Unit 2 carbohydrate metabolism

Biochemistry

1. Carbohydrate
Metabolism 1
Mrs. Kulkarni DipaliM.
Assistant Professor,
Yash Institute of Pharmacy,
Aurangabad.

2. Glycolysis –Pathway, energetics, significance
Citric acid cycle-Pathway, energetics, significance
HMP shunt and its significance;
Glucose-6-Phosphate dehydrogenase(G6PD) deficiency
Contents

3. METABOLIC
PATHWAYS
CATABOLICPATHWAYS
Are involved in oxidative
breakdown of larger
complexes.
Theyareusually
exergonicinnature
ANABOLICPATHWAYS
Areinvolvedinthe
synthesis of
compounds.
Theyareusually
endergonicinnature.

4. CHARACTERISTICSOFMETABOLISM
1.Metabolicpathwaysare mostly
irreversible
2.Everymetabolicpathwayhasa
committed firststep.
3.All metabolicpathwaysare regulated.
4.Metabolicpathwaysin eukaryoticcells
occur inspecificcellular locations.

5. GLYCOLYSIS
Glycolysiscomesfroma mergeroftwoGreekwords:
Glykys=sweet
Lysis=breakdown/splitting
ItisalsoknownasEmbden-Meyerhof-Parnaspathway
orEMPpathway.

6. INTRODUCTION
•GLYCOLYSISisthe sequence of10 enzyme-catalyzed
reactionsthatconvertsglucose intopyruvatewith
simultaneousproductionon ofATP.
•Inthisoxidativeprocess,1mol ofglucoseispartially
oxidisedto2 molesof pyruvate.
•Thismajor pathwayof glucose metabolismoccursin
thecytosolofallcell.
•Thisuniquepathwayoccursaerobicallyaswellas
anaerobically&doesn’t involvemolecularoxygen.

7. •It also includesformationofLactatefromPyruvate.
•Theglycolyticsequenceofreactionsdifferfrom
speciestospeciesonlyin the mechanismofits
regulation&inthesubsequentmetabolicfateof
thepyruvateformed.
•Inaerobic organisms,glycolysisisthepreludeto
Citricacid cycleand ETC.
•Glycolysisis the centralpathwayforGlucose
catabolism.

8. Glucose
Extracellular
matrix&cellwall
polysachharide.
Glycogen,
Starch,
Sucrose
Pyruvate
Ribose-5-
phosphat
e
Oxidationvia
pentose phosphate
pathway
Synthesis of
structuralpolymers
storage
Oxidation
viaglycolysis
Majorpathwaysof
glucoseutilization.

10. TWOPHASES OFGLYCOLYSIS
•Glycolysisleadstobreakdownof6-Cglucose
intotwomoleculesof3-Cpyruvatewiththe
enzymecatalyzedreactionsbeingbifurcated
orcategorizedinto2phases:
1.Phase1-preparatoryphase
2.Phase2-payoffphase.

11. PREPARATORYPHASE
•It consists of the 1
st5 steps of glycolysis in which
theglucoseisenzymaticallyphosphorylatedbyATP
toyieldFructose-1,6-biphosphate.
•Thisfructuse-1,6-biphosphateisthen splitinhalfto
yield 2 molecules of 3-carbon containing
Glyceraldehyde-3-phosphate/dihyroxyacteone
phosphate.

12. •Thusthefirstphaseresultsincleavageof the
hexosechain.
•Thiscleavagerequiresan investmentof2ATP
moleculestoactivatethe glucosemoleandprepare
itforits cleavage into3-carboncompound.

14. PAYOFFPHASE
•Thisphase constitutesthelast5 reactionsof
Glycolysis.
•This phasemarksthereleaseof ATPmolecules
during conversion of Glyceraldehyde-3-phosphtae
to2molesofPyruvate.
•Here 4 molesof ADPare phosphorylatedtoATP.
Although4moles ofATPareformed,thenetresult
isonly 2 molesofATPpermole ofGlucose oxidized,
since2 molesofATPareutilizedin Phase1.

