Review on Carbohydrates

8/8/2019 Review on Carbohydrates

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REVIEWREVIEW

ofof

CARBOHYDRATESCARBOHYDRATES


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CARBOHYDRATESCARBOHYDRATES Hydrates of carbon [Hydrates of carbon [CCnn((HH22OO))mm]]

PolyPolyhydroxyaldehydehydroxyaldehyde or polyor polyhydroxyketonehydroxyketone, or, or

substance that gives these compounds on hydrolysissubstance that gives these compounds on hydrolysis

Most abundant organic compound in the plant worldMost abundant organic compound in the plant world

Chemically made up of skeletalChemically made up of skeletal CC,,HH which is usuallywhich is usually

2x the number of C, highly variable number of2x the number of C, highly variable number ofOO,,

occasionaloccasional NN && SS

Linked to many lipids and proteinsLinked to many lipids and proteins


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FUNCTIONS ofFUNCTIONS of

CARBOHYDRATESCARBOHYDRATES Storehouses of chemical energyStorehouses of chemical energy

Glucose,starch, glycogenGlucose,starch, glycogen

Structural components for supportStructural components for support

Cellulose, chitin, GAGsCellulose, chitin, GAGs

Essential components of nucleic acidsEssential components of nucleic acids

DD--ribose, 2ribose, 2--deoxydeoxy--DD--riboseribose

Antigenic determinantsAntigenic determinants Fucose, DFucose, D--galactose, Dgalactose, D--glucose, Nglucose, N--acetylacetyl--DD--

glucosamine, Dglucosamine, D--acetylacetyl--DD--galactosaminegalactosamine


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SPECIFIC CARBOHYDRATESSPECIFIC CARBOHYDRATESMonosaccharidesMonosaccharides

GlucoseGlucose ((dextrose, grape sugar, bloodsugardextrose, grape sugar, bloodsugar)) Can be stored as glycogenCan be stored as glycogen

Most metabolically important monosaccharideMost metabolically important monosaccharide

FructoseFructose ((levuloselevulose))

GalactoseGalactose ((brain sugar)brain sugar)

MannoseMannose Targets lysosomal enzymes to their destinationsTargets lysosomal enzymes to their destinations

Directs certain proteins from Golgi body to lysosomesDirects certain proteins from Golgi body to lysosomes


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DisaccharidesDisaccharides

SucroseSucrose ((table sugar, cane sugartable sugar, cane sugar,,saccharosesaccharose))

glucose & fructoseglucose & fructose linkedlinked11--22

LactoseLactose ((milk sugarmilk sugar)) glu & galglu & gallinkedlinked 11--44

MaltoseMaltose ((malt sugarmalt sugar)) 2 glucose2 glucose linkedlinked

11--44

TrehaloseTrehalose ((mycosemycose)) 2 glucose2 glucose linkedlinked

11--11 GentiobioseGentiobiose ((amygdaloseamygdalose)) 2 glucose2 glucose

linkedlinked 11--66

CellobioseCellobiose 2 glucose2 glucose linkedlinked 11--44


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CLASSES OF CARBOHYDRATESCLASSES OF CARBOHYDRATES

Number of CNumber of C Triose, tetroses, pentose, hexose, heptuloseTriose, tetroses, pentose, hexose, heptulose

Number of saccharide unitsNumber of saccharide units

Monosaccharides, disaccharides, oligosaccharides (2 to 10Monosaccharides, disaccharides, oligosaccharides (2 to 10

units), polysaccharidesunits), polysaccharides

Position of carbonyl (C=O) groupPosition of carbonyl (C=O) group

AldoseAldose if terminally locatedif terminally located

Ketose

Ketose if centrally locatedif centrally located

Reducing propertyReducing property

Reducing sugarsReducing sugars (all monosaccharides)(all monosaccharides)

Nonreducing sugarsNonreducing sugars (sucrose)(sucrose)


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STRUCTURALPROJECTIONS OFSTRUCTURALPROJECTIONS OF

MONOSACCHARIDESMONOSACCHARIDES

FISCHERFISCHER by Emil Fischerby Emil Fischer

((Nobel Prize in Chemistry1902Nobel Prize in Chemistry1902))

22--D representation for showingD representation for showing

the configuration of a stereocenterthe configuration of a stereocenter

Horizontal lines project forwardHorizontal lines project forward

while vertical lines project towardswhile vertical lines project towardsthe rearthe rear

DD ((RR oror++) or) orLL ((SS oror--))


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HAWORTHHAWORTH by Walter Haworthby Walter Haworth

((Nobel Prize in Chemistry1937Nobel Prize in Chemistry1937))

A way to view furanose (5A way to view furanose (5--membered ring) andmembered ring) and

pyranose (6pyranose (6--membered ring) forms ofmembered ring) forms of

monosaccharidesmonosaccharides

The ring is drawn flat and viewed through its edgeThe ring is drawn flat and viewed through its edge

with the anomeric carbon on the the right and thewith the anomeric carbon on the the right and the

oxygen atom on the rearoxygen atom on the rear


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ANOMERIC CARBONANOMERIC CARBON


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CHAIR & BOATCHAIR & BOAT

CONF

ORM

ATION

SCON

FO

RMAT

ION

S


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AMINO SUGARSAMINO SUGARS


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REDUCING PROPERTYREDUCING PROPERTY

KetoseKetose

EnediolEnediol

AldoseAldose

AldonateAldonate

H

H C OH

C = O

R

OH-

OH-

Oxidizing

agent

HO H

C

C

R OH

O H

C

H C OH

R

O O-

C

H C OH

R


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AABBOO ANTIGENSANTIGENS

NN--acetylacetyl-- DD--galactosegalactose NN--acetylacetyl--

DD--galactosaminegalactosamine DD--glucosamineglucosamine

FucoseFucose

DD--galactosegalactose DD--galactosegalactose NN--acetylacetyl--

DD--glucosamineglucosamine

FucoseFucose

DD--galactosegalactose NN--acetylacetyl--

DD--glucosamineglucosamine

FucoseFucose

TYPE ATYPE A

TYPEBTYPEB

TYPE OTYPE O


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POLYSACCHARIDESPOLYSACCHARIDES STARCHSTARCH

Storage carbohydrate in plantsStorage carbohydrate in plants

Two principal parts areTwo principal parts are amyloseamylose (20(20--25%) &25%) &

amylopectinamylopectin (75(75--80%) which are completely80%) which are completelyhydrolyzed to Dhydrolyzed to D--glucoseglucose

