SM_c22BiosynthesisofAminoAcidsNucleotidesandRelatedMolecules

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1、c22BiosynthesisofAminoAcidsNucleotidesandRelatedMolecules.qxd12/12/124:21PMPageS-258chapterBiosynthesisofAminoAcids,Nucleotides,and22RelatedMolecules1.ATPConsumptionbyRootNodulesinLegumesBacteriaresidingintherootnodulesofthepeaplantconsumemorethan20%oftheATPproducedbytheplan

2、t.SuggestwhythesebacteriaconsumesomuchATP.AnswerBacteriaintherootnodulesmaintainasymbioticrelationshipwiththeplant:theplantsuppliesATPandreducingpower,andthebacteriasupplyammoniumionbyreducingatmosphericnitrogen.ThisreductionrequireslargequantitiesofATP.2.GlutamateDehydrogen

3、aseandProteinSynthesisThebacteriumMethylophilusmethylotro-phuscansynthesizeproteinfrommethanolandammonia.RecombinantDNAtechniqueshaveim-provedtheyieldofproteinbyintroducingintoM.methylotrophustheglutamatedehydrogenasegenefromE.coli.Whydoesthisgeneticmanipulationincreasethepr

4、oteinyield?AnswerThesynthesisofproteinrequiresthesynthesisofaminoacids.Thetransferofnitro-genfromanammoniumiontocarbonskeletons—thatis,aminoacidsynthesis—canbecarriedoutintwoways:(1)combinationoftheNH3withglutamatetoformglutamine,catalyzedbyglutaminesynthetaseand(2)reductive

5、aminationofa-ketoglutaratetoformglutamate,cat-alyzedbyglutamatedehydrogenase.Thelatterprocess,whichispromotedbytheintroduc-tionoftheE.colienzyme,isespeciallyimportantbecauseglutamateistheaminogroupdonorinalltransaminationreactions.3.PLPReactionMechanismsPyridoxalphosphatecan

6、helpcatalyzetransformationsoneortwocar-bonsremovedfromthecarbonofanaminoacid.Theenzymethreoninesynthase(seeFig.22–17)promotesthePLP-dependentconversionofphosphohomoserinetothreonine.Suggestamechanismforthisreaction.AnswerAlinkbetweenenzyme-boundPLPandthephosphohomoserinesub

7、strateisfirstformed,withrearrangementtogeneratetheketimineatthecarbonofthesubstrate.Thisactivatesthecarbonforprotonabstraction,leadingtodisplacementofthephosphateandformationofadoublebondbetweentheandcarbons.Arearrangement(beginningwithprotonabstractionatthepyridoxalcarb

8、onadjacenttothesubstrateaminonitrogen)movesthedoublebondbetweentheandcarb