Chapter23TCACycleOverviewofTCAcycleAllTCAcyclereactionsSummaryoftheTCAcyclePhysiologicalfunctionsoftheTCAcycleTheGlyoxylateCycleAnapleroticreactionsoftheTCAcycleRegulationoftheTCAcycleOutlineOverviewofTCAcycleakaCitricAcidCycle,KrebsCycle

Acommonmetabolicpathwayforglucose,aaandfattyacidPyruvate(actuallyacetate)fromglycolysisisdegradedtoCO2

SomeATPisproducedMoreNADHismadeNADHgoesontomakemoreATPinelectrontransportandoxidativephosphorylationAllroadsleadtoTCAcylceEntryintotheTCACyclePyruvate

Dehydrogenase(PDH)ComplexPyruvateisoxidatively

decarboxylatedtoformacetyl-CoAPyruvatedehydrogenaseusesTPP,CoASH,lipoicacid,FADandNAD+Pyruvatedehydrogenase(E1)Dihydrolipoamide

transacetylase(E2)Dihydrolipoamidedehydrogenase(E3)lipoicacidanditslinkagewithE2Pyruvate

DehydrogenaseComplexofE.coliThefivereactionsofthePDHmultienzymecomplexArsenicCompoundsArePoisonousinpartbecausetheySequesterLipoamideAllTCAcyclereactionsFormationofcitrateCatalyzedbyCitrateSynthase

AnotherexamplefortheinducedfitmodelOAA,thefirstsubstratetobindtotheenzyme,inducealargeconformationalchange,creatingabindingsiteforthesecondsubstrate,acetyl-CoA.Whencitroyl-CoAformsontheenzymesurface,anotherconformationalchangebringsthesideofacrucialAspresidueintopositiontocleavagethethioester.Thismechanismdecreasesthelikelihoodofprematureandunproductivecleavageofthethioesterbondofacetyl-CoAFormationofCitrateandFluorocitrateConformationalchangesincitratesynthaseTheOpenConformationTheclosed,substrate-bindingconformationIsomerizationofCitratetoIsocitrateCatalyzedbyAconitase

CitrateisapoorsubstrateforoxidationSoaconitaseisomerizescitratetoyieldisocitratewhichhasasecondary

-OH,whichcanbeoxidizedNotethestereochemistryoftheRxn:aconitaseremovesthepro-RHofthepro-Rarmofcitrate!Aconitaseusesaniron-sulfurclusterIsomerizationofCitratetoIsocitrateAmechanismforaconitasereactionStereochemistryoftheAconitaseReactionOxidativeDecarboxylationofIsocitrateCatalyzedbyIsocitrate

Dehydrogenase(IDH)ClassicNAD+chemistry(hydrideremoval)followedbyadecarboxylation

IsocitratedehydrogenaseisalinktotheelectrontransportpathwaybecauseitmakesNADHKnowthemechanism!ThereactionmechanismofIDHAsecondoxidativedecarboxylationCatalyzedby

-Ketoglutarate

DehydrogenaseComplexThisenzymeisnearlyidenticaltopyruvatedehydrogenase-structurallyandmechanisticallyFivecoenzymesused-TPP,CoASH,Lipoicacid,NAD+,FADYouknowthemechanismifyourememberpyruvatedehydrogenaseAnothertargetforarseniccompoundsOxidativeDecarboxylationofα-KetoglutarateAsubstrate-levelphosphorylationCatalyzedbySuccinyl-CoA

Synthetase

AnucleosidetriphosphateismadeItssynthesisisdrivenbyhydrolysisofaCoAesterThemechanisminvolvesaphosphohistidine

Thesubstrate-levelphosphorylationThereactionmechanismofsuccinyl-CoA

synthetaseAnoxidationinvolvingFADCatalyzedbySuccinate

DehydrogenaseThisenzymeisactuallypartoftheelectrontransportpathway(ComplexII)intheinnermitochondrialmembraneTheelectronstransferredfromsuccinatetoFAD(toformFADH2)arepasseddirectlytoubiquinone(UQ)intheelectrontransportpathwayDehydrogenationofSuccinateHydrationacrossthedoublebondCatalyzedbyFumarasetrans-additionoftheelementsofwateracrossthedoublebondTheactualmechanismisnotknownforcertainHydrationacrossthedoublebondAnNAD+-dependentoxidationCatalyzedbyMalate

DehydrogenaseThecarbonthatgetsoxidizedistheonethatreceivedthe

-OHinthepreviousreactionThisreactionisenergeticallyexpensive,

Go'=+30kJ/molThisandtheprevioustworeactionsformareactiontriadthatwewillseeoverandover!RegenerationofOAATheFateofCarboninTCACarboxylCofacetateturnstoCO2onlyinthesecondturnofthecycle(followingentryofacetate)MethylCofacetatesurvivestwocyclescompletely,buthalfofwhat'sleftexitsthecycleoneachturnafterthat.TCACycleSummaryTotalrxn:Acetyl-CoA+3NAD＋+FAD+GDP+Pi+2H2O→2CO2+3NADH+FADH2+GTP+2H＋+CoAOneAcetyl-CoAthroughthecycleproducestwoCO2,oneATP,fourreducedcoenzymesTwoH2OsareusedassubstratesAbsolutelydependsonO2FunctionoftheTCACycleProducesMoreATPsAsasourceofbiosyntheticprecursorsAcommonfinalmetabolicpathwayforglucose,aasandfattyacidsSomeImmediatesactaseffectorstoregulateothermetabolicpathwaysProducesCO2StoichiometryofCoenzymeReductionandATPFormationDuringtheCompleteOxidationofGlucoseAmphibolicfunctionsofthecitricacidcycleAnapleroticReactionsTheGlyoxylateCycleAvariantofTCAforplantsandbacteria

Acetate-basedgrowth-netsynthesisofcarbohydratesandotherintermediatesfromacetate-isnotpossiblewithTCAGlyoxylatecycleoffersasolutionforplantsandsomebacteriaandalgaeTheCO2-evolvingstepsarebypassedandanextraacetateisutilizedIsocitrate

lyaseandmalate

synthasearetheshort-circuitingenzymesGlyoxylate