AmyloseAmylose is composed ofis composed ofcontinuous, unbranchedcontinuous, unbranched

chain ofchain of4000 D4000 D--glucoseglucose linked vialinked via 11--4 bonds4 bonds

AmylopectinAmylopectin is a chain ofis a chain of10,000 D10,000 D--glucoseglucose units linkedunits linked

viavia 11--4 bonds but4 bonds but branchingbranching of 24of 24--30 glucose units is30 glucose units is

started viastarted via 11--6 bonds6 bonds


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GLYCOGENGLYCOGEN

EnergyEnergy--reserve carbohydrate in animalsreserve carbohydrate in animals

Highly branched containing approximatelyHighly branched containing approximately101066 glucose units linked viaglucose units linked via 11--4 bonds &4 bonds &

11--6 bonds6 bonds

WellWell--nourished adult stores 350 g. of itnourished adult stores 350 g. of it

equally divided between the liver andequally divided between the liver and

musclesmuscles


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CELLULOSECELLULOSE

Plant skeletal polysaccharidePlant skeletal polysaccharide

Linear chain of 2200 glucose units linkedLinear chain of 2200 glucose units linkedviavia 11--4 bonds4 bonds

High mechanical strength is due toHigh mechanical strength is due to

aligning of stiff fibers where hydroxyl formaligning of stiff fibers where hydroxyl form

hydrogen bondinghydrogen bonding


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ACIDIC POLYSACCHARIDESACIDIC POLYSACCHARIDES

Also called mucopolysaccharides (MPS) orAlso called mucopolysaccharides (MPS) orglycosaminoglycans (GAG)glycosaminoglycans (GAG)

Polymers which contain carboxyl groups and/orPolymers which contain carboxyl groups and/or

sulfuric ester groupssulfuric ester groups

Structural and functional importance in connectiveStructural and functional importance in connective

tissuestissues

Interact with collagen to form loose or tight networksInteract with collagen to form loose or tight networks


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ACIDIC POLYSACCHARIDESACIDIC POLYSACCHARIDES HYALURONIC ACIDHYALURONIC ACID

Simplest GAGSimplest GAG

Contains 300Contains 300--100,000 repeating units of100,000 repeating units ofDD--glucuronicglucuronicacid and Nacid and N--acetylacetyl--DD--glucosamineglucosamine

Abundant in embryonic tissues, synovial fluid, and theAbundant in embryonic tissues, synovial fluid, and thevitreous humor to hold retina in placevitreous humor to hold retina in place

Joint lubricant & shock absorberJoint lubricant & shock absorber

HEPARINHEPARIN Heterogeneous mixture of variably sulfonated chainsHeterogeneous mixture of variably sulfonated chains

Stored in mast cells of the liver, lungs and the gutStored in mast cells of the liver, lungs and the gut

NaturallyNaturally--occurring anticoagulant by acting asoccurring anticoagulant by acting asantithrombin III and antithromboplastinantithrombin III and antithromboplastin

Composed of two disaccharide repeating units A & B;Composed of two disaccharide repeating units A & B; A isA is LL--iduronic acididuronic acid--22--sulfate linked to 2sulfate linked to 2--deoxydeoxy--22--sulfamidosulfamido--DD--

galactosegalactose--66--sulfatesulfate

BB isis DD--glucuronic acid betaglucuronic acid beta--linked to 2linked to 2--deoxydeoxy--22--sulfamidosulfamido--DD--glucoseglucose--66--sulfatesulfate


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HEPARAN SULFATEHEPARAN SULFATE

CHONDROITIN SULF

ATECHONDROITIN SULF

ATE Most abundant in mammalian tissuesMost abundant in mammalian tissues

Found in skeletal and soft connective tissuesFound in skeletal and soft connective tissues

Composed of repeating units ofComposed of repeating units ofNN--acetyl galactosamine sulfate linkedacetyl galactosamine sulfate linked

beta1beta1--4 to glucuronic acid4 to glucuronic acid

KERATAN SULFATEKERATAN SULFATE

DERMATAN SULFATEDERMATAN SULFATE Found in skin, blood vessels,heart valves, tendons, aorta, spleenFound in skin, blood vessels,heart valves, tendons, aorta, spleen

and brainand brain

The disaccharide repeating units areThe disaccharide repeating units are LL--iduronic acid and Niduronic acid and N--

acetylgalactosamineacetylgalactosamine--44--sulfate with small amounts of Dsulfate with small amounts of D--glucuronic acidglucuronic acid


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GLYCOLYSISGLYCOLYSIS

The specific pathway by which the bodyThe specific pathway by which the bodygets energy from monosaccharidesgets energy from monosaccharides

First stage is ACTIVATIONFirst stage is ACTIVATION

At the expense ofAt the expense of2ATP2ATPss glucose isglucose isphosphorylatedphosphorylated

Step #1Step #1

formation of glucoseformation of glucose--66--phosphatephosphate

Step # 2Step # 2

isomerization to fructoseisomerization to fructose--66--phosphatephosphate


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Step # 3Step # 3

Secondphosphate group is attached to yield fructoseSecondphosphate group is attached to yield fructose--1,61,6--bisphosphatebisphosphate

Second stage is CSecond stage is C66 to 2 molecules of Cto 2 molecules of C33

Step # 4Step # 4

FructoseFructose--1,61,6--bisphosphate is broken down into two C3bisphosphate is broken down into two C3fragmentsfragments

glyceraldehydeglyceraldehyde--33--phosphate (Gphosphate (G--33--P) andP) and

dihydroxyacetone phosphate (DHAP)dihydroxyacetone phosphate (DHAP)

Only GOnly G--33--P is oxidized in glycolysis. DHAP is convertedP is oxidized in glycolysis. DHAP is convertedto Gto G--33--P as the latterdiminishes.P as the latterdiminishes.


