt t2
thousandglucoseunits looks like a branched
tree (20-30 glucoseunits per branch).
Starches are hydrolysed by amylase
(pancreaticor salivary)to liberatedextrins,and
finally maltoseand glucoseunits.Amylaseacts
specificallyon a (1 -+ 4) glycosidicbonds.
Dextrins
Dextrins are the breakdown products of
starchby the enzyme amylaseor dilute acids.
Starch is sequentially hydrolysed through
different dextrins and, finally, to maltose and
glucose.The variousintermediates(identifiedby
iodine colouration) are soluble starch (blue),
amylodextrin (violet), erythrodextrin (red) and
achrodextrin (no colour).
Inulin
fnulin is a polymerof fructosei.e., fructosan.
It occursin dahlia bulbs,garlic,onion etc. lt is
a low molecularweight (around5,000) poly-
saccharideeasilysoluble in water. Inulin is not
utilized by the body. lt is used for assessing
kidney function through measurement of
glomerular filtration rate (GFR).
Glycogen
Clycogen is the carbohydrate reserve in
animals,henceoften referredro asanimal starch.
It is present in high concentration in liver,
followed by muscle,brainetc.Clycogenis also
found in plantsthat do not possesschlorophyll
(e.9.yeast,fungi).
The structureof glycogenis similarto that of
amylopectin with more number of branches.
Glucoseis the repeatingunit in glycogenjoined
togetherby u (1 + 4) glycosidicbonds,and a
(1 + 6) glycosidic bonds at branching points
(Fi9.2.1Q.The molecularweight (up to 1 x 108)
and the numberof glucoseunits (up to 25,000)
vary in glycogendependingon the sourcefrom
which glycogenis obtained.
29. 22 BIOCHEMISTRY
Fiq.2.14: Structureofglycogen(A)Generalstructure
(B)Enlargedat a branchpoint.
Cellulose
Celluloseoccursexclusivelyin plantsand it is
the most abundant organic substancein plant
kingdom. lt is a predominantconstituentof
plant cell wall. Celluloseis totally absent in
animal body.
Cellulose is composed of p-D-glucose units
linked by 9 0 -+ 4) glycosidic bonds(Fi9.2.1fl.
Cellulosecannot be digestedby mammals-
includingman-due to lack of the enzymethat
cleavesB-glycosidicbonds(a amylasebreakscr
bondsonly). Certainruminantsand herbivorous
animalscontainmicroorganismsin thegutwhich
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
importancein human nutrition. lt is a major
constituentol fiber, the non-digestablecarbo-
hydrate.The functions of dietary fiber include
decreasing the absorption of glucose and
cholesterolfrom 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)
glycosidicbonds.lt isa structuralpolysaccharide
found in the exoskeletonof some invertebrates
e.g. insects,crustaceans.
When the polysaccharidesare composedof
differenttypesof sugarsor their derivatives,they
are referred to as heteropolvsaccharidesor
heteroglycans.
MUCOPOLYSACCHARIDES
Mucopolysaccharidesare heteroglycansmade
up of repeatingunitsof sugarderivatives,namely
amino sugarsand uronic acids.Theseare more
commonly known as glycosaminoglycans
(GAG).Acetylatedamino groups,besidessulfate
and carboxyl groups are generally present in
CAC structure.The presenceof sulfate and
carboxyl groups contributesto acidity of the
molecules, making them acid mucopoly,-
saccharides.
Someof the mucopolysaccharidesare found
in combination with proteins to forrn
mucoproteins or mucoids or proteoglycans
(Fig.2.l6l.Mucoproteinsmay containup to 95o,
carbohydrate and 5o/"protein.
S-D-Glucose
T
N
T
Ot
(B)
9H2OH
uqt, CH2oH
y'-O., , F--o. ,r4-Or
- (+ i) - r+ ,L^_K. X^_-oJ, ,./'o-'- J - L-/ " --l
Fig. 2.15 : Structureof cellulose(The repeat:r; --
'
may be several thousands).
30. CARBOHYDRATES 23
Fig. 2.16 : Diagrammaticrepresentationof a
prateoglycan complex.
Mucopolysaccharidesareessentialcomponents
of tissue structure.The extracellularspacesof
tissue (particularlyconnective tissue-cartilage,
skin, blood vessels,tendons)consistof collagen
and elastinfibersembeddedin a matrixor ground
substance.Thegroundsubstanceis predominantly
composedof CAC.
The importantmucopolysaccharidesincluoe
hyaluronicacid, chondroitin4-sulfate,heparin,
dermatansulfateand keratansulfate(Fig.Z.'[1.
j'i' ,ir:r:, | '.i. :,{,tiiiEl'l
Hyaluronicacid is an importantGAC found
in the groundsubstanceof synovialfluid of joints
and vitreoushumor of eyes.it is also presentas
a ground substancein connectivetissues,and
forms a gel aroundthe ovum. Hyaluronicacid
servesas a lubricantand shock absorbantin
joints.
BToMEDtCAt/ CLtft|ICALCO$CEpTS
Hyaluronicacid
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
Rq'-
CF
practice.
Fructoseis obundantly found in the semen which is utilized by the spermsfor energy.
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 playsa 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.
The mucopolysaccharideheparin is an onticoagulant(preuentsblood clotting).