16. STEP1:PHOSPHORYLATION
•GlucoseisphosphorylatedbyATPtoformsugar
phosphate.
•Thisisanirreversible reaction&is catalyzedby
hexokinase.
•Thusthereaction canberepresentedas follows:
Glucose
Glucose-6-phosphate
Hexokinase
ATP
ADP

17. STEP2:ISOMERIZATION
•It isareversiblerearrangementofchemicalstructureof
carbonyloxygenfromC1toC2,formingaKetosefromthe
Aldose.
•Thus,isomerizationof thealdose Glucose6-phosphate
givestheketose,Fructose-6-phoshphate.
Glucose-6-phosphate
Phosphoglucoisomerase
Fructose-6-phosphate

18. STEP3:PHOPHORYLATION
•Here the Fructose-6-phosphate is phosphorylated
byATPtofructose-1,6-bisphosphate.
•Thisisanirreversiblereaction andiscatalyzedby
phosphofructokinaseenzyme.
Fructose-6-phosphate
Fructose-1,6-bisphosphate
ATP
ADP
Phosphofructokinase

19. STEP4:BREAKDOWN
•Thissixcarbonsugariscleavedtoproducetwo3-C
molecules:glyceradldehyde-3-phosphate(GAP)&
dihydroxyacetonephosphate(DHAP).
•ThisreactioniscatalyzedbyAldolase.
Glyceraldehyde-3-
phosphate
Dihydroxyacetone
phosphateTriose phosphate
isomerase
Fructose-1,6-
bisphosphate
Aldolase

20. STEP5:ISOMERIZATION
•Dihydroxyacetonephosphateis oxidizedtoform
Glyceraldehyde-3-phosphate.
•This reactioniscatalyzedby triosephosphate
isomeraseenzyme.
Glyceraldehyde-3-phosphate
Dihydroxyacetonephosphate
Triosephosphate
isomerase
2
2

21. STEP6
•2moleculesofGlyceraldehyde-3-phosphateare
oxidized.
•Glyceraldehyde-3-phosphatedehydrogenase
catalyzestheconversionofGlyceraldehyde3-
phosphateinto1,3-bisphosphoglycerate.
Aldehyde Carboxylicacid
Carboxylic
acid
Ortho-
phosphate
Acyl-
phosphate
product
Joining)

22. Resultantreaction
Glyceraldehyde-3-phosphate
1,3-bisphosphoglycerate
Glyceraldehyde-3-phosphate
dehydrogenase
2NAD⁺+2Pi
2NADH+2H⁺
2
2

23. STEP7
•The transferofhigh-energyphosphategroupthat
wasgenerated earlier toADP,formATP.
•Thisphosphorylationi.e.additionof phosphateto
ADPtogiveATPis termedassubstratelevel
phosphorylation as the phosphate donor is the
substrate1,3-bisphosphoglycerate(1,3-BPG).
•Theproduct ofthisreactionis2molecules of
3-phosphoglycerate.

24. 1,3-
bisphosphoglycerate
3-phosphoglycerate
Phosphoglycerate
kinase
2ADP
2ATP
FIRSTSUBSTRATE LEVEL
PHOSPHORYLATION
2
2

25. STEP8
•The remaining phosphate-esterlinkagein 3-
phosphoglycerate,ismovedfromcarbon3to
carbon 2 ,because of relatively low free energy of
hydrolysis,toform2-phosphoglycerate(2-PG).
3-phosphoglycerate
2-phosphoglycerate
Phosphoglycerate
mutase
2
2

26. STEP 9:DEHYDRATION OF2-PG
•Thisisthesecondreactioninglycolysis where a
high-energyphosphatecompoundisformed.
•The2-phosphoglycerateisdehydratedbytheaction
of enolase to phosphoenolpyruvate(PEP). This
compoundisthephosphateesteroftheenol
tautomerofpyruvate.
•Thisisareversiblereaction.