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ATPATP--YIELDING Third stageYIELDING Third stageStep # 5Step # 5

GlyceraldehydeGlyceraldehyde--33--phosphate is oxidized to 1,3phosphate is oxidized to 1,3--

bisphosphoglycerate; hydrogen of aldehyde isbisphosphoglycerate; hydrogen of aldehyde is

removedby NADremovedby NAD++

Step # 6Step # 6

Phosphate from the carboxyl group is transferredPhosphate from the carboxyl group is transferred

to the ADP yielding ATP and3to the ADP yielding ATP and3--

phosphoglyceratephosphoglycerate

Step # 7Step # 7

Isomerization of3Isomerization of3--phosphoglycerate to 2phosphoglycerate to 2--

phosphoglyceratephosphoglycerate


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Step # 8Step # 8

Dehydration of 2Dehydration of 2--phosphoglycerate tophosphoglycerate tophosphoenolpyruvate (PEP)phosphoenolpyruvate (PEP)

Step # 9Step # 9

Removal of the remaining phosphate to yieldRemoval of the remaining phosphate to yieldATP andpyruvateATP andpyruvate

Step # 10Step # 10Reductive decarboxylation of pyruvate toReductive decarboxylation of pyruvate to

produce ethanol andCOproduce ethanol andCO22


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REACTIONS OF GLYCOLYSISREACTIONS OF GLYCOLYSIS

STEPSTEP REACTIONREACTION ENZYMEENZYME REACTIONREACTION

TYPETYPE

G inG in

kJ/molkJ/mol

11 Glucose + ATPGlucose + ATP

GG--66--P + ADP + HP + ADP + H++

HexokinaseHexokinase PhosphorylPhosphoryl

transfertransfer--33.533.5

22 GG--66--PP FF--66--PP PhosphoglucosePhosphoglucoseisomeraseisomerase

IsomerizationIsomerization --2.52.5

33 FF--66--P + ATPP + ATP

FF--1,61,6--BP+ ADP + HBP+ ADP + H++

PhosphofructoPhosphofructo--

kinasekinase

PhosphorylPhosphoryl

transfertransfer--22.222.2


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STEPSTEP REACTIONREACTION ENZYMEENZYME REACTION TYPEREACTION TYPE G inG in

kJ/kJ/

molmol

44 FF--1,61,6--BPBP DHAP + GAPDHAP + GAP AldolaseAldolase Aldol cleavageAldol cleavage --1.31.3

55 DHAPDHAP GAPGAP TrioseTriose

phosphatephosphate

isomeraseisomerase

IsomerizationIsomerization +2.5+2.5

66 GAP+Pi + NADGAP+Pi + NAD++

1,31,3--BPG + NADH + HBPG + NADH + H++

GlyceraldehydeGlyceraldehyde

--33--PhosphatePhosphateDehydrogenaseDehydrogenase

PhosphorylationPhosphorylation

coupled tocoupled tooxidationoxidation

+2.5+2.5

77 1,31,3--BPG + ADPBPG + ADP

33--phosphoglycerate +ATPphosphoglycerate +ATP

PhosphoglycerPhosphoglycer--

ate kinaseate kinase

PhosphorylPhosphoryl

transfertransfer

+1.3+1.3

88 33--phosphoglyceratephosphoglycerate

22--phosphoglyceratephosphoglycerate

PhosphoglycePhosphoglyce--

rate mutaserate mutase

Phosphoryl shiftPhosphoryl shift +0.8+0.8

99 22--phosphoglyceratephosphoglycerate

PEP + HOHPEP + HOH

EnolaseEnolase DehydrationDehydration --3.33.3

1010 PEP + ADP+ H+PEP + ADP+ H+ pyruvate + ATPpyruvate + ATP Pyruvate kinasePyruvate kinase PhosphorylPhosphoryl

transfertransfer

--16.716.7


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CITRIC ACID CYCLECITRIC ACID CYCLESTEPSTEP REACTIONREACTION ENZYMEENZYME PROSTHETPROSTHET

IC GROUPIC GROUP

REACTIONREACTION

TYPETYPE

GGoo

inin

kJ/kJ/

molmol

11 acetylCoA + oxaloacetateacetylCoA + oxaloacetate ++HOHHOH

citrate + CoA + Hcitrate + CoA + H++CitrateCitrate

synthasesynthase

CondensationCondensation --31.431.4

2a2a CitrateCitrate ciscis--aconitate + HOHaconitate + HOH AconitaseAconitase FeFe--SS DehydrationDehydration +8.4+8.4

2b2b CisCis--Aconitate + HOHAconitate + HOH isocitrateisocitrate AconitaseAconitase FeFe--SS HydrationHydration --2.12.1


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IC GROUPIC GROUP

REACTIONREACTION

TYPETYPE

GGoo

inin

kJ/kJ/

molmol

33 Isocitrate + NAD+Isocitrate + NAD+

--ketoglutarate + COketoglutarate + CO22 ++

NADHNADH

IsocitrateIsocitrate

DehydroDehydro--

genasegenase

DecarboxylatiDecarboxylati

on & oxidationon & oxidation

-- 8.48.4

44 --ketoglutarate + NAD+ CoAketoglutarate + NAD+ CoA

succinyl CoA +succinyl CoA + COCO22 ++

NADHNADH

--

ketoglutaketogluta--

raterate

dehydrodehydro--

genasegenase

complexcomplex

LipoicLipoic

acid, FAD,acid, FAD,

TPPTPP

DecarboxylaDecarboxyla--

tion &tion &

oxidationoxidation

--30.130.1

55 Succinyl CoA +Pi + GDPSuccinyl CoA +Pi + GDP

succinate + GTP + CoAsuccinate + GTP + CoA

SuccinylSuccinyl

CoACoA

synthetsynthet--

asease

SubstrateSubstrate--

levellevel

phosphorylaphosphoryla--

tiontion

--3.33.3


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IC GROUPIC GROUP

REACTIONREACTION

TYPETYPE

GGoo inin

kJ/kJ/

molmol

66 Succinate +FAD (enzymeSuccinate +FAD (enzyme--bound)bound)

fumarate + FADHfumarate + FADH22

(enzyme(enzyme--bound)bound)