The suruiual of Antarctic lish below -2"C is attributed to the antit'reeze glycoproteins.
streptomycin is a glycosideemployed in the treatment oJ tuberculosis.
!3:.
[j-
IF
s: -;--
s' -sS't/:-
-'-
31. 24 BIOCHEMISTFIY
Hyaluronic acid is composed of alternate
units of D-glucuronic acid and N-acetyl
D-glucosamine.These two molecules form
disaccharideunits held togetherby 0 (t -+ S)
glycosidic bond (Fi9,2,15).Hyaluronic acid
containsabout 250-25,000 disaccharideunits
(heldby p 1 -+ 4 bonds)with a molecularweight
uo to 4 million.
Hyaluronidase is an enzyme that breaks
(B1 -+ 4 linkages)hyaluronic acid and other
CAC. This enzyme is present in high
concentrationin testes,seminalfluid, and in
certainsnakeand insectvenoms.Hyaluronidase
of semen is assignedan important role in
fertilization as this enzyme clears the gel
(hyaluronicacid) around the ovum allowing a
better penetration of sperm into the ovum.
Hyaluronidaseof bacteriahelps their invasion
into the animaltissues.
Ghondroitin sulfates
Chondroitin 4-sulfate (Greek: chondros-
cartilage) is a major constituent of various
mammalian tissues(bone, cartilage,tendons,
heart,valves,skin,corneaetc.).Structurally,it is
comparablewith hyaluronicacid. Chondroitin
4-sulfateconsistsof repeatingdisaccharideunits
composedof D-glucuronicacid and N-acetyl
D-galactosamine4-sulfate(Fig.2.lV.
Chondroitin5-sulfateis alsopresentin many
tissues.As evident from the name, the sulfate
group is found on C6 insteadof Ca.
Heparin
Heparin is an anticoagulant(preventsblood
clotting)thatoccursin blood,lung,liver,kidney,
spleenetc. Heparin helps in the releaseof the
enzyme lipoprotein lipase which helps in
clearingthe turbidityof lipemic plasma.
Heparin is composedof alternatingunits of
N-sulfoD-glucosamine6-sulfateand glucuronate
2-sulfate(Fi9.2.17).
Dermatan sulfate
The name dermatansulfateis derived from
the fact that this compoundmostlyoccursin the
skin. lt is structurallyrelated to chondroitin
D-Glucuronicacid N-Acetylglucosamine
Hyaluronic acid
H NH-CO-CH3
N-Acetylgalactosamine
4-sulfate
Chondroitin 4-sulfate
o-
D-Glucuronate-2-sulfateN-Sulfoglucosamine
6-sulfate
Heparin
t
-o'r
H NH_CO_CH.
N-Acetylgalactosamine
4-sulfate
Dermatansulfate
H NH_CO :-
N-Acetylglucosamine
6-sulfate
Keratansulfate
Fiq.2.17 : Structuresof commonglycosaminogi',-;-: -
D-Glucuronicacid
H O-SO;
-o-so3
H NH-SOa
qH2oH
o
the disaccharidesas repeatingunits.
32. Ghapter 2 : CAFIBOHYDHATES 25
Glycosaminoglycan Composition Tissuedistribution Function(s)
Hyaluronicacid D-Glucuronicacid,
N-acetylglucosamine
Connectivetissue,synovialfluid,
vitroushumor
Servesasalubricant.and
shockabsorber.Promotes
woundhealing
Chondroitinsulfate D-Glucuronicacid,
N-acetylgalactosamine
4-sulfate
Cartilage,bone,skin,bloodvessel
walls
Helpstomaintainthestructure
andshapesoftissues
Heparin D-Glucuronate2-sulfate,Blood,lung,liver,kidney,spleen
N-sulfoglucosamine
6-sulfate
Actsasananticoagulant
Dermatansulfate L-lduronicacid,N-acetyl-
galactosamine4-sulfate
Bloodvesselvalves,heartvalves, Maintainstheshapesoftissues
skin
Keratansulfate D-Galactose,N-acetyl-
glucosamine6-sulfate
Cartilage,cornea,connective
tissues
Keepscorneatransparent
4-sulfate.The only differenceis that there is an
inversion in the configuration around C5 of
D-glucuronic acid to form L-iduronic acid
(Fi9.2.1V.
Keratan sulfate
It is a heterogeneousCAG with a variable
sulfate content, besides small amounts of
mannose, fructose, sialic acid etc. Keratan
sulfateessentiallyconsistsof alternatingunitsof
D-galactosamine and N-acetylglucosamine
6-sulfate.
A summaryof the glycosaminoglycanswith
regardto composition,distributionand functions
is given in Table 2.3.
Several proteins are covalently bound to
carbohydrateswhich are referredto as glyco-
proteins. The carbohydrate content of
glycoproteinvariesfrom 1o/oto 90o/oby weight,
Sometimes the term mucoprotein is used for
glycoprotein with carbohydrateconcentration
more than 4"/o. Clycoproteins are very widely
distributedin the cells and perform variety of
functions.Theseincludetheir role as enzymes,
hormones,transportproteins,structuralproteins
and receptors.A selectedlist of glycoproteins
and their major functionsis given in Table2.4.
The carbohydratesfound in glycoproteins
include mannose, galactose, N-acetyl-
glucosamine, N-acetylgalactosamine,xylose,
L-fucoseand N-acetylneuraminicacid (NANA).
