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Chapter24PhosphatePentosePathway(PPP)OverviewofPPPAllPPPreactionsPhysiologicalfunctionsofPPPRegulationofPPPOutlinePentosePhosphatePathwayAka:Pentoseshunt,Hexose

monophosphateshuntandPhosphogluconatepathwayItoccursinthecytosol.Twooxidativeprocessesfollowedbyfivenon-oxidativestepsOperatesactiveinthecytosolofliverandadiposecellsCellsneedaconstantsupplyofenergyNADH,NADPHandATPATP-energycurrencyNADPH-reducingpowerGlucose-->NADH-->ATPGlucose-->NADPH-->biosynthesis(reductive)PPPProvidesNADPHandribose-5-PPentosePhosphatePathwayAllthePPPreactionsGlucose-6-PDehydrogenase:Irreversible1ststep-highlyregulated(inhibitedbyNADPH)Gluconolactonase:Uncatalyzedreactionhappenstoo6-PhosphogluconateDehydrogenase:AnoxidativedecarboxylationOxidativePhaseRegulatoryenzymeTheenzymeishighlyspecificforNADP+;theKmforNAD+is1000greaterthanforNADP+.Step1:DehydrogenationofGlucose-6-phosphateStep2:HydrolysisofLactoneStep3:Dehydrogenationof6-phosphogluconateTransketolase:transferoftwo-carbonunitsTransaldolase:transfersathree-carbonunitTheNonoxidative

PhaseStep4andStep5:Conversionofribulose5-phosphatetoxylulose5-phosphateorribose5-phosphateVariationsonthePPPMoreribose-5-PthanNADPHisneededBothribose-5-PandNADPHareneededMoreNADPHthanribose-5-PisneededNADPHandATPareneeded,butribose-5-PisnotRapidlydividingcellsrequiremoreribose5-phosphatethanNADPHTheneedforNADPHandribose5-phosphateisbalancedMoreNADPHisneededthanribose5-phosphate;FattyacidsynthesisinadiposecellsThecellneedsbothNADPHandATPTheUseofNADPHBiosynthesisfattyacidsphotosynthesisDNAsynthesisRedoxregulationofcellularprocessescontrolofcellularredoxstatedisulfidebondformationantioxidantdefenseGlutathioneandNADPHGlutathionePeroxidasecatalyzesdegradationoforganichydroperoxidesbyreduction,astwoglutathionemolecules(representedasGSH)areoxidizedtoadisulfide.2GSH+ROOH

GSSG+ROH+H2OGlutathionePeroxidaseusesthetraceelementseleniumasfunctionalgroup.Theenzyme'sprimarystructureincludesananalogofcysteine,selenocysteine,withSereplacingS.RegenerationofreducedglutathionerequiresNADPHproducedinthePentosePhosphatePathway.GlutathioneReductasecatalyzes:GSSG+NADPH+H+

→2GSH+NADP+

GeneticdeficiencyofGlucose-6-PDehydrogenasecanleadtohemolyticanemia,duetoinadequate[NADPH]withinredbloodcells.EffectofpartialdeficiencyofGlucose-6-PDehydrogenaseisexacerbatedbys

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