SuccinateSuccinate

dehydrodehydro--

genasegenase

FAD, FeFAD, Fe--SS OxidationOxidation 00

77 Fumarate + HOHFumarate + HOH LL--malatemalate FumaraseFumarase HydrationHydration --3.83.8

88 LL--malate + NAD+malate + NAD+oxaloacetate + NADH + Hoxaloacetate + NADH + H++

MalateMalatedehydrodehydro--

genasegenase

OxidationOxidation +29.7+29.7


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REGULATION OF TCA CYCLEREGULATION OF TCA CYCLEPyruvate

Acetyl CoA

Citrate

Isocitrate

-Ketoglutarate

Succinyl

CoA

Succinate

Fumarate

Malate

Oxaloacetate

-ATP, acetyl CoA & NADH

- ATP & NADH

+ ADP

- ATP, succinylCoA & NADH

-KGD

ICD


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BIOSYNTHETIC ROLESBIOSYNTHETIC ROLES

OF TCA CYCLEOF TCA CYCLE

Pyruvate

Acetyl CoA

Citrate

Isocitrate

-Ketoglutarate

Succinyl

CoA

Succinate

Fumarate

Malate

Oxaloacetate

Aspartate

Other amino

acids,

purines &

pyrimidines

Porphyrins,

heme,

chlorophyll

Glutamate

Other

amino

acids &purines

Fatty acids,

sterols


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NOTES TO REMEMBERNOTES TO REMEMBER

The unusual thing about theThe unusual thing about the structure of Nstructure of N--acetylmuramic acidacetylmuramic acid compared to othercompared to othercarbohydrates is thecarbohydrates is the presence of a lactic acidpresence of a lactic acid

side ch

ain.side ch

ain.

Cell walls ofCell walls ofplantsplants areare cellulosiccellulosic (polymer of(polymer ofDD--glucose);glucose); bacterialbacterial cell walls consist mainlycell walls consist mainly

ofofpolysaccharide crosslinked to peptidepolysaccharide crosslinked to peptidethrough murein bridgesthrough murein bridges; and; and fungalfungal cell wallscell wallsareare chitinouschitinous ((polymer of Npolymer of N--acetylacetyl----DD--glucosamineglucosamine))


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Glycogen and starch differ mainly in theGlycogen and starch differ mainly in the degreedegree

of chain branchingof chain branching..

EnantiomersEnantiomers are nonsuperimposable,are nonsuperimposable, mirrormirror--

image stereoisomersimage stereoisomers differing configurationdiffering configuration

on all carbonson all carbons whilewhile diastereomersdiastereomers areare

nonsuperimposablenonsuperimposable nonmirrornonmirror--imageimage

stereoisomers differing only on twostereoisomers differing only on two

carbonscarbons..

Fischer projection of glucoseFischer projection of glucose hashas 4 chiral4 chiral

centerscenters while itswhile its Haworth projectionHaworth projection hashas 55

chiral centerschiral centers..


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Sugar phosphateSugar phosphate is anis an ester bondester bond

formedformed between a sugarhydroxyl andbetween a sugarhydroxyl and

phosphoric acidphosphoric acid..

AAglycosidic bondglycosidic bond is anis an acetalacetal which canwhich can

bebe hydrolyzed to regenerate the twohydrolyzed to regenerate the twooriginal sugarhydroxylsoriginal sugarhydroxyls..

AAreducing sugarreducing sugaris one that has ais one that has a freefreealdehyde groupaldehyde group thatthat can be easilycan be easily

oxidized.oxidized.


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MajorMajorbiochemical roles of glycoproteinsbiochemical roles of glycoproteins

areare signal transductionsignal transduction as hormones,as hormones,

recognition sites for external moleculesrecognition sites for external molecules inineukaryotic cell membranes, andeukaryotic cell membranes, and defensedefense asas

immunoglobulins.immunoglobulins.

LL--sorbitolsorbitol is made byis made by reducing Dreducing D--glucoseglucose..

ArabinoseArabinose is ais a ribose epimerribose epimer, thus,, thus, itsitsderivativesderivatives araara--AA andand araara--CC ififsubstitutedsubstituted

for ribosefor ribose act asact as inhibitors in reactions ofinhibitors in reactions of

ribonucleosidesribonucleosides..


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Two best precursors for glycogenTwo best precursors for glycogen areare

glucoseglucose andand fructosefructose..

CelluloseCellulose because of thebecause of the -- bondingbondingisis linearlinear

as to structure andas to structure and structuralstructural as to role whileas to role while

starchstarchbecause ofbecause of--bondingbondingcoilscoils withwith

energy storageenergy storage role.role.

TheThe highly branched nature of glycogenhighly branched nature of glycogen

gives rise to agives rise to a number of available glucosenumber of available glucose

molecules at a timemolecules at a time upon hydrolysis toupon hydrolysis to

provide energy. Aprovide energy. Alinearlinearone providesone provides oneone

glucose at a timeglucose at a time..


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The enzymeThe enzyme --amylaseamylase is anis an exoglycosidaseexoglycosidasedegrading polysaccharides from the ends. Thedegrading polysaccharides from the ends. Theenzymeenzyme --amylaseamylase is anis an endoglycosidaseendoglycosidasecleaving internal glycosidic bonds.cleaving internal glycosidic bonds.