NANA is an importantsialicacid (SeeFig.2,l1).
Antifreeze glycoproteins : The Antarctic fish
live below -2oC, a temperatureat which the
Glycoprotein(s) Major function(s)
Collagen
Hydrolases,proteases,
glycosidases
Ceruloplasmin
lmmunoglobulins
Synovialglycoproteins
Thyrotropin,erylhropoietin
Bloodgroupsubstances
Fibronectin,laminin
Intrinsicfactor
Fibrinogen
Structure
Enzymes
Transport
Defenseagainstinfection
Lubrication
Hormones
Antigens
Cell-cellrecognitionand
adhesion
Absorptionofvitamin8,,
Bloodclotting
33. 26 ElIOCHEMISTF|Y
blood would {reeze.lt is now known that ihese
fish contain antifreezeglycogtrateinwhich lower
the freezingpoint of waterand interferewith tne
crystalformationof ice. Antifreezegiycoproteins
consistof 50 repeatingunits of the tripeptide,
alanine-alawine-threonine. Each threonine
residue is bound to B-galactosyl(1 + 3) o(
N-acetylgalactosamine.
ri#i .f*iCA# '?
r,.4F.!"r.Ii $:F"r1.*fi {
"'.3
i4 t: * :il
The blood group antigens (of erythrocyte
membrane) contain carbohydratesas glyco-
proteinsor glycolipids.N-,A.cetylgaiactosamine,
galactose,fucose,sialic acid etc. are found in
the blood group substances.The carbohydrate
contentalso playsa determinantrole in blood
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 ytoiysaccharides.The monosacchsridesare further diuided into dit't'erentcategories
bqsedan the presenceaf t'wnctionalgroups {oldosesar ketoses)and the number of
carbon atoms (trioses,tetroses,pentases,hexosesand heptcses).
3. Glyceraldehyde{triose) is the simplestcarbohydrateand is chosen as a reJerenceto
write the cont'iguratian of all other rnonasaccharides(D- anc L- forms). It' two
rnonosaccharidesdiffer in their structurearound o singlecarbonatom, they ore known
as eplmers.Glucoseand galactoseare C4-epimers.
4. D'Glucose is the most predominant naturally occurring aldosdmonosaccharide.
Giucoseexisfscs a and p anemerswith dit'Jerentopticalrotations. The interconuersion
of a and B anomericforms with changein theopticalrotatianis knoun asmutsratation.
5. Manosaccharidespariicipate in seuercl recctions"Theseinclude oxidation, reduction.
dehydration, asazone formetion etc. Formatian ol esters and glycosides by
manosacchqridesis af specialsignificanceln biochemical reactions.
6. Among the oligosacchqrides,disoccharidesare the most common. Theseinclude the
reducing disaccharidesnamely lactose(rnilk sugar)and maltase(malt sugar)and the
non-reducingsucrose(canesugar).
7. Palysacclwridesare the poiymersot' monosaccharidesor their deriuatiues,held together
by glycosidic bonds.Homopalysaccharidessre compasedot' 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 animalsrespectiuelg.
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. Theyprouide the mstrix or grownd substanceof extracellular tissuespacesin
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),caualently bound to protein. Seueralenzyrnes,
hormanes,structura! proteinsand cellular receptorsare in fact glycoproteins.
34. Ghapter 2 : CAFIBOHYDHATES
I. Essayquestions
1. Define and classifycarbohydrateswith suitableexamples.Add a note on the functionsof
carbohydrates.
2. Describethe structureand functionsof mucopolysaccharides.
3. Cive an accountof the structuralconfigurationof monosaccharides,with specialreferenceto
glucose.
4. Discussthe structureand functionsof 3 biochemicallyimportantdisaccharides.
5. Definepolysaccharidesand describethe structureof 3 homopolysaccharides.
Short notes
(a)Epimers,(b)Mutarotation,(c)Osazoneformation,(d)Clycosidicbond,(e)Sugarderivatives,(fl
Anomers,(g)Enediol,(h)Amino su8ars,(i) Inversionof sucrose,(j) Deoxysugars.
Fill in the blanks
1. Namea non-reducingdisaccharide
2. The carbohydratethat is taken as a referencefor writing the configurationof others
3. lf two monosaccharidesdifferin configurationarounda singlecarbonatom,they are known
as
27
II.
III.
4.
5.
6.
7.
B.
9.
10.
The s and B cyclicformsof D-glucoseare referredto as
The non-carbohydratemoietyfound in glycosidesis known as
Cive an exampleof a glycosideantibiotic
Theglycosidicbondsat the branchingpointsin the structureof starchare
The polysaccharideemployedfor the assessmentof kidneyfunction
The glycosaminoglycanthat servesas a lubricantand shockabsorbantof joints
Namethe sialicacid,mostlyfound in the structureof glycoproteinsand glycolipids
IV. Multiple choice questions
11. Riboseand deoxyribosedifferin structurearounda singlecarbon,namely
(a)Cr (b)Cz (c)C: (d)Cq.
12. One of the followingis not an aldose
(a)Clucose(b)Calactose(c) Mannose(d) Fructose.
13. The glycosaminoglycanthat servesas an anticoagulant
(a) Heparin(b) Hyaluronicacid (c)Chondroitinsulfate(d) Dermatansulfate.
