清华大学生物化学课件氨基酸分解和尿素生成

Chapter18AminoAcidOxidationandtheProductionofUrea,1.Thesurplusaminoacidsinanimalscanbecompletelyoxidizedorconvertedtootherstorablefuels,Aminoacidsinexcess(fromdiet,proteinturnover)canneitherbestored,norexcreted,butoxidizedtoreleaseenergyorconvertedtofattyacidsorglucose.Animalsalsoutilizeaminoacidforenergygenerationduringstarvationorindiabetesmellitus(糖尿病).Microorganismscanalsouseaminoacidsasanenergysourcewhenthesupplyisinexcess.Plantsalmostneveruseaminoacidsasanenergysource(neitherfattyacids).,2.Dietaryproteinsaredigestedintoaminoacidsinthegastrointestinaltract,Pepsincleavespolypeptidesintosmallerpeptidesinstomach(N-terminalsideofPhe,TrpandTyrresidues).Trypsin(C-terminalsideofArgandLys)andchymotrypsin(C-terminalsideofPhe,TrpandTyr)furthercleavethepeptidesinsmallintestine.Carboxypeptidaseandaminopeptidasecleavethesmallpeptidesintoaminoacids,whicharethenabsorbedandeventuallydeliveredtoliver.,Partofthehumandigestive(gastrointestinal)tract,3.Theaminogroupsandthecarbonskeletonsofaminoacidstakeseparatebutinterconnectedpathways,Theaminogroupisreusedorexcreted,asammonia,urea(viatheureacycle)oruricacid.Thecarbonskeletons(a-ketoacids)generallyfindtheirwaytothecitricacidcycleforfurtheroxidationorconversion.Thedegradationofthecarbonskeletonsmaybeverycomplicatedbutsimilartothatoffattyacidsinsomecases.,Overviewofaminoacidcatabolisminmammals,4.Liveristhemajorsiteofaminoaciddegradationinvertebrates,Glutamate,a-ketoglutarateandpyruvatecollectsaminogroups(informsofGln,GluandAla)tolivermitochondriaforfurtherprocessing.TheexcessNH4+isexcreteddirectlyinbonyfishes,orasureainmostterrestrialvertebrates,orasuricacidinbirdsandterrestrialreptiles.Thecarbonsinbothureaanduricacidarehighlyoxidized(withmostoftheenergyextracted).,Overviewofcatabolismofaminogroupsinvertebrateliver*,Excretoryformofnitrogen,Differentaminotransferasescatalyzethetransferoftheaminogroupfromanaminoacidtoa-ketoglutaratetoformGluandaa-ketoacid,5.PLPfacilitatesthetransaminationandothertransformationsofaminoacids,Pyridoxalphosphate(PLP),beingderivedfromvitaminB6(i.e.,pyridoxine)andtheprostheticgroupforalltheaminotransferases,actsasatemporarycarrieroftheaminogroups.PLPisboundcovalentlytotheactivesitesofaminotransferasesviastructuraldetermination.PLPacceptsandthendonatesanaminogroupbyformingaSchiffbasewiththeamino-donatingaminoacidandamino-acceptinga-ketoacid(beinga-ketoglutarateinmanycases).,PLPisboundcovalentlytotheactivesitesofaminotransferasesthroughSchiff-baselinkage.,PLPboundtoaspartateaminotransferase,Activesiteofaspartateaminotransferase,Thereactionoccursviaatypicalping-pongmechanism:thefirstproductleavesbeforethesecondsubstrateenters.Aninterconversionbetweenanaldimine（醛亚胺)andaketimine（酮亚胺）isproposedtooccurviaaquinonoid（醌型的）intermediateduringthetransaminationreaction.PLPactinavarietyofotherreactionsatthea,b,andgcarbonsofaminoacids,includingracemizations,decarboxylationsattheacarbon,eliminationandreplacementreactionsatthebandgcarbons.,6.Glutamatecollectsanddeliversfreeammoniatotheliver,Freeammoniaisproducedintissues(e.g.,bythedirectdeaminationofSerandThr,deaminationofnucleotides)andistoxictoanimals,thusneedtobetransportedtotheliverbyfirstconvertingtoanontoxiccompound,Gln.Glutaminesynthetasecatalyzesthetwo-stepadditionofammoniatoGlutoyieldGln,withATPconsumedtoactivatetheg-COOH.Thisreactionalsorepresentshowfixednitrogeninbacteriaenterbiomolecules;Glnisthesourceofaminogroupsinavarietyofbiosyntheticreactions.,Ammoniatransportintheformofglutamine,7.Alaninetransportsammoniafrommuscletotheliver,Pyruvateisanabundantproductfrommuscleglycolysisandcantakeanaminogroup(fromGlu)toformAla(alternatively,bereducedtolactate).Alanine