生物化学课件：氨基酸代谢

氨基酸代谢第一节Thenitrogencycle

Nitrogenexistspredominantlyinanoxidizedstateintheenvironment,occurringprincipallyasN2intheatmosphereorasnitrateion(NO3-)inthesoilsandoceans.Itsacquisitionbybiologicalsystemsisaccompaniedbyitsreductiontoammoniumion(NH4+)andtheincorporationofNH4+intoorganiclinkageasaminogroup.ThereductionofNO3-toNH4+occursingreenplants,variousfungi,andcertainbacteriainatwo-stepmetabolicpathwayknownasnitrateassimilation.第一节Thenitrogencycle

TheformationofNH4+fromN2gasistermednitrogenfixation.N2fixationisanexclusivelyprokaryoticprocess.Noanimalsarecapableofeithernitrogenfixationornitrateassimilation.Animalsreleaseexcessnitrogeninareducedform,eitherasNH4+orasorganicnitrogenouscompoundssuchasurea.ThereleaseofNoccursbothduringlifeandasaconsequenceofmicrobialdecompositionfollowingdeath.第一节Thenitrogencycle

Variousbacteriareturnthereducedformsofnitrogenbacktotheenvironmentbyoxidizingthem.TheoxidationofNH4+toNO3-isperformedbynitrifyingbacteria.NitratenitrogenalsoreturnstotheatmosphereasN2asresultofthemetabolicactivityofdenitrifyingbacteria.第一节ThenitrogencycleDietaryproteinsaredigestedintoaminoacidsinthegastrointestinal(胃肠)tractviatheactionofpepsin,trypsin,chymotrypsin,carboxypeptidasesandaminopeptidases.SourcesofaminoacidsforanimalsProteins(butnotpepsin)unfoldedAbsorbedastri-&dipeptides,andaminoacidsDegradation&absorptionofdietaryproteinsPepsin:thefirstenzymediscovered(18thcentury).proteasesEssentialaminoacidsAminoacidscannotbestoredinanimals:excessbeingcompletelyoxidizedtoreleaseenergyorconvertedtostorablefuels(fattyacidsorcarbohydrates).

Overallfateofexcessaminoacids第二节Aminoaciddegradation

1.氧化脱氨基

氨基酸在酶的作用下脱去氨基生成相应酮酸的过程，叫氧化脱氨基作用。一.氨的去路

Glu+NAD(P)+H2O

a-KG+NH4+

+NADH(P)

+H+

2.脱氢酶作用-GDH一.氨的去路

3.转氨基作用一.氨的去路

转氨基作用是α-氨基酸和α-酮酸之间氨基的转移作用。一种α-氨基酸的α-氨基借助转氨酶（transaminase）的催化作用转移到α-酮酸的羰基上，结果生成新的酮酸，而原来的α-酮酸则形成相应的氨基酸。

3.转氨基作用谷丙转氨酶催化的转氨基作用机理

一.氨的去路

4.联合脱氨作用(转氨酶-谷氨酸脱氢酶)谷氨酸

α-酮戊二酸

丙氨酸

丙酮酸

转氨酶

谷氨酸脱氢酶

NAD(P)+H+

NAD(P)+联合脱氨基作用

PLP

一.氨的去路

在氨基酸脱羧酶催化下进行脱羧作用，生成一个伯胺类化合物和CO2，其反应可以用下式表示二.

脱羧基作用

PLPactsasatemporarycarrierofaminogroupsattheactivesitesofallaminotransferases.PLPfacilitatesseveraldifferenttypesoftransformationaroundthea-carbonofaminoacids.PLPisderivedfromvitaminB6(pyridoxine，吡哆醇)吡哆醛磷酸磷酸吡哆胺SerumaminotransferaseshavebeenusedasclinicalmarkersoftissuedamagesDamagedheartorlivercellsleakaminotransferases.Bloodaspartateaminotransferaseandalanineaminotransferaseareusuallyexaminedforindicationsofillness.三.氨基酸碳架的分解

氨基酸脱羧酶

1.进入TCA循环

Oxidationofthecarbonskeletonsofaminoacidsinmammals2.再合成为氨基酸谷氨酸＋丙酮酸α-酮戊二酸＋丙氨酸谷氨酸＋草酰乙酸α-酮戊二酸＋天冬氨酸三.氨基酸碳架的分解

3.转变为糖和脂肪

当体内不需要将α-酮酸再合成氨基酸，并且体内的能量供给充足时，α-酮酸可以转变为糖或脂肪。例如，用氨基酸饲养患人工糖尿病的狗，大多数氨基酸可使尿中的葡萄糖的含量增加，少数几种可使葡萄糖及酮体的含量同时增加。在体内可以转变为糖的氨基酸称为生糖氨基酸，按糖代谢途径进行代谢；能转变为酮体的氨基酸称为生酮氨基酸。

三.氨基酸碳架的分解

硝酸盐还原分两步进行：第一步在硝酸还原酶（nitratereductase,NR）催化下，由NAD（P）H提供1对电子，硝酸盐被还原为亚硝酸盐，第二步是在亚硝酸还原酶（nitritereductase,NiR）下，由还原型铁氧还蛋白（Fdred）提供3对电子，使亚硝酸盐（NO2-）还原成氨。

第三节Nitratereduction硝酸盐还原分两步进行：第一步在硝酸还原酶（nitratereductase,NR）催化下，由NAD（P）H提供1对电子，硝酸盐被还原为亚硝酸盐，第二步是在亚硝酸还原酶（nitritereductase,NiR）下，由还原型铁氧还蛋白（Fdred）提供3对电子，使亚硝酸盐（NO2-）还原成氨。

第三节NitratereductionAmmoniumentersorganiclinkageviathreemajorreactionsthatarefoundinallcells.Theenzymesmediatingthesereactionsare：(1)Cabamoyl-phosphatesynthetaseI(氨甲酰磷酸合成酶)(2)Glutamatedehydrogenase（谷氨酸脱氢酶）,(3)Glutaminesynthetase（谷氨酰氨合成酶）.