14. The followingpolysaccharideis composedof B-glycosidicbonds
(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.
35. Lirpirdls
fl ?"'-o-
i
R--c-o1H
frCH2-H R3
The Jat speaks :
"ffith uater, I say, 'Touch menot':
T'otlte tongue,I am tasteful;
IY'ithin limits, I am datiful;
fn excess,I am dangerous!"
I ipids (Creek: lipos-fat) are of Breat
L importance to the body as the chief
concentratedstorageform of energy, besides
their role in cellularstructureand variousother
biochemicalfunctions.As such. lioids are a
heterogeneous group of compounds ano,
therefore,it is rather difficult to define them
preciselv.
Lipidsmay be regarded as organic substances
relatively insoluble in water, soluble in organic
solvents (alcohol, ether etc.), actually or
potentially related to fatty acidsand utilized by
the living cells.
Unlike the polysaccharides,proteins and
nucleic acids,lipids are not polymers.Further,
lipidsare mostlysmall molecules.
Lipids are broadlyclassified(modifiedfrom
Bloor) into simple, complex, derived and
miscellaneouslipids,whicharefurthersubdivided
into differentgroups
1. Simple lipids: Estersof fatty acids with
alcohols.Theseare mainly of two types
(a) Fatsand oils (triacylglycerols): Theseare
estersof fatty acids with glycerol. The
difference between fat and oil is only
physical.Thus,oil is a liquid while fat is
a solid at room temperature.
(b) Waxes: Estersof fattyacids(usuallylong
chain)with alcoholsotherthan glycerol.
These alcohols may be aliphatic or
alicyclic.Cetylalcoholis mostcommonly
found in waxes.
2. Complex(or compound)lipids: Theseare
estersof fatty acids with alcohols containing
additional groups such as phosphate,
nitrogenousbase, carbohydrate,protein etc
They are furtherdividedas follows
(a) Phospholipids:They containphosphor,c
acid and frequentlya nitrogenousbase
This is in addition to alcohol and fai:.
acids.
28
36. Chapter 3 : LIPIDS 29
(i) Glycerophospholipids: Thesephospho-
lipids containglycerolas the alcohol
e.9.,lecithin,cephalin.
(ii) Sphingophospholipids: Sphingosineis
the alcohol in this group of phospho-
lipidse.g.,sphingomyelin.
(b) Glycolipids: Theselipids contain a fatty
acid, carbohydrateand nitrogenousbase.
The alcohol is sphingosine,hence they
are also called as glycosphingolipids.
Clycerol and phosphateare absente.g.,
cerebrosides,gangliosides.
(c) Lipoproteins: Macromolecularcomplexes
of lipids with proteins.
(d) Other complexlipids: Sulfolipids,amino-
lipidsand lipopolysaccharidesareamong
the othercomplex lipids.
3. Derived lipids: Theseare the derivatives
obtainedon the hydrolysisof group 1 and group
2lipids which possessthe characteristicsof
lipids.Theseincludeglycerolandotheralcohols,
fatty acids,mono- and diacylglycerols,lipid (fat)
soluble vitamins, steroid hormones, hydro-
carbonsand ketonebodies.
4. Miscellaneouslipids: These include a
large number of compounds possessingthe
characteristics of lipids €.g., carotenoids,
squalene,hydrocarbonssuch as pentacosane(in
bees wax), terpenesetc.
NEUTRAT LIPIDS: The lipids which are
unchargedare referredto asneutrallipids.These
are mono-, di-, and triacylglycerols,cholesterol
and cholesterylesters.
Functions of lipids
Lipids performseveralimportantfunctions
1. They are the concentratedfuel reserveof
the body (triacylglycerols).
2. Lipids are the constituentsof membrane
structure and regulate the membrane
permeability(phospholipidsand cholesterol).
3. They serve as a source of fat soluble
vitamins(4, D, E and K).
4. Lipidsare importantas cellularmetabolic
regulators(steroidhormonesand prostaglandins).
5. Lipidsprotectthe internalorgans,serveas
insulatingmaterialsand give shapeand smooth
appearanceto the body.
Fatty acids are carboxylic acids with
hydrocarbonside chain. They are the simplest
form of lipids.
Occurrence
Fattyacidsmainly occur in the esterifiedform
as major constituentsof variouslipids.They are
also present as free (unesterified)fatty acids.
Fattyacidsof animalorgin are much simplerin
structure in contrast to those of plant origin
which oftencontaingroupssuch asepoxy,keto,
hydroxy and cyclopentanerings.
Even and odd carbon fatty acids
Most of the fatty acids that occur in natural
lipids are of even carbons(usually 14C-2OC).
This is due to the fact that biosynthesisof fatty
acidsmainly occurswith the sequentialaddition
of 2 carbon units. Palmitic acid (l6C) and
stearicacid (l$C) are the most common. Among
the odd chain fatty acids, propionic acid (3C)
and valericacid (5C)are well known.
Saturated and unsaturated
fatty acids
Saturatedfatty acids do not contain double
bonds,while unsaturatedfattyacidscontainone
or more double bonds. Both saturated and
unsaturatedfatty acids almost equally occur in
the natural lipids. Fatty acids with one double
bond are monounsaturated,and thosewith 2 or
more double bonds are collectivelv known as
polyunsaturated fafty acids (PIJFA).