27. 2-phosphoglycerate
Phosphoenolpyruvate
H₂O
Enolase
2
2

28. STEP10:TRANSFEROFPHOSPHATE
FROMPEP toADP
•Thislaststepis theirreversibletransferofhigh
energy phosphorylgroupfrom
phosphoenolpuruvatetoADP.
•Thisreaction iscatalyzedbypyruvatekinase.
•This isthe2
ndsubstratelevelphosphorylation
reactioninglycolysiswhichyields ATP.
•Thisisa non-oxidativephosphorylationreaction.

29. Phosphoenolpyruvate
Pyruvate
2
2
Pyruvatekinase
SECOND
SUBSTRATE LEVEL
PHOSPHORYLATION
2ADP
2ATP

30. OVERALL BALANCE SHEET OF
GLYCOLYSIS
•Each molecule of glucose gives 2 molecules of
Glyceraldehyde-3-phosphate.Therefore,thetotal
inputof all 10 reactionscanbe summarizedas:
Glucose + 2ATP+2Pi+2NAD⁺+ 2H⁺+4ADP
2Pyruvate+2H⁺+4ATP+2H₂O+ 2NADH+2ADP
On cancelling the common terms fromthe above
equation,we getthe netequationforGlycolysis:

31. Glucose+2Pi+2ADP+ 2NAD⁺
2Pyruvate+2NADH+2ATP+2H⁺+ 2H₂O
THUSTHESIMULTANEOUSREACTIONSINVOLVED IN
GLYCOLYSISARE:
Glucoseisoxidizedto Pyruvate
NAD⁺isreducedtoNADH
ADPisphosphorylatedtoATP

32. ENERGYYIELDINGLYCOLYSIS:
STEPNO.REACTION CONSUMPTIONofATPGAINofATP
1 1 -
3
Glucose glucose-6-phosphate
Fructose-6-phosphate
fructose-1,6-biphosphate
1 -
7 - 1x2=2
10
1,3-diphosphoglycerate
3-phosphoglycerate
Phosphoenolpyruvate pyruvate - 1x2=2
2 4
Netgain ofATP=4-2=2

39. TCACycle
⚫AlsoknownasKrebscycle
⚫TCAcycle essentiallyinvolvestheoxidationof
acetyl CoAtoCO
2andH
2O.
⚫TCAcycle–thecentral metabolicpathway
⚫The TCA cycle is the final common oxidative
pathwayforcarbohydrates,fats,aminoacids.

40. ⚫TCAcyclesuppliesenergy&alsoprovidesmany
intermediatesrequiredforthesynthesisofamino
acids,glucose,hemeetc.
⚫TCAcycle is themostimportantcentralpathway
connectingalmostalltheindividualmetabolic
pathways.

41. ⚫Definition
⚫Citric acid cycleorTCAcycleortricarboxylicacid
cycle essentially involves the oxidation of acetyl
CoAtoCO
2 &H
2O.
⚫Location oftheTCAcycle
⚫Reactions ofoccurinmitochondrialmatrix,in
closeproximitytotheETC.

43. Pyruvate
Cis-Aconitase
Succinate
Oxaloacatete
PDH
CO
2,NADH+H
+
AcetylCoA
NAD
+
ɑ-Ketoglutarate
CO
2,NADH+H
+
NAD
+
Iso-citrate
NAD
+
NADH+H
+
OxalosuccinateGTP
GDP+Pi SuccinylCoA
Fumarate
FAD
FADH
2
NAD
+
NADH+H
+
Malate
-H
2O
Citrate
synthase
Citrate
Aconitase
Aconitase
SDH
Fumarase
TCA

44. ReactionsofTCAcycle
⚫Oxidativedecarboxylationofpyruvatetoacetyl
CoAbyPDHcomplex.
⚫Thisstepisconnectinglinkbetweenglycolysisand
TCAcycle.

45. ReactionsofTCACycle
⚫Step:1Formation ofcitrate
⚫Oxaloacetate condenses with acetyl CoA to form
Citrate,catalysedbytheenzymecitratesynthase
⚫Inhibitedby:
⚫ATP,NADH,Citrate-competitiveinhibitorof
oxaloacetate.

