FattyAcidBiosynthesis：脂肪酸的生物合成

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1、Chapter25:FattyAcidBiosynthesisCH234BruceA.Hathaway1AcetylCoenzymeAAcetylCoenzymeAisthestartingmaterialforfattyacidbiosynthesis.Itisderivedfrommetabolismofcarbohydrates,fatsandsomeaminoacids.Thetwo-carbonpieceprovidesthecarbonsforthenewfattyacid.AcetylCoenzymeA23AcetylC

2、oenzymeA(AcCoA)OverviewofMetabolismAndEnergyProduction4Step1:PreparationofMalonylCoAAcetylCoAreactswithbicarbonatetoformMalonylCoA.Thisisenergy-requiring,sohydrolysisofanATPisusedtodrivethereaction.MalonylCoAismorereactivethanAcetylCoA.5Step2:TransthioesterifactionBothA

3、cetylCoAandMalonylCoAareconnectedtoAcetylCarrierProtein(ACP),whichisanotherthiol.Onethiolreplacesanothertomakeadifferentthioester.ACPshuttlesthemoleculestothedifferentenzymesneededtodothefollowingreactions.6Step3:C-CBondFormationMalonylACPlosesCO2tomakeacarbonanion.Anac

4、etylACPreactswiththecarbonaniontogiveafour-carbonβ-ketoester.7Step4:ReductionoftheKetoneTheketoneisreducedtoanalcohol.Onlyonestereoisomerisproduced:why?NADPHfunctionssimilartoNaBH4inthisreaction.8NADPHNADPHissimilartoNADH.Ithasanextraphosphate,whileNADHhasanH.Metabolism

5、usesNADH.BiosynthesisusesNADPH.9ReductionoftheKetonewithNADPHNADPHdonatesahydrideH-toreducetheketone:theH+comesfromwater,ultimately.Onlyoneenantiomerisformed.10Step5:DehydrationoftheAlcoholThealcoholisspecificallydehydratedtothetransalkene.Again,onlyoneisomerisformed.11

6、Step6:ReductionoftheAlkeneNADPH+H+reducestheC=Ctoanalkane.Wehavenowmadeafour-carbon“fattyacid”.12ElongationofFattyAcidsTheButyroyl-ACPreplacestheAcetyl-ACPinStep3,tomakea6-carbonketoacid.13ElongationofFattyAcidsThe6-carbonketo-acidisreduced,dehydrated,andreducedagain(st

7、eps4-6),tomakethesix-carbonfattyacid.Thiscanreactwithmoremalonyl-ACP…14ElongationofFattyAcidsThecycleofsteps3-6isrepeatedfivemoretimesuntilthe16-carbonfattyacidismade.Forsomereason,thisenzymecomplexonlymakespalmiticacid.Longerfattyacidsandunsaturatedfattyacidsaresynthes

8、izedinotherlocations.15EnergyRequirementsToaddtwocarbonstoAcetylCoA,itrequires1ATP(tomakeamalonylCoA)and2NADPH