Nomenclature of fatty acids
The namingof a fatty acid (systematicname)
is basedon the hydrocarbonfrom which it is
derived. The saturatedfatty acids end with a
suffix -anoic (e.g., octanoic acid) while the
unsaturatedfatty acids end with a suffix -enoic
37. 30 BIOCHEMISTF|Y
(e.9., octadecanoic acid). In addition to
systematicnames/ fatty acids have common
nameswhich are more widely used (Iable J. l).
Numbering of carbon atoms : lt startsfrom
the carboxylcarbonwhich is takenas number1.
The carbonsadjacentto this (carboxylC) are2,
3, 4 and so on or alternatelya, F, T and so on.
The terminalcarbon containingmethyl group is
known omega (or) carbon. Starting from the
methylend, the carbonatomsin a fattyacid are
numberedas omega 1, 2, 3 etc. The numbering
of carbon atoms in two different ways is given
below
7654321
cH3 - cH2 - cH2- cH2-cH2 - cH2 - COOH
01 a2 o)3 ()4 ol5 (t)6
Length of hydrocarbon
cha:n of fatty acids
Dependingon the length of carbon chains,
fatty acids are categorizedinto 3 groups-short
chain with less than 6 carbons; medium chain
with 8 to 14 carbons and long cfiain with 16 to
24 carbons.
Shorthand representation
of latty aclds
lnstead of writing the full structures,
biochemists employ shorthand notations (by
numbers)to representfatty acids. The general
rule is that the total numberof carbonatomsare
written first,followed by the nunrberof double
bonds and finally the (firstcarbon) position of
Common Name Systematicname Abbreviationx Structure
l.Saturatedfattyaclds
Aceticacid
Propionicacid
Butyricacid
Valericacid
Caproicacid
Caprylicacid
Capricacid
Lauricacid
Myristicacid
Palmiticacid
Stearicacid
Arachidicacid
Behenicacid
Lignocericacid
Ethanoicacid
n-Propanoicacid
n-Butanoicacid
n-Pentanoicacid
n-Hexanoicacid
n-Octanoicacid
n-Decanoicacid
n-Dodecanoicacid
n-Tetradecanoicacid
n-Hexadecanoicacid
n-Octadecanoicacid
n-Eicosanoicacid
n-Docosanoicacid
n-Tetracosanoicacid
CHsCO0H
CHgCHzCOOH
CHs(CHz)z0O0H
CHo(CHz)gCOOH
CHs(CHe)+COOH
CHe(CHz)oCOOH
CHs(CHz)eC0OH
CHs(CHz)roCOOH
CHs(CHzhzCOOH
CHg(CHz)t+CO0H
CHs(CHz)roC0OH
CHg(CHz)reCOOH
CHs(CHz)zo00OH
CH3(CHz)zzCOOH
2:0
3:0
4:0
F.n
6:0
8:0
10:0
12:0
14:0
16:0
18:0
20:0
22:0
24:0
ll. Unsaturatedfattyacids
Palmitoleicacid
Oleicacid
Linoleicacid**
Linolenicacid*x
Arachidonicacid
cr1s9-Hexadecenoicacid
cls-9-Octadecenoicacid
cls,cls-9,12-Octadeca-
dienoicacid
Allce9,12,15-0cta-
decatrienoicacid
Allcls-5,8,11,14-
16:1;9
18:1;9
18:2;9,12
18:3;9,12,'15
20:4;5,8,11,14
CHg(CHz)sCH=CH(CHz)zCOOH
CHs(CHz)zCH=CH(CHz)zCOOH
CHg(CHz)+CH=CHCHzCH=CH(CHz)zCOOH
CHoCHzCH=CHCHzCH=CHCHzCH
=CH(CHz)zCO0H
CHg(CHz)+CH=CHCHzCH=CHCHzCH
Elc0:a!tr3e!o!1ci1___ __=9H9'tcl=_cl9F!)49oli
* Totalnunberofcarbonatons,followedbythenumberotdoublebondsandthefirctcarbonposrtionotthedoublebond(s).
** Essentialfawacids.
38. Ghapten 3 : LIPIDS 31
double bonds, startingfrom the carboxyl end.
Thus,saturatedfattyacid, palmiticacid iswritten
as.l 6:0, oleic acid as 18:1;9, arachidonic
acid as 20 : 4; 5, 8, 11, 14.
There are other conventionsof representing
the double bonds.Ae indicatesthat the double
bond is between9 and 10 of the fatty acid. o 9
representsthe double bond position(9 and 10)
from the <oend. Naturallyoccurringunsaturated
fatty acids belongto ro 9, ol 6 and o 3 series.
a 3 series Linolenicacid(18 : 3;9, 12, 15)
a 6 series Linoleicacid ('l8 : 2; 9, 12) and
arachidonic acid (20 : 4; 5, 8,
11, 14)
ro9 series Oleicacid(18 : 1; 9)
The biochemically important saturatedand
unsaturated fatty acids are given in the
Table 3.1.
The fatty acidsthat cannotbe synthesizedby
the body and, therefore, should be supplied in
the diet are known asessentialfattyacids(EFA).
Chemically, they are polyunsaturated fatty
acids, namely linoleic acid (18 : 2; 9, 12) and
Iinolenic acid (18 : 3; 9, 12, 15). Arachidonic
acid (20 :4;5,8, 11,14) becomesessential,if
its precursorlinoleic acid is not providedin the
diet in sufficientamounts.The structuresof EFA
are given in the Table3.1.
