大二上学期文件-生物化学biochemistry lecture3citric acid cycletca

Lecture3CitricAcidCycleTricarboxylicacidcycle(TCAcycleorKrebsZhangSchoolofLifeScience&: : p332-HANSADOLFHansKrebs,

1953NobelLaureateinforhisdiscoveryofthecitricacidPlaceofBirth:Hildesheim,GermanyResidence:GreatBritainAffiliation:SheffieldProductionofAcetyl-ReactionsoftheCitricAcidRegulationoftheCitricAcidTheGlyoxylateCycle(乙醛酸循环）TCATakeplace－－Eukaryotes:mitochondriaProkaryotes:cytosolcellularrespirationcellularrespiration--Ratherthanbeingreducedtolactate,ethanol,orsomeotherfermentationproduct,thepyruvateproducedbyglycolysisisfurtheroxidizedtoH2Oand重ProductionofAcetyl-CoA-----PyruvateAMajorEntryRouteforCarbonintotheCitricAcid 酸脱氢重enzymes(E1,E2andPyruvateDehydrogenaseDihydrolipoyltransacetylase(E2)二氢硫辛酰转乙酰基fiveThiaminePyrophosphate焦磷酸硫胺素辅羧酶LipoicAcid硫辛酸-lipoamide硫辛酰重Thereactivecarbon

TPPisthecoenzymeformofvitaminB1(thiamine)catalyzedbyPyruvateDehydrogenasetwoofthethreecarbonsofpyruvatearecarriedtransientlyonTPPintheformofahydroxyethyl羟乙基or“activeacetaldehyde乙醛groupwhichissubsequentlyreleasedasacetaldehyde羟乙基羟乙基

TheTPPcarbanion负碳离子actsasanucleophile亲核attackingthecarbonylgroupofpyruvate.strongtendencytoformanewbondisgenerallyunstable②Decarboxylationproducesacarbanion负thatisstabilizedbythethiazolium噻唑ring.③Protonationtoformhydroxyethyl④releaseof⑤AprotondissociatestoregeneratetheCryoelectron冷冻电子显微镜术micrographofPDHcomplexesisolatedfrombovinekidney.ThreedimensionalimageofPDHE1E1moleculesAnumberofE3subunits(red)arealsoboundtothecore,wheretheswingingarmonE2canreachtheiractivesites.ofE2(blue)reachesoutwardtotouchtheactivesitesThelipoylThecore(green)consistsof24moleculesofE2,arrangedin8 E1acceptsatwocarbonaldehyde（乙醛）fromthedecarboxylationofpyruvate.Thealdehydegroupistransferredtothefirstlipoamide（硫辛酰胺）armofE2.TheacetylgroupislinkedtoCoASH,formingacetyl-E3oxidizesthereducedlipoamidearmbytransferrin hydrogenstoFAD,formingFADH2FADH2isoxidizedbyNAD+,formingFADandNADH+Reactionsofcentralmetabolicpathway中心代谢途generatesNADHandproducesGTPvia evelManymetabolicprocessesuseintermediatesofTCAintheir重formcitrateTheTCAHasEight①Steps1,3,and4areessentiallyirreversibleinthecellFourstepsinthisprocessareoxidations,energyof and③Step5maybeeitherATPor

重TheCitricAcidCycleHasEightPhase1:IntroductionandLossofTwoCarbonStep1:IntroductionofTwoCarbonAtomsasAcetyl-Step3:GenerationofCO2byanNAD+LinkedDehydrogenaseStep4:GenerationofaSecondCO2byaMultienzymeComplexPhase2:RegenerationofOxaloacetate（草酰乙酸Step5:ASubstra evelPhosphorylationStep7:Hydration水合作用ofaCarbon-CarbonDoubleStep8:ADehydrogenationthatRegeneratesStep1:IntroductionofTwoCarbonAtomsasAcetyl-柠檬酸合酶citrateTCA限速Thelarge,negativestandardfree-energychangeofthecitratesynthasereactionisessentialtotheoperationofthecycletheconcentrationofoxaloacetate草酰乙酸isnormallyverylow(10-重Structureofcitrateopenformoftheenzymeclosedformwithboundoxaloacetateandastable ogofacetyl-CoA.Theflexible ofeachsubunitundergoesalargeconformationalchangeonbindingoxaloacetatecreatingabindingsiteforacetyl-CoA.CitricacidisproducedindustriallybygrowingthefungusAspergillus曲霉nigerinthepresenceofaninexpensivesugarsourceusuallybeetmolasses(甜菜糖蜜CultureconditionsaredesignedtoinhibitthereactionsofthecitricacidcyclesuchthatcitrateStep2:Isomerizationof顺顺乌头柠檬 异柠檬叔醇化合