,oncegetstoliver,willtransferitsaminogrouptoa-ketoglutarateoroxaloacetatetoreformpyruvate.Pyruvateisreconvertedtoglucoseinliverviathegluconeogenesispathway,whichwillbetransportedbacktomuscleforenergysupply.Thisiscalledglucose-alaninecycle,complementingtheCoricycleforlivertocontinuouslyprovideglucosetovigorouslycontractingskeletalmuscles.,Glucose-alaninecycle,Coricycle,8.GlnandGlureleasesNH4+inlivermitochondria,TheglutaminaseinlivermitochondriacatalyzestheconversionofGlntoGlu,releasingNH4+(fromthesidechainamideofGln).Theglutamatedehydrogenase(ahexamericallostericenzyme)therecatalyzestheoxidativedeaminationofGlu,releasingNH4+,withreleasedelectronscollectedbyeitherNAD+orNADP+.GlutamatedehydrogenaseisallostericallyactivatedbyADPandGDP,butinhibitedbyATPandGTP,i.e.,aloweringoftheenergychargeacceleratestheoxidationofaminoacids.,Oxidativedeamination,Glutamatedehydrogenase,9.NH4+inhepatocytesisconvertedintoureainmostterrestrialvertebratesforexcretion,HansKrebsandKurtHenseleitrevealedin1932(fiveyearsbeforethecitricacidcyclewaselucidated)thatthisconversionisaccomplishedviaacyclicpathway,calledtheureacycle.ThetwonitrogenatomsarederivedfromAspandNH4+,withthecarbonatomcomingfromCO2.,Ornithine（鸟氨酸),actingasthecarrierofthecarbonandnitrogenatoms,hasarolesimilartothatofoxaloacetateinthecitricacidcycle.Theureacyclespansthemitochondriaandcytosol.Fourphosphoanhydridebonds(equivalenttofourATPmolecules)arehydrolyzedfortheproductionofeachurea.2NH4+HCO3-+3ATP4-+H2OUrea+2ADP3-+4Pi2-+AMP2-+5H+,Theureacycletakesplaceinthemitochondriaandcytosoloflivercells,10.Theconversionofammoniatoureatakesfiveenzymaticsteps,Firstacarbamoylphosphate(氨基甲酰磷酸,anactivatedcarbamoylgroupdonor)isproducedinthemitochondrialmatrixusingoneNH4+,oneHCO3-,andtwoATPmolecules,withthecatalysisofcarbamoylphosphatesynthetaseI(isozymeIIexistsinthecytosol,actinginpyrimidinesynthesis).,Theureacyclebeginsinthemitochondrialmatrixwhenacarbamoylgroupistransferredtoornithinetoformcitrulline(瓜氨酸),inareactioncatalyzedbyornithinetranscarbamoylase.Citrullinethenmovesintothecytosol.TheaminogroupofoneAspcondenseswiththeureidogroupofcitrullinetoformargininosuccinate,inanATP-dependentreactioncatalyzedbyargininosuccinatesynthetase,viaacitrullyl-AMPintermediate.,TheargininosuccinateisthencleavedtoformArgandfumarate,inareactioncatalyzedbyargininosuccinatelyase.Argisthencleavedtoformureaandregenerateornithineatthesametime,catalyzedbyarginase.FumarateentersmitochondriaandisreconvertedtoAsp:convertedtooxaloacetatefirstviathecitricacidcycle(generatingoneNADH),thentakeanaminogroupfromGluviaatransaminationreaction.,Ureacycleandreactionsthatfeedaminogroupsintothecycle,ornithinetranscarbamoylaseargininosuccinatesynthetaseargininosuccinatelyasearginase,Theureacycleandthecitricacidcyclearelinkedviatheaspartate-arginino-succinateshunt,“Krebsbicycle”,Linksbetweentheureacycleandcitricacidcycle,11.Therateofureasynthesisiscontrolledattwolevels,Allostericregulationofenzymeactivity(fast):N-acetylglutamate,synthesizedfromacetyl-CoAandglutamatebythecatalysisofN-acetylglutamatesynthase,positivelyregulatescarbamoylphosphatesynthetaseIactivity.Generegulationofenzymeamount(slow):synthesesofthefiveliverenzymesinvolvedinureasynthesisareincreasedduringstarvation(whenenergyhastobeobtainedfrommuscleproteins!)orafterhighproteinuptake.,CarbamoylphosphatesynthetaseIispositivelyregulatedbytheenigmaticN-acetylglutamate,12.Geneticdefectsoftheureaproductionenzymesleadtohyperammonemiaandbraindamage,Geneticdefectsofallfiveureacycleenzymeshavebeenrevealed.Highlevelsofammonialeadtomentaldisorderorevencomaanddeath.Ingeniousstrategiesforcopingwiththedeficiencieshavebeendevisedbasedonathoroughunderstandingoftheunderlyingbiochemistry.,StrategyI:dietcontrol,providetheessentialaminoacidsintheira-ketoacidforms.StrategyII:whenargininosuccinatelyaseisdeficient,ingestingasurplusofArgwillhelp(ammoniawillbecarriedoutofthebodyintheformofargininosuccinate,insteadofurea).StrategyIII:whencarbamoylphosphatesynthetase,ornithinetranscarbamoylase,orargininosuccinatesynthetasearedeficient,theammoniacanbeeliminatedbyingestingcompounds(e.g.,benzoateorphenylacetate),whichwillbeexcretedafteracceptingammonia.,Treatmentfordeficienciesinureacycleenzymes,13.Thecarbonskeletonsoftheaminoacidsareconverted(funneled)intosevenmajormetabolicintermediates,Intermediatesinclude:acetoacetyl-CoA,acetyl-CoA(whichcanbeconvertedtofattyacidsandketonebodies,theseaminoacidsarethusketogenic)Pyruvate,a-ketoglutarate,succinyl-CoA,fumarate,oxaloacetate(allcanbeconvertedtoglucose,theseaminoacidsarethusglucogenic),Aminoacidshavinglongcarbonchainsoftendegradeinwayssimilartofattyacidoxidation.Aminoacidshavingshortchainsoftendegradeintopyruvateorcitricacidintermediatesdirectly.Someoftheconversionsareverycomplicated,usingavarietyofcofactors.,Thecarbonskeletonsofaminoacidsareoxidizedviathecitricacidcycle*,14.Someaminoacidsareconvertedtointermediatesofcitricacidcyclebysimpleremovaloftheaminogroups,AlapyruvateAspoxaloacetateGlua-ketoglutarateAsnAsp(viaadeaminationreactioncatalyzedbyasparaginase)GlnGlu(viaadeaminationreactioncatalyzedbyglutaminase),15.Acetyl-CoAisformedfromthedegradationofmanyaminoacids,Trp,Ala,Ser,Gly,ThrandCysareconvertedtopyruvatefirstbeforebeingconvertedtoacetyl-CoA.PortionsofthecarbonskeletonsofTrp,Phe,Try,Lys,Leu,andIleareconverted(viaacetoacetyl-CoAordirectly)toacetyl-CoA.SomeofthefinalstepsinthedegradationofLeu,Lys,andTrpresemblestepsinfattyacidoxidation.Tryptophandegradationisthemostcomplex.,Trp,Thr,Ala,Ser,GlyandCysareconvertedtopyruvate,beforebeingconvertedtoacetyl-CoA,Trp,Phe,Tyr,Lys,Leu,andIleareconvertedtoacetyl-CoA,Intermediatesoftryptophandegradationareprecursorsforsynthesizingotherbiomolecules,Arg,His,Glu,GlnandProareconvertedtoa-ketoglutarate,Met,Ile,ThrandValareconvertedtosuccinyl-CoA,AspandAsnareconvertedtooxaloacetate,16.Leu,Ile,andValaredegradedviareactionssimilartofattyacidoxidation,Theyareoxidizedprimarilyinmuscle,adipose,kidneyandbraintissue,notinliver.Thesamebranched-chainaminotransferaseandbranched-chaina-ketoaciddehydrogenasecomplexcatalyzethefirsttwodegradativereactions(transaminationandoxidativedecarboxylation)ofallthesethreeaminoacidsinextrahepatictissues.Thea-ketoaciddehydrogenasecomplexhasasimilarstructureandcatalyzesessentiallythesametypeofreactionaspyruvatedehydrogenasecomplexandthea-ketoglutaratedehydrogenasecomplex.,CatabolicpathwaysforVal,IleandLeu,Absentinliver,17.Afewgeneticdiseasesarerelatedtodefectsinaminoacidcatabolism,Defectsinenzymesofaminoaciddegradationleadtoaccumulationofspecificintermediates,manyofwhichcausediseases.Deficiencyofphenylalaninehydroxylasecausesphenylketonuria(PKU,苯丙酮尿症),oneoftheearliesthumangeneticdefectsofmetabolism(firstrevealedin1934intestingmentallyretardedpatients,whoseurineturnedolive-greenonadditionofFeCl3).TheaccumulatedPheisconvertedtophenylpyruvate(viaanaminotransferase)whichcausedthecolorchangeofurine.,PheandTyrareeventuallyconvertedtofumarateandacetoacetyl-CoA,(PKU),PhenylalaninethatisaccumulatedinPKUpatientsisconvertedtootheralternativeproducts.,AnotherdiseaserelatedtoPhe/Tyrcatabolismandofhistoricalinterestisalkaptonuria(尿黑酸症).ArchibaldGarroddiscoveredthattheconditionistransmittedasasinglerecessiveinheritabletraitandcouldbetracedtotheabsenceofasingleenzyme(inearly1900s),thushintedthedirectrelationbetweengenesandenzymes.Homogentisatedioxygenasewasf