Biochemical basis for essentiality: Linoleic
acid and linolenic acid are essentialsince
humans lack the enzymesthat can introduce
double bonds beyond carbons9 to 10.
Functionsof EFA: Essentialfatty acids are
required for the membrane structure and
function, transportof cholesterol,formation of
lipoproteins,preventionof fatty liver etc. They
are also needed for the synthesisof another
important group of compounds, namely
eicosanoids(Chapter 32.
Deficiency of EFA: The deficiency of EFA
results in phrynoderma or toad skin,
characterizedby the presenceof hornyeruptions
H..ar(CHz)zCOOH
H'c'1cHr;rcu,
Oleic acid
(clsform)
Fig. 3.1 : Cis-trans isomerism in
unsaturated fattv acids.
on the posteriorand lateralpartsof limbs,on the
back and buttocks,lossof hair and poor wound
healing.
lsomerism in
unsaturated fatiy aeids
Unsaturated fatty acids exhibit geometric
isomerismdependingon the orientationof the
groupsaround the 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
configuration. Thus oleic acid is a cis isomer
while elaidic acid is a transisomer,as depicted
in Fig.3.1. Cis isomersare lessstablethan frans
isomers. Most of the naturally occurring
unsaturatedfatty acids exist as crs isomers.
In the cis isomericform, there is a molecular
binding at the double bond. Thus, oleic acid
exists in an L-shapewhile elaidic acid is a
straightchain.Increasein the numberof double
bonds will cause more bends (kinks) and
arachidonicacid with 4 doublebondswill have
a U-shape.lt is believed that cis isomersof fatty
acids with their characteristic bonds will
compactlypack the membranestructure.
Hydroxy fatty acids: Someof the fatty acids
are hydroxylated.p-Hydroxybutyricacid, one of
the ketonebodiesproducedin metabolism,is a
simple example of hydroxy fatty acids.
Cerebronic acid and recinoleic acid are long
chain hydroxy fatty acids.
Cyclic fatty acids: Fatty acids with cyclic
structuresare ratherraree.g./ chaulmoogric acid
found in chaulmoogra oil (used in leprosy
treatment)containscyclopentenylring.
Elaldicacid
(fransform)
39. 32 BIOCHEMISTFIY
U
A CH2-O-C Fl,
ltl
R2-C-O-CH O
ttl
cH2-o-c-R3
Triacylglycerol
o
cH2-o-c -B
t-
HO_CH
I
cH20H
1-Monoacylglycerol
o
o
Rz-C
cH2-o-c-R,
-o-cH
I
cH2oH
1,2-Diacylglycerol
O CH,_OH
ill
R-C-O-CH
I
cH2oH
2-Monoacylglycerol
Fig. 3.2 : General structures of acylglycerols
(For palmitoyl R = CtsHati for stearoyl R = C.rzHssiFor linoleoyl R = qtHsi
Eicosanoids:Thesecompoundsare relatedro
eicosapolyenoicfatty acids and include prosta-
glandins,prostacyclins,leukotrienesand throm-
boxanes.Theyarediscussedtogether(Chapter32).
Triacylglycerols (formerly triglycerides) are
the estersof glycerol with fatty acids. The fats
and oils thatarewidely distributedin both plants
and animals are chemically triacylglycerols.
They are insolublein water and non-polarin
characterand commonly known as neutralfats.
Fatsas stored fuel : Triacylglycerolsare the
most abundantgroup of lipids that primarily
function as fuel reservesof animals. The fat
reserveof normal humans (men 2Oo/o,women
25% by weigh$ is sufficientto meet the body's
caloric requirementsfor 2-3 months.
Fats primarily occur in adipose tissue:
Adipocytes of adipose tissue-predominantly
found in the subcutaneouslayer and in the
abdominalcavity-are specializedfor storageof
triacylglycerols.The fat is storedin the form of
globulesdispersedin the entirecytoplasm.And
surprisingly,triacylglycerolsarenot the structural
componentsof biological membranes.
Structures of acylglycerols: Monoacyl-
glycerols, diacylglycerolsand triacylglycerols,
respectivelyconsistingof one, two and three
moleculesof fatty acidsesterifiedto a molecule
of glycerol,are known (Fi5.3.2).Among these,
triacylglycerols are the most important
biochemically.
Simpletriacylglycerolscontainthe sametype
of fattyacid residueat all the threecarbonse.g.,
tristearoylglycerolor tristearin.
Mixed triacylglycerols are more common.
They contain2 or 3 different typesof fattyacid
residues.In general,fatty acid attachedto C1 is
saturated,that attached to C2 is unsaturated
while that on C3 can be either.Triacylglycerols
are named according to placement of acyl
radicalon glycerole.9.,'l
,3-palmitoyl2-linoleoyl
glycerol.
Triacylglycerols of plants, in general, have
higher content of unsaturated fatty acids
compared to that of animals.
$tereospecific numbering
of glycerol
The structureof glycerolgivesan impression
thatcarbons1 and 3 are identical.Thisis not true
in a 3-dimensionalstructure.In orderto represent
the carbonatomsof glycerolin an unambiguous
manner, biochemists adopt a stereospecific
numbering(sn)and prefixglycerolwith sn.