Dietary fibersDietary fibers bindbind toxic substances in the guttoxic substances in the gutandand decreases the transit timedecreases the transit time, so harmful, so harmfulcompounds such as carcinogens are removed fromcompounds such as carcinogens are removed fromthe body more quickly than would be the case withthe body more quickly than would be the case withlowlow--fiber diet.fiber diet.

TheThe sugar portions of the blood groupsugar portions of the blood groupglycoproteinsglycoproteins are theare the source of the antigenicsource of the antigenicdifference.difference.

CC li kili ki b t d t l l i thb t d t l l i th


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CrossCross--linkinglinking can be expected to play a role in thecan be expected to play a role in thestructures ofstructures ofcellulose and chitincellulose and chitin where mechanicalwhere mechanicalstrength is afforded bystrength is afforded by extensive hydrogen bondingextensive hydrogen bonding..

Converting a sugar to an epimerConverting a sugar to an epimerrequiresrequires inversioninversionof configuration at a chiral centerof configuration at a chiral center. This can only be. This can only bedone bydone by breaking and reforming covalent bondsbreaking and reforming covalent bonds..

Vitamin CVitamin C is ais a lactone (a cyclic ester)lactone (a cyclic ester) with awith a doubledoublebond between two of the ring carbonsbond between two of the ring carbons. The. Thepresence of a double bond makes itpresence of a double bond makes it susceptible tosusceptible toair oxidation.air oxidation.

The sequence ofThe sequence ofmonomers in a polysaccharidemonomers in a polysaccharide isisnot genetically codednot genetically coded and in this sense does notand in this sense does notcontain any informationcontain any information unlike the nucleotideunlike the nucleotidesequencesequence..


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Glycosidic bondsGlycosidic bonds can be formed between the sidecan be formed between the sidechain hydroxyls of serine or threonine residueschain hydroxyls of serine or threonine residues

and th

e sugarh

ydroxylsand th

e sugarh

ydroxyls. In addition, there is a. In addition, there is apossibility of ester bonds forming between thepossibility of ester bonds forming between the sidesidechain carboxyl groups of aspartate or glutamatechain carboxyl groups of aspartate or glutamateand the sugarhydroxylsand the sugarhydroxyls..

InIn glycolysisglycolysis, reactions that, reactions that require ATPrequire ATP are:are:

1.1. phosphorylation of glucosephosphorylation of glucose ((HK,GKHK,GK))

2.2. phosphorylation of fructosephosphorylation of fructose--66--phosphatephosphate ((PFKPFK))

Reactions thatReactions that produce ATPproduce ATP are:are:1.1. transfer of phosphate from 1,3transfer of phosphate from 1,3--

bisphosphoglycerate to ADPbisphosphoglycerate to ADP ((PGKPGK))

2.2. transfer of phosphate from PEP to ADPtransfer of phosphate from PEP to ADP ((PKPK))


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InIn glycolysisglycolysis, reactions that, reactions that require NADHrequire NADH are:are:

1.1. reduction of pyruvate to lactatereduction of pyruvate to lactate ((LDHLDH))

2.2. reduction of acetaldehyde to ethanolreduction of acetaldehyde to ethanol((alcoholdehydrogenasealcoholdehydrogenase))

Reactions thatReactions that require NADrequire NAD are:are:

1.1. oxidation of Goxidation of G--33--P to give 1,3P to give 1,3--DPGDPG ((GG--33--PDPD))

NADHNADH--linked dehydrogenaseslinked dehydrogenases areare LDH, ADH & GLDH, ADH & G--33--PD.PD.

The purpose of theThe purpose of the step th

at produces lactatestep th

at produces lactate is tois toreduce pyruvatereduce pyruvate soso that NADH can be oxidized tothat NADH can be oxidized toNAD+ needed for the step catalyzed byNAD+ needed for the step catalyzed byglyceraldehydeglyceraldehyde--33--phosphatephosphate..


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AldolaseAldolase catalyzes thecatalyzes the reverse aldol condensationreverse aldol condensationof fructoseof fructose--1,61,6--bisphosphate to glyceraldehydebisphosphate to glyceraldehyde--33--phosphate and DHAPphosphate and DHAP..

TheThe energy releasedenergy released by all the reactions ofby all the reactions ofglycolysisglycolysis isis 184.5 kJ mol glucose/mol184.5 kJ mol glucose/mol. The energy. The energyreleased by glycolysisreleased by glycolysis drives the phosphorylation ofdrives the phosphorylation oftwo ADP to ATP for each molecule of glucosetwo ADP to ATP for each molecule of glucose,,trapping 61.0 kJ mol/glucosetrapping 61.0 kJ mol/glucose. The estimate of. The estimate of33%33%efficiencyefficiency comes from the calculation (61.0/184.5) xcomes from the calculation (61.0/184.5) x100 = 33%.100 = 33%.

There is aThere is a net gain of two ATP molecules pernet gain of two ATP molecules perglucoseglucose molecule consumed inmolecule consumed in glycolysisglycolysis. The. Thegross yield of 4 ATPs per glucosegross yield of 4 ATPs per glucose molecule, but themolecule, but thereactions of glycolysis require two ATP per glucose.reactions of glycolysis require two ATP per glucose.


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PyruvatePyruvate can be converted tocan be converted to lactatelactate,, ethanolethanol oror

acetylCoAacetylCoA..

TheThe free energy ofhydrolysis of a substratefree energy ofhydrolysis of a substrate is theis the

energeticenergetic driving force in substratedriving force in substrate--levellevel

phosphorylationphosphorylation. An example is the conversion of. An example is the conversion of

glyceraldehydeglyceraldehyde--33--phosphate to 1,3phosphate to 1,3--bisphosphoglyceratebisphosphoglycerate..

Coupled reactions in glycolysisCoupled reactions in glycolysis are those reactionsare those reactionscatalyzed bycatalyzed by hexokinase, phosphofructokinase,hexokinase, phosphofructokinase,

glyceraldehydeglyceraldehyde--33--phosphate dehydrogenase,phosphate dehydrogenase,

phosphoglycerokinase,phosphoglycerokinase, andandpyruvate kinase.pyruvate kinase.