46. Steps2 &3
Citrateis isomerizedtoisocitrate
⚫Citrateis isomerizedtoisocitratebytheenzyme
aconitase
⚫Thisisachievedin atwostagereaction of
dehydrationfollowedbyhydrationthroughthe
formationof anintermediate-cis-aconiase

47. Steps4&5
Formationof-ketoglutarate
⚫Isocitrate dehydrogenase (ICDH) catalyses the
conversionof(oxidativedecarboxylation)ofisocitrate
tooxalosuccinate&thento-ketoglutarate.
⚫TheformationofNADH&theliberationofCO
2
occureatthisstage.
⚫Stimulated(cooperative)byisocitrate,NAD
+
,Mg
2+
,
ADP,Ca
2+
(linkswithcontraction).
⚫InhibitedbyNADH&ATP

48. Step:6Conversionof-ketoglutarate
to succinylCoA
⚫Occurs through oxidative decarboxylation,
catalysed by -ketoglutarate dehydrogenase
complex.
⚫-ketoglutaratedehydrogenaseisanmultienzyme
complex.
⚫AtthisstageofTCAcycle,secondNADHis
produced&the secondCO
2isliberated.

49. Step:7
Formationofsuccinate
⚫Succinyl CoAis convertedtosuccinateby
succinate thiokinase.
⚫Thisreactioniscoupledwiththephosphorylation
ofGDPtoGTP.
⚫This isasubstrate level phosphorylation.
⚫GTPis convertedtoATPbytheenzymenucleoside
diphosphatekinase.

50. Step:8
Conversionofsuccinatetofumarate
⚫Succinate isoxidizedbysuccinatedehydrogenase
tofumarate.
⚫This reactionresults intheproductionofFADH
2.
⚫Step:9 Formationofmalate:Theenzyme
fumarasecatalyses theconversionoffumarateto
malate withthe additionof H
2O.

51. Step:10
Conversionofmalateto
oxaloacetate
⚫Malateisthenoxidizedtooxaloacetatebymalate
dehydrogenase.
⚫Thethird&finalsynthesisofNADHoccursatthis
stage.
⚫Theoxaloacetateisregeneratedwhichcan
combine withanothermoleculeof acetylCoA&
continuethe cycle.

52. Regenerationof
oxaloacetate
⚫TheTCAcyclebasicallyinvolvesthe oxidationof
acetyl CoA to CO
2with the simultaneous
regenerationofoxaloacetate.
⚫Thereisnonetconsumptionofoxaloacetateorany
otherintermediatein the cycle.

53. SignificanceofTCA
cycle
⚫CompleteoxidationofacetylCoA.
⚫ATPgeneration.
⚫Finalcommonoxidativepathway.
⚫Integrationofmajormetabolicpathways.
⚫Fatisburned onthewickofcarbohydrates.
⚫Excess carbohydratesare convertedasneutralfat
⚫No netsynthesis of carbohydratesfromfat.
⚫Carbonskeletonof aminoacidsfinallyentertheTCAcycle.

54. RequirementofO
2byTCA
cycle
⚫ThereisnodirectparticipationofO
2inTCAcycle.
⚫Operatesonlyunderaerobicconditions.
⚫This is due to, NAD
+
& FAD required for the
operationof the cyclecanbe regeneratedinthe
respiratory chainonlyinpresenceofO
2.
⚫Therefore,citric acidcycleisstrictlyaerobic.

55. EnergeticsofTCA
Cycle
⚫Oxidationof3NADHby ETCcoupled with
oxidative phosphorylation results in the synthesis of
9ATP.
⚫FADH
2leadstotheformationof2ATP.
⚫Onesubstratelevelphosphorylation.
⚫Thus,a totalof12ATPareproducedfromone
acetylCoA.

56. Regulation ofTCACycle
⚫Threeregulatoryenzymes
1.Citratesynthase
2.Isocitratedehydrogenase
3.α-ketoglutaratedehydrogenase

57. •HMP pathway or HMP shunt is also called as
pentosephosphate pathwayorphosphogluconate
pathway.
•This is an alternative pathway to glycolysis and
TCAcyclefortheoxidationofglucose.
•HMPshuntismoreanabolicinnature.