6n,on
no-C'.-H
6tr,ot
sn-GfcJrol
40. C*rapter'3: LIPIDS 33
It should be noted that C1 and C3 are
different. Cells possess enzymes that can
distinguish these two carbons. Thus
glycerokinasephosphorylatessn-3(andnot sn-l)
glycerolto give sn-glycerol3-phosphate.
PROPERTIESOF TRIACYLGTYCEROLS
A few importantpropertiesof triacylglycerols,
which have biochemical relevance, are
discussedbelow
1. Hydrolysis: Triacylglycerols undergo
stepwiseenzymatichydrolysisto finally liberate
free fatty acids and glycerol. The processof
hydrolysis,catalysedby lipasesis importantfor
digestionof fat in the gastrointestinaltract and
fat mobilizationfrom the adiposetissues.
2. Saponification: The hydrolysisof triacyl-
glycerolsby alkalito produceglyceroland soaps
is known as saoonification.
Triacylglycerol+ 3 NaOH ---------+
Clycerol+ 3 R-COONa(soaps)
3. Rancidity: Rancidityis the term used to
represent the deterioration of fats and oils
resultingin an unpleasanttaste.Fatscontaining
unsaturatedfatty acids are more susceptibleto
ranciditv.
Rancidity occurs when fats and oils are
exposed to air, moisture, light, bacteria etc.
Hydrolytic rancidity occurs due to partial
hydrolysis of triacylglycerols by bacterial
enzymes.Oxidativerancidityis due to oxidation
of unsaturatedfatty acids. This results in the
formation of unpleasant products such as
dicarboxylic acids, aldehydes, ketones etc.
Rancid fats and oils are unsuitablefor human
consumotion.
Antioxidants : The substanceswhich can
preventthe occurrenceof oxidativerancidityare
known as antioxidants. Trace amounts of
antioxidantssuch as tocopherols(vitamin E),
hydroquinone,gallic acid and c,-naphtholare
addedto the commercialpreparationsof fatsand
oilsto preventrancidity.Propylgallate,butylated
hydroxyanisole(BHA) and butylated hydroxy-
toluene(BHT)are the antioxidantsused in food
preservation.
a. tipid peroxidation in vivo: In the living
cells, lipids undergo oxidation to produce
peroxidesand free radicalswhich can damage
the tissue.Thefreeradicalsarebelievedto cause
inflammatory diseases, ageing, cancer/
atherosclerosisetc. lt is fortunatethat the cells
possessantioxidantssuchasvitamin E,urateand
superoxidedismutaseto prevent in vivo lipid
peroxidation (Chapter 34).
Tests to check purity
of fats and oils
Adulterationof fatsand oils is increasingday
by day. Several tests are employed in the
laboratoryto check the purity of fats and oils.
Some of them are discussedhereunder
lodine number: lt is defined as the grams
(number) of iodine absorbedby 100 g of fat or
oil. lodine number is usefulto know the relative
unsaturationof fats,and is directly proportional
to the content of unsaturatedfatty acids. Thus
lower is the iodine number,lessis the degreeof
unsaturation.The iodine numbersof common
oils/fatsare given below.
FaUoil lodine number
Coconutoil
Butter
Palmoil
Oliveoil
Groundnutoil
Cottonseedoil
Sunfloweroil
Linseedoil
7- 10
25- 28
4C- 55
80- 85
85- 100
100- 110
125- 135
175-200
Determinationof iodinenumberwill help to
know the degreeof adulterationof a given oil.
Saponificationnumber: lt is defined as the
mg (number) of KOH required to hydrolyse
(saponify)one gram of fat or oiL Saponification
number is a measureof the averagemolecular
sizeof the fattyacidspresent.Thevalueis higher
for fats containing short chain fatty acids. The
saponificationnumbersof a few fatsand oils are
given below
Humanfat : 195-200
Butter :230-240
Coconutoil : 250-260
41. 34 ElIOCHEMISTRY
Reichert-Meissl(RM) number: lt is definedas
the number of ml 0.1 N KOH required to
completelyneutralizethe soluble volatile fatty
acidsdistilledfrom 5 g fat. RM number is useful
in testingthe purity of buttersince it containsa
goodconcentrationof volatilefattyacids(butyric
acid, caproicacid and caprylicacid).This is in
contrastto other fats and oils which have a
negligibleamount of volatile fatty acids. Butter
hasa RM numberin the range25-30,while it is
lessthan I for mostotheredibleoils. Thusany
adulteration of hutter can be easily tested by
this sensitiveRM number.
Acid number : lt is definedas the numberof
mg of KOH requiredto completely neutralize
freefatty acidspresentin one gramfat or oil. In
normalcircumstances,refinedoils shouldbe free
from any free fatty acids. Oils, on
decomoosition-due to chemical or bacterial
contamination-yield freefatty acids.Therefore,
oils with increasedacid number are unsafefor
humanconsumption.
These are complex or compound lipids
containingphosphoricacid,in additionto fatty
acids,nitrogenousbaseand alcohol(Fig.3.3).
There are two classesof phospholipids
1. Clycerophospholipids(or phosphoglyce-
rides)that contain glycerolas the alcohol.
2. Sphingophospholipids(or sphingomyelins)
that containsphingosineas the alcohol.