Iso mesIso mes allo forallo for s btle control of the en mes btle control of the en me toto


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IsozymesIsozymes allow forallow forsubtle control of the enzymesubtle control of the enzyme totorespond to different cellular needs. For example, in therespond to different cellular needs. For example, in theliver,liver, LDH is most often used to convert lactate toLDH is most often used to convert lactate topyruvate, but the reaction is often reversed in thepyruvate, but the reaction is often reversed in the

musclesmuscles. Having a. Having a different isozymedifferent isozyme in the liver andin the liver andthe muscle allows for thosethe muscle allows for those reactions to bereactions to beoptimizedoptimized..

FructoseFructose--1,61,6--bisphosphate can only undergo thebisphosphate can only undergo thereactions of glycolysisreactions of glycolysis. The components of the. The components of thepathway up to this point can have other metabolicpathway up to this point can have other metabolicfates.fates.

TheThe physiologically irreversible glycolytic stepsphysiologically irreversible glycolytic stepsare thoseare those catalyzed by HK, PFK and PKcatalyzed by HK, PFK and PK. Thus, they. Thus, theyareare controlling points in glycolysis.controlling points in glycolysis.


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HexokinaseHexokinase isis inhibited by glucoseinhibited by glucose--66--phosphatephosphate..PhosphofructokinasePhosphofructokinase isis inhibited by ATP and citrateinhibited by ATP and citrate..Pyruvate kinasePyruvate kinase isis inhibited is inhibited by ATP,inhibited is inhibited by ATP,

acetylCoA and alanine.acetylCoA and alanine.

PhosphofructokinasePhosphofructokinase isis stimulated by AMP andstimulated by AMP andfructosefructose--2,62,6--bisphosphatebisphosphate..

Pyruvate kinasePyruvate kinase isis stimulated by AMP and fructosestimulated by AMP and fructose--1,61,6--bisphosphatebisphosphate..

AnAn isomeraseisomerase is a general term for an enzyme thatis a general term for an enzyme that

ch

anges th

e form of a substrate with

out ch

angingch

anges th

e form of a substrate with

out ch

angingits empirical formulaits empirical formula..

AAmutasemutase is an enzyme thatis an enzyme that moves a functionalmoves a functionalgroup such as a phosphate to a new location in agroup such as a phosphate to a new location in a

substrate moleculesubstrate molecule..

TheThe glucokinase has a higher Km for glucose thanglucokinase has a higher Km for glucose than


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TheThe glucokinase has a higher Km for glucose thanglucokinase has a higher Km for glucose thanhexokinasehexokinase. Thus, under conditions of. Thus, under conditions oflow glucoselow glucose,,thethe liver will not convert glucose to glucoseliver will not convert glucose to glucose--66--phosphatephosphate, using a substrate that is needed, using a substrate that is needed

elsewhere. When theelsewhere. When the glucose concentration becomesglucose concentration becomeshigherhigher, however,, however, glucokinase will function to helpglucokinase will function to helpphosphorylate glucose so that it can be stored asphosphorylate glucose so that it can be stored asglycogenglycogen..

TheThe net yield of ATPnet yield of ATP from glycolysis is the same,from glycolysis is the same, 22ATPATP, when, when either fructose, mannose, andeither fructose, mannose, andgalactose is usedgalactose is used. The energetics of the conversion. The energetics of the conversionof hexoses to pyruvate are the same regardless ofof hexoses to pyruvate are the same regardless of

hexose type.hexose type.

TheThe net yield of ATPnet yield of ATP is 3 fromis 3 from glucose derived fromglucose derived fromglycogenglycogen because the starting material is glucosebecause the starting material is glucose--11--phosphate. One of the priming reactions is no longerphosphate. One of the priming reactions is no longerused.used.


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AAreaction with a negativereaction with a negative GGoo isisthermodynamically possiblethermodynamically possible under standardunder standardconditions.conditions.

Individuals whoIndividuals who lack the gene that directs thelack the gene that directs thesynthesis of the M form of the enzyme PFKsynthesis of the M form of the enzyme PFK cancancarry on glycolysis in their livers but suffer musclecarry on glycolysis in their livers but suffer muscleweaknessweakness because they lack the enzyme in muscle.because they lack the enzyme in muscle.

The reaction ofThe reaction of22--PG to PEPPG to PEP is ais a dehydrationdehydration (loss of(loss ofwater) rather than a redox reaction.water) rather than a redox reaction.

TheThe hexokinasehexokinase moleculemolecule changes shapechanges shapedrastically on binding to substratedrastically on binding to substrate, consistent with, consistent withthethe induced fit theoryinduced fit theory of an enzyme adapting itself toof an enzyme adapting itself toits substrate.its substrate.


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ATPATP is anis an inhibitor of several steps of glycolysisinhibitor of several steps of glycolysis asaswell as other catabolic pathways. The purpose ofwell as other catabolic pathways. The purpose ofcatabolic pathways is to produce energycatabolic pathways is to produce energy, and high, and high

levels ofATP mean the cell already has sufficientlevels ofATP mean the cell already has sufficientenergy.energy. GG--66--P inhibits HKP inhibits HK and is an example ofand is an example ofproduct inhibitionproduct inhibition. If G. If G--66--P level is high, it mayP level is high, it mayindicate that sufficient glucose is available fromindicate that sufficient glucose is available fromglycogen breakdown or that the subsequent enzymaticglycogen breakdown or that the subsequent enzymatic

steps of glycolysis are going slowly. Either way theresteps of glycolysis are going slowly. Either way thereis no reason to produce more Gis no reason to produce more G--66--P.P.

PhosphofructokinasePhosphofructokinase isis inhibited by a specialinhibited by a special

effector molecule, fructoseeffector molecule, fructose--2,62,6

--bisp

h

osph

atebisph

osph

ate,,whose levels are controlled by hormones. It is alsowhose levels are controlled by hormones. It is alsoinhibited by citrateinhibited by citrate, which indicates that there is, which indicates that there issufficient energy from the TCA cycle probably from fatsufficient energy from the TCA cycle probably from fator amino acid catabolism.or amino acid catabolism.