58. •ItisconcernedwiththebiosynthesisofNADPH&
pentoses.
•About10% of glucose enteringin this
pathway/day.
•Theliver&RBCmetaboliseabout30% ofglucose
bythispathway.

59. Locationofthepathway
•Theenzymesarelocatedinthecytosol.
•The tissues such as liver, adipose tissue, adrenal
gland, erythrocytes, testes & lactating mammary
gland,arehighlyactiveinHMPshunt.
•Mostofthesetissuesareinvolvedinbiosynthesisof
fattyacidsandsteroidswhicharedependentonthe
supplyofNADPH.

60. HMPshunt-uniquemultifunctional
pathway
•It startswithglucose6-phosphate.
•NoATPis directlyutilizedorproducedin HMP
shunt
•It is multifunctionalpathway,several
interconvertiblesubstancesproduced,whichare
proceedindifferentdirectionsinthemetabolic
reactions

61. Reactionsofthepathway
•Reactionsofthepathway:
•DividedintoTwophasesoxidative&non–oxidative.
•Oxidativephase
•Step:1
•Glucose6-phosphateisoxidisedbyNADP-dependent
Glucose6-phosphatedehydrogenase(G6PD),6-
phosphogluconolactoneis formed.
•NADPHisformedin thisreactionandthisisaratelimiting
step.

62. •Step:2
•6-phosphogluconolactoneishydrolysedbygluconolactone
hydrolaseto form 6-phosphogluconate.
•Step:3
•The next reaction involving the synthesis of NADPH and is
catalysedby 6 –phosphogluconatedehydrogenaseto
produce3keto6–phosphogluconatewhichthenundergoes
decarboxylationtogiveribulose5–phosphate.

63. Non-OxidativePhase
•Step:4
•The ribulose-5-phosphateisthenisomerizedto
ribose-5-phosphateorepimerisedtoxylulose-5-
phosphate
•Step:5Transketolasereaction
•Transketolaseis athiaminepyrophosphate(TPP)
dependentenzyme.

64. •Ittransferstwo-carbonunitfromxylulose5-
phosphatetoribose5-phosphatetoforma7-
carbonsugar,sedoheptulose7-phosphateand
glyceraldehyde3–phosphate.

65. •Step:6Transaldolasereaction
•Transaldolasebringsabout thetransferofa3–
carbonfragmentfromsedoheptulose7-phosphate
toglyceraldehyde3-phosphatetogivefructose6-
phosphate&4–carbonerythrose4–phosphate.

66. •Step:7 SecondtransketolaseReaction
•Inanothertransketolasereactiona2–carbonunit
istransferredfromxylulose5–phosphateto
erythrose4–phosphatetoformfructose6–
phosphate& glyceraldehyde3–phosphate.
•Fructose6–phosphate&glyceraldehyde 3 –
phosphate are further metabolized by glycolysis &
TCAcycle.

67. HMP-Shuntpathway
Glucose6-phosphate
NADP+
Glucose6P-
dehydrogenase
Mg
+2
NADPH+H
+
Ribulose5-phosphate
6-phosphoglucanolactone
Glucanolactone
hydrolase
6-phosphogluconate
NADP+
CO
2,NADPH+H
+
Phosphogluconate
dehydrogenase
Mg
+2

68. Ribulose5-phosphate
Xylulose5-phosphate
Sedoheptolose7-
phosphate
Erythrose 4-
Phosphate
•Ribose5-
phosphate
•Transketola
se, TPP
Glyceralde
hyde3-
•phosphat
e
Xylulose5-phosphate
Fructose6-
Phosphate
Glyceraldehyde3-
phosphate
Fructose6-
Phosphate
Transketolase,TPP

69. SignificanceofHMPShunt
•HMPshuntisuniqueingenerating twoimportantproducts-
pentosesandNADPH
•Importanceofpentoses:
InHMPshunt,hexoses are convertedinto pentoses, the
mostimportantbeingribose5–phosphate.
•Thispentose oritsderivativesareusefulforthesynthesisof
nucleicacids(DNA& RNA)
•ManynucleotidessuchasATP,NAD
+,FAD&CoA

70. ImportanceofNADPH
•NADPH is required for the bio synthesis of fatty
acidsandsteroids.
•NADPHis used in the synthesis of certain amino
acidsinvolvingthe enzymeglutamate
dehydrogenase.
•FreeradicalScavenging
•The free radicals (super oxide, hydrogen peroxide)
arecontinuouslyproducedinallcells.