1.i t
".t
.;:i r,. : . ,,,.i., i-l,
Clycerophospholipidsare the major lipids
thatoccur in biologicalmembranes.Theyconsist
of glycerol 3-phosphateesterifiedat its C1 and
C2 with fatty acids. Usually, C1 contains a
saturated fatty acid while C2 contains an
unsaturatedfatty acid.
1. Phosphatidicacid : This is the simplest
phospholipid. lt does not occur in good
concentration in the tissues. Basically,
phosphatidicacid is an intermediatein the
synthesisof triacylglycerolsand phospholipids.
The other glycerophospholipidscontaining
differentnitrogenousbasesor other groupsmay
be regardedas the derivativesof phosphatidic
acid.
2. Lecithins (phosphatidylcholine)zTheseare
the mostabundantgroupof phospholipidsin the
cell membranes.Chemically,lecithin (Creek :
lecithos-egg yolk) is a phosphatidicacid with
choline as the base. Phosphatidylcholines
represent the storage form of hody's choline.
*
BtoMEDtCAL/ CLtNtCAt CONCEpTS
os Lipids are important to the body as constituentsof membranes,sourceol 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 subcutoneoustissueand around certaln orgons
serueos thermal insulators,
se The unsaturatedfatty acids-linoleicand linolenic acid-<re essentiolto humans, the
deficiencyof which cousesphrynodermo or toad skin.
s The cyclicfatty acid, namelychoulmoogricocid,isemployedin the treatmentof leprosy.
og Fqts and oils on exposureto ah; moisture, bacteriaetc. 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.
43. 36 BIOCHEMISTF|Y
(a) Dipalmitoyl lecithin is an important
phosphatidylcholinefoundin lungs,lt isa
surface active agent and prevents the
adherence of inner surface of the
lungsdue to surfacetension.Respiratory
distresssyndromein infantsis a disorder
characterizedby the absenceof dipalmitoyl
lecithin.
(b) Lysolecithinis formed by removalof the
fatty acid either at C, or C, of lecithin.
3. Cephafins (phosphatidylethanolamine):
Ethanolamineis the nitrogenousbasepresentin
cephalins,Thus,lecithinandcephalindifferwith
regardto the base.
4. Phosphatidylinositol: The steroisomer
myo-inositolis attachedto phosphatidicacid to
givephosphatidylinositol(Pl).Thisisan important
comDonentof cell membranes.The action of
certain hormones(e.9.oxytocin, vasopressin)is
mediatedthroughPl.
5. Phosphatidylserine:The amino acid
serineis presentin this group of glycerophos-
pholipids.Phosphatidylthreonineis alsofound in
certaintissues.
6. Plasmalogens: When a fatty acid is
attachedby an etherlinkageat C1 of glycerolin
the glycerophospholipids, the resultant
compound is plasmalogen. Phosphatidal-
ethanolamineis the most imoortantwhich is
similarin structureto phosphatidylethanolamine
but for the ether linkage(in place of ester).An
unsaturatedfatty acid occurs at C1. Choline,
inositoland serinemay substituteethanolamine
to give other plasmalogens.
Z. Cardiolipin: lt is so named as it was first
isolated from heart muscle. Structurally, a
cardiolipin consists of two molecules of
phosphatidicacid held by an additionalglycerol
through phosphategroups. lt is an important
componentof inner mitochondrialmembrane.
Cardiolipin is the only phosphoglyceridethat
possessesantigenic properties.
Sphingomyelins
Sphingosineis an amino alcohol presentin
sphingomyelins(sphingophospholipids).They do
notcontainglycerolat all. Sphingosineisattached
by an amide linkageto a fatty acid to produce
ceramide.The alcohol group of sphingosineis
bound to phosphorylcholinein sphingomyelin
structure(Fig.3.3).Sphingomyelinsare important
constituentsof myelin and are found in good
quantityin brain and nervoustissues.
Action of phospholipases
Phospholipasesare a group of enzymesthat
hydrolysephospholipids.Thereare four distinct
phospholipases(Ar, 42, C and D), eachone of
them specificallyactson a particularbond. For
details,refer lipid metabolism(Chapter l4).
Functions of phospholipids
Phospholipidsconstitutean importantgroup
of compound lipids that performa wide variety
of functions
1. In associationwith proteins,phospholipids
form the structural components of membranes
and regulatemembranepermeability.
2. Phospholipids (lecithin, cephalin and
cardiolipin)in the mitochondriaare responsible
for maintaining the conformation of electron
transportchain components,and thus cellular
respiration.
3. Phospholipidsparticipatein the absorption
of fat from the intestine.
4. Phospholipids are essential for the
synthesisof different lipoproteins,and thus
participate in the transport of lipids.
5. Accumulationof fat in liver(fattyliver)can
be preventedby phospholipids,hence they are
regarded as lipotropic factors.
6. Arachidonicacid,an unsaturatedfattyacid
liberated from phospholipids, serves as a
precursorfor the synthesisof eicosanoids(prosta-
glandins,prostacyclins,thromboxanesetc.).
7. Phospholipidsparticipatein the reverse
cholesteroltransport and thus help in the
removalof cholesterolfrom the body.
8. Phospholipidsact as surfactants(agenL.
lowering surface tension). For instance
dipalmitoylphosphatidylcholineis an importar:
fung surfactant. Respiratory distresssyndrome ^
infantsis associatedwith insufficientproductio^
of this surfactant.