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PKPK is alsois also inhibited byinhibited by acetylCoAacetylCoA, the, the presence ofpresence ofwhich indicates that fatty acids are being used towhich indicates that fatty acids are being used togenerate energygenerate energy for the citric acid cycle.for the citric acid cycle.

TheThe main function of glycolysismain function of glycolysis is tois to feed carbonfeed carbonunits to the TCA cycleunits to the TCA cycle. When these carbon. When these carbonskeletons can come from other sources, glycolysis isskeletons can come from other sources, glycolysis isinhibited to spare glucose for other purposes.inhibited to spare glucose for other purposes.

Thiamine pyrophosphate (TPP)Thiamine pyrophosphate (TPP) is a coenzyme in theis a coenzyme in thetransfer of 2transfer of 2--carbon unitscarbon units. It is required for. It is required forcatalysis bycatalysis bypyruvate decarboxylase in alcoholic fermentationpyruvate decarboxylase in alcoholic fermentation. The. The

important part of TPP is the fiveimportant part of TPP is the five--membered ring wheremembered ring wherea C is found between an S and Na C is found between an S and N. This. This carbon forms acarbon forms acarbanion and is extremely reactive, making it able tocarbanion and is extremely reactive, making it able toperform nucleophilic attack on carbonyl groupsperform nucleophilic attack on carbonyl groupsleading to decarboxylation of several compounds inleading to decarboxylation of several compounds in

different pathways.different pathways.


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TPPTPP is a coenzymeis a coenzyme required in the reactionrequired in the reactioncatalyzed by pyruvate carboxylasecatalyzed by pyruvate carboxylase. Because this. Because thisreaction is a part of thereaction is a part of the metabolism of ethanolmetabolism of ethanol, less, less

will be available to serve as a coenzyme in thewill be available to serve as a coenzyme in thereactions of other enzymes that require it.reactions of other enzymes that require it.

Animals that have been run to deathAnimals that have been run to death havehaveaccumulated large amounts of lactic acidaccumulated large amounts of lactic acid in theirin theirmuscle tissue, accounting for themuscle tissue, accounting for the sour taste of theirsour taste of theirmeat.meat.

Conversion of glucose to lactate rather than pyruvateConversion of glucose to lactate rather than pyruvate

recycles NADHrecycles NADH..

TheThe formation of fructoseformation of fructose--1,61,6--bisphosphatebisphosphate is theis thecommitted step in glycolysiscommitted step in glycolysis. It is also one of the. It is also one of the

energyenergy--requiring steps of the said pathway.requiring steps of the said pathway.


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AApositivepositive GGoo does not necessarily meandoes not necessarily mean

that thethat the reaction has a positivereaction has a positive GG..

Substrate concentrationsSubstrate concentrations cancan make amake anegativenegative G out of a positiveG out of a positive GGoo..

TheThe entire pathwayentire pathway can be looked atcan be looked at as aas alarge coupled reactionlarge coupled reaction. Thus,. Thus, if the overallif the overall

pathway has a negativepathway has a negative G, an individual stepG, an individual step

may be able to have a positivemay be able to have a positive G andtheG andthepathway can still continue.pathway can still continue.

InIn glycogen storageglycogen storage the reactions thatthe reactions that require ATPrequire ATP are:are:


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InIn glycogen storageglycogen storage, the reactions that, the reactions that require ATPrequire ATP are:are:

1.1. formation of UDPformation of UDP--glucose from glucoseglucose from glucose--11--phosphatephosphateand UTP (indirect requirement since ATP is neededand UTP (indirect requirement since ATP is neededto regenerate UTP)to regenerate UTP) (UDP(UDP--glucose phosphorylase)glucose phosphorylase)

2.2. regeneration of UTPregeneration of UTP (nucleoside phosphate kinase)(nucleoside phosphate kinase)

3.3. carboxylation of pyruvate to oxaloacetatecarboxylation of pyruvate to oxaloacetate (pyruvate(pyruvatecarboxylase)carboxylase)

Reactions thatReactions that produce ATPproduce ATP areare NONENONE..

Three differencesThree differences between NADPHbetween NADPH andand NADHNADH

1.1. phosphate at 2 position of ribosephosphate at 2 position of ribose in NADPHin NADPH

2. NADH is2. NADH is produced in oxidative reactions that yield ATPproduced in oxidative reactions that yield ATPwhile NADPH is awhile NADPH is a reducing agentreducing agent in biosynthesis.in biosynthesis.

3.3. Different enzymes use NADH as a coenzymeDifferent enzymes use NADH as a coenzyme comparedcomparedto those that require NADPH.to those that require NADPH.


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InIn glycogen storageglycogen storage, there is, there is no reactionno reaction

that requires acetylCoAthat requires acetylCoA butbut biotin isbiotin is

required in the carboxylation of pyruvaterequired in the carboxylation of pyruvateto oxaloacetate.to oxaloacetate.

TheThe four fates of glucosefour fates of glucose--66--phosphatephosphate are:are:

Converted to glucose (gluconeogenesis)Converted to glucose (gluconeogenesis)

Converted to glycogen (glycogenesis)Converted to glycogen (glycogenesis)

Converted to pentose phosphatesConverted to pentose phosphates Hydrolyzed to pyruvate (glycolysis)Hydrolyzed to pyruvate (glycolysis)


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InIn making equal amounts of NADPH and pentosemaking equal amounts of NADPH and pentosephosphatesphosphates, it, it only involves oxidative reactionsonly involves oxidative reactions. In. In makingmakingmostly or purely NADPHmostly or purely NADPH, the, the use of oxidative reactions,use of oxidative reactions,

transketolase and transaldolase reactions, andtransketolase and transaldolase reactions, andgluconeogenesisgluconeogenesis are required. Inare required. In making mostly or onlymaking mostly or onlypentose phosphatespentose phosphates, needed reactions are, needed reactions are transketolase,transketolase,transaldolase, andglycolysistransaldolase, andglycolysis in reversein reverse..