仲醇化合（可氧化重乌头Theiron-sulfurcenteractsinbothsubstratebindingandThreeCysresiduesoftheenzymebindthreeironthefourthironisboundtooneofthecarboxylgroupsofcitrateandalsointeractsnoncovalentlywithahydroxylgroupofcitrate.Abasicresidue(:B)ontheenzymehelpstopositionthecitrateintheactivesite.Step3:GenerationofCO2byanNAD+LinkedATP，NADHinhibittheactivityofisocitrate异柠檬 草酰琥珀 α-酮戊二Isocitratedehydrogenase异柠檬酸脱酮酸的形成可促 C-C键的断裂，有利于脱羧作用的进CoenzymeNAD+

Mitand重Step GenerationofaSecondCO2byaMultienzymeα-酮戊二琥珀酰α-酮戊二琥珀酰 -KDdehydrogenaseDihydrolipoamidetransacetylase(E2)二氢硫辛酰转琥珀酰Dihydrolipoamidedehydrogenase(E3)二氢硫辛酰脱fiveThiaminePyrophosphate焦磷酸硫胺素辅羧酶 重Phase2:RegenerationofOxaloacetate（草酰乙酸Step5:ConversionofSuccinyl-CoAto琥珀酰－CoA底物水平磷酸化OnlyoneinInanimal:Inplantandmicroorganism:琥珀酰 琥珀Differentfrom 重①aphosphorylgroupreplacestheCoAofsuccinyl-CoAboundtotheenzyme,formingahigh-energyacyl酰基磷②thesuccinylphosphatedonatesitsphosphorylgrouptoaHisresidueontheenzyme,formingahigh-energyphosphohistidylenzyme③thephosphorylgroupistransferredfromtheHisresiduetotheterminalphosphateofGDP(orADP),formingGTP(orATP).Step6:OxidationofSuccinateto琥珀 延胡索琥珀酸脱氢succinate -AFlavin-DependentMalonate(丙二酸 ogofsuccinate,isastrongcompetitiveOnlyenzymeofTCAismembrane-重Step7:HydrationofFumarateto延胡索酸延胡索L-苹果 重Step8:OxidationofMalateto 重ADehydrogenationThatRegenerates苹果酸脱L-苹果 草酰乙TheEnergyofOxidationsintheForeachacetyl-CoAoxidizedbythecitricacid311ATPor重Atwo-carbonacetylgroupenteredthecyclebycombiningwithoxaloacetate.TwocarbonatomsemergedfromthecycleasCO2theoxidationofisocitrateandα-ketoglutarate.thetwocarbonatomsappearingasCO2arenotthesametwocarbonsthatenteredintheformoftheacetyl重EnergeticsoftheCitricAcid重Besidesacetyl-anycompoundthatgivesrisetoafour-orfive-carbonintermediateofthecitricacidcycle—canbeoxidizedbythecycleTheTCAisamphibolic(双重代谢的)servinginbothcatabolismandanabolism------cycleintermediatescanbedrawnoffandusedasthestartingmaterialforavarietyofbiosynthetic重RegulationofPyruvateDehydrogenaseandtheCitricAcidCycleregulatedintwoprimarycontrollingtheentryoffuelintothecyclecontrollingkeyreactionswithinthe重ControllingtheentryoffuelintothePyruvatedehydrogenase---majorregulatorypointforentryofmaterialsintoTCATheenzymeisregulatedallostericallyandbycovalentmodification.AllostericRegulationE2-inhibitedbyacetyl-CoA,activatedbyCoA-SHE3-inhibitedbyNADH,activatedbyNAD+.ATP:allostericinhibitoroftheAMP:重CovalentRegulation重Keyreactionswithinthecycle:allostericregulationisocitrateActivatedbyADP,inhibitedbyPhosphorylationofoneserineresidueintheenzymepreventsbindingofisocitrateα－ketoglutarateInhibitedbysuccinyl-CoAand重4.Glyoxylate somebacteria(inclu