71. •These willdestroyDNA,proteins,fattyacids &all
biomolecules& inturncellsaredestroyed.
•The free radicals are inactivated by the enzyme
systemscontainingSOD,POD&glutathione
reductase.
•ReducedGSH isregeneratedwiththe helpof
NADH.

72. •ErythrocyteMembraneintigrity
•NADPHisrequiredbytheRBCtokeepthe
glutathioneinthereducedstate.
•In turn,reducedglutathionewilldetoxifythe
peroxides&freeradicalsformedwithintheRBC.
•NADPH,glutathione&glutathionereductase
togetherwillpreservetheintigrityofRBC
membrane.

73. •PreventionofMet-Hemoglobinemia
•NADPHisalsorequiredtokeeptheironof
hemoglobininthereduced(ferrous)state&to
preventtheaccumulationofmet-hemoglobin.
•Met-hemoglobincannotcarrytheoxygen.

74. •DetoxificationofDrugs
•Mostofthedrugsandotherforeignsubstances are
detoxified by the liver microsomal P450 enzymes,
withthehelpofNADPH.
•LensofEye:
•MaximumconcentrationofNADPHisseeninlens
ofeye.
•NADPHisrequiredforpreservingthe
transparencyoflens.

75. •Macrophagebactericidalactivity:
NADPH is required for the production of reactive
oxygenspecies(ROS)bymacrophasestokill
bacteria.
•AvailabilityofRibose:
Ribose &Deoxy–riboseare requiredforDNA&
RNAsynthesis.

76. •Ribose is also necessary for nucleotide co –
enzymes.
•Reversalof non–oxidativephaseispresentinall
tissues,bywhichribosecouldbemadeavailable.
•WhataboutATP
ATPisneitherutilizednorproducedbytheHMP
shunt.
•Cells do not use the shunt pathway for energy
production.

77. RegulationofHMPShunt
⚫The entryofglucose6-phosphateinto the pentose
phosphatepathwayiscontrolledbythe cellular
concentrationofNADPH
⚫NADPHisa stronginhibitorofglucose6-phosphate
dehydrogenase(G6PD)
⚫NADPHisusedinvariouspathways,inhibitionis
relieved &theenzymeisacceleratedtoproduce
moreNADPH

78. ⚫The synthesis of glucose 6-phosphate
dehydrogenase isinducedbytheincreased
insulin/glucagonratioaftera highcarbohydrate
meal.

79. Glucose-6-phosphatedehydrogenasedeficiency(G6PD)
•Itisaninheritedsex –linkedtrait.
•Itismore severe inRBC.
•DecreasedactivityofG6PDimpairsthe
synthesisof NADPHin RBC.
•Thisresultsintheaccumulationofmet
hemoglobin &peroxidesinerythrocytes
leading tohemolysis.

80. •Thedeficiencyismanifestedonlywhenexposedto
certaindrugsortoxins,e.g.intakeofantimalarial
druglikeprimaquine&ingestionoffava
beans(favism) &sulphadrugsalsoparecipitatethe
hemolysis

81. G6PDdeficiency&malaria
•G6PD deficiency is associated with resistance to malaria
(caused byplasmodium infection)
•Theparasite requiresreducedglutathioneforitssurvival,
whichwillnot be availableinadequateamountsin
deficiencyof G6PD.
•Met–hemoglobinemia
•G6PDdeficient persons will show increased Met –
hemoglobinin circulation,eventhoughcyanosis maynot
be manifested.

82. References
•TextbookofBiochemistry–USatyanarayana
•TextbookofBiochemistry–DMVasudevan