Transketolase catalyzes the transfer of 2Transketolase catalyzes the transfer of 2--carbon unitcarbon unit, whereas, whereastransaldolase catalyzes the transfer of a 3transaldolase catalyzes the transfer of a 3--carbon unitcarbon unit..

It is essential that theIt is essential that the mechanisms that activate glycogenmechanisms that activate glycogensynthesis also deactivate glycogen phosphorylasesynthesis also deactivate glycogen phosphorylase becausebecausethey both occur in the same cell compartment.they both occur in the same cell compartment. If both are on atIf both are on atthe same time, a futile ATPhydrolysis resultsthe same time, a futile ATPhydrolysis results. On/off. On/offmechanism is highly efficient in its control.mechanism is highly efficient in its control.

UDPGUDPG i l bi th ii l bi th i t f l tt f l t


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UDPGUDPG, in glycogen biosynthesis,, in glycogen biosynthesis, transfers glucose totransfers glucose tothe growing glycogen moleculethe growing glycogen molecule..

Glycogen synthaseGlycogen synthase isis subject to covalentsubject to covalentmodification and to allosteric controlmodification and to allosteric control. The enzyme is. The enzyme isactive in its phosphorylated form and inactive whenactive in its phosphorylated form and inactive whendephosphorylateddephosphorylated..

AMPAMP is anis an allosteric inhibitor of glycogen synthaseallosteric inhibitor of glycogen synthase,,whereaswhereas ATP and glucoseATP and glucose--66--phosphatephosphate areareallosteric activatorsallosteric activators..

InIn gluconeogenesisgluconeogenesis,, biotin is the molecule to whichbiotin is the molecule to whichcarbon dioxide is attached to the process of beingcarbon dioxide is attached to the process of beingtransferred to pyruvatetransferred to pyruvate. The reaction. The reaction producesproducesoxaloacetateoxaloacetate, which then undergoes further reactions, which then undergoes further reactionsof gluconeogenesis. Biotin isof gluconeogenesis. Biotin is not used innot used inglycogenesis and PPP.glycogenesis and PPP.

In gluconeogenesis glucoseIn gluconeogenesis glucose 66 phosphate isphosphate is


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In gluconeogenesis, glucoseIn gluconeogenesis, glucose--66--phosphate isphosphate is

dephosphorylated to glucose (last step); in glycolysis,dephosphorylated to glucose (last step); in glycolysis,

GG--66--P isomerizes to fructoseP isomerizes to fructose--66--phosphate (early step).phosphate (early step).

TheThe Cori cycleCori cycle is a pathway in which there isis a pathway in which there is cycling ocycling o

glucose due to glycolysis in muscle andglucose due to glycolysis in muscle and

gluconeogenesis in livergluconeogenesis in liver. The. The blood transports lactateblood transports lactate

from muscle to liver and glucose from liver to musclefrom muscle to liver and glucose from liver to muscle..

There is aThere is a net gain of 3net gain of 3, rather than 2,, rather than 2, ATP whenATP when

glycogenglycogen, not glucose,, not glucose, is the starting material ofis the starting material ofglycolysisglycolysis..


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Control mechanisms are important in metabolism.Control mechanisms are important in metabolism.They are:They are: Allosteric control (takes place in msec)Allosteric control (takes place in msec)

Covalent control (takes place from s to min)Covalent control (takes place from s to min)

Genetic control ( longer time scale)Genetic control ( longer time scale)

Enzymes, like all catalysts, speed up the forward andEnzymes, like all catalysts, speed up the forward andreverse reaction to the same extent. Having differentreverse reaction to the same extent. Having differentcatalysts is the only way to ensure independentcatalysts is the only way to ensure independentcontrol over the rates of the forward and the reversecontrol over the rates of the forward and the reverseprocess.process.

The glycogen synthase is an exergonic reactionThe glycogen synthase is an exergonic reactionoverall because it is coupled to phosphate esteroverall because it is coupled to phosphate esterhydrolysis.hydrolysis.

Increasing the level of ATP is favorable to bothIncreasing the level of ATP is favorable to both


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Increasing the level ofATP is favorable to bothIncreasing the level ofATP is favorable to bothgluconeogenesis and glycogen synthesis.gluconeogenesis and glycogen synthesis.

Decreasing the level of fructoseDecreasing the level of fructose--1,61,6--bisphosphatebisphosphatewould tend to stimulate glycolysis, rather thanwould tend to stimulate glycolysis, rather thangluconeogenesis and glycogen synthesis.gluconeogenesis and glycogen synthesis.

If a cell needs NADPH, all the reactions of the PPPIf a cell needs NADPH, all the reactions of the PPP

take place. If a cell needs ribosetake place. If a cell needs ribose--55--phosphate, thephosphate, theoxidative portion of the pathway can be bypassed andoxidative portion of the pathway can be bypassed andonly the nonoxidative reshuffling reactions take place.only the nonoxidative reshuffling reactions take place.The PPP does not have a significant effect on the ATPThe PPP does not have a significant effect on the ATPsupply of a cell.supply of a cell.

GlucoseGlucose--66--phosphate is expectedly oxidized to aphosphate is expectedly oxidized to alactone rather than an openlactone rather than an open--chain ester because thechain ester because thelatter is easy to hydrolyze.latter is easy to hydrolyze.


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In the PPP resshuffling reactions, without anIn the PPP resshuffling reactions, without an

isomerase, all the sugars involved are ketoisomerase, all the sugars involved are keto

sugars that are not substrates forsugars that are not substrates fortransaldolase.transaldolase.

Sugar nucleotides (UDPG) have twoSugar nucleotides (UDPG) have twophosphates which when hydrolyzed drivesphosphates which when hydrolyzed drives

towards the polymerization of glycogen. Thus,towards the polymerization of glycogen. Thus,

they are fit for glycogenesis.they are fit for glycogenesis.


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Review on Carbohydrates

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