第四节AmmoniumassimilationNH4+inhepatocytes(肝细胞)isconvertedintoureaforexcretionviatheureacycleinmostterrestrialvertebratesUreaisformedfromammonia,CO2(asbicarbonate)andAsp.ThepathwaywasalsodiscoveredbyHansKrebsin1932(fiveyearsbeforehediscoveredthecitricacidcycle).FourATPmoleculesareconsumedtoproduceeachurea.

Carbamoyl-phosphatesynthetaseIcatalyzesoneofthestepsintheureacycle.TwoATPareconsumed,oneintheactivationofHCO3-forreactionwithammonium,andtheotherinthephosphorylationofthecarbamateformed:1.Carbamoyl-phosphatesynthetaseI

NH4++HCO3-+2ATPH2N-CO-O-PO3-+2ADP+Pi+2H+

N-acetylglutamateisanessentialallostericactivatorforthisenzyme第四节AmmoniumassimilationThesynthesisofCarbamoyl（氨甲酰）

phosphaterequirestwoactivationsteps,consumingtwoATPmolecules:oneforactivatingHCO3-,theothertophosphorylatecarbamate.

ananhydride1.Carbamoyl-phosphatesynthetaseI

该反应消耗2个ATP分子中的两个高能磷酸键，其中1个是用于活化HCO3-，另1分子ATP则用于磷酸化氨甲酰基。第四节AmmoniumassimilationFumarateisconvertedbacktoAspviaapartialusageofthecitricacidcycle.TherateofureasynthesisiscontrolledattwolevelsAllosteric（别构）regulation:N-acetylglutamate,bybindingtoasitewhichhydrolyzes（水解）Glninanotherisozyme,positivelyregulatescarbamoylphosphatesynthetaseIactivity.Generegulation:synthesesoftheureacycleenzymesareallincreasedduringstarvation(whenenergyhastobeobtainedfrommuscleproteins!)orafterhighproteinuptake.Theratesoftranscriptionofthefivegenesencodingtheenzymesareincreased.GeneticdefectsoftheureacycleenzymesleadtohyperammonemiaandbraindamageHighlevelsofammonialeadtomentaldisorderorevencomaanddeath.Ingeniousstrategiesforcopingwiththedeficiencieshavebeendevisedbasedonathoroughunderstandingoftheunderlyingbiochemistry.StrategyI:dietcontrol,providetheessentialaminoacidsintheira-ketoacidforms.StrategyII:whenargininosuccinatelyaseisdeficient,ingestingasurplusofArgwillhelp(ammoniawillbecarriedoutofthebodyintheformofargininosuccinate,insteadofurea).StrategyIII:whencarbamoylphosphatesynthetaseI,ornithinetranscarbamoylase,orargininosuccinatesythetasearedeficient,theammoniacanbeeliminatedbyingestingcompounds(e.g.,benzoateorphenylacetate),whichwillbeexcretedafteracceptingammonia.

Glutamatedehydrogenasecatalyzesthereductiveaminationofa-ketoglutaratetoyieldglutamate.Reducedpyridinemucleotides(NADHorNADPH)providethereducingpower:2.Glutamatedehydrogenase(GDH)

NH4++a-ketoglutarate+NADPH+H+

glutamate+NADP++H2O第四节AmmoniumassimilationTheglutamatedehydrogenasereaction第四节Ammoniumassimilation3.Glutaminesynthetase(GS)GlutaminesynthetasecatalysestheATP-dependentamindationofthe-carboxylgroupofglutamatetoformglutamine.GSactivitydependsonthepresenceofdivalentcationssuchasMg2+.GlutamineisamajordonorinthebiosynthesisofmanyorganicNcompoundsandGSactivityistightlyregulated.

GDHandGSareresponsibleformostoftheammoniumassimilatedintoorganiccompounds.第四节Ammoniumassimilation谷氨酰胺合成酶谷氨酰胺合成酶第四节AmmoniumassimilationTheGlutaminesynthetaseisaprimaryregulatorypointinnitrogenmetabolism:beingregulatedbyatleasteightallostericeffectorsandreversibleadenylylation.

Thebacterial

glutaminesynthetasehas12subunitsarrangedastworingsofhexamers.Activesites

Tyr397(adenylylationsite)Theglutaminesynthetaseisaccumulativelyinhibitedbyatleast8allostericeffectors,mostlyendproductsofglutaminemetabolism.Glutamatesynthasecatalyesthereductiveaminationofa-ketoglutaratesuingtheamide-NofglutamineastheNdonor:Glutamatesynthase(GOGAT)

Reductant+a-KG+Gln2Glu+oxidizedredctant

第四节AmmoniumassimilationTheglutamatesynthasereaction谷氨酸合酶第四节AmmoniumassimilationOnlycertainbacteriacanfixN2intoammoniaRhizobiaCyanobacteria蓝细菌根瘤菌第5节NitrogenfixationThedinitrogenase(固氮酶)complexincertainbacteria(diazotrophs)catalyzestheconversionofN2(azote,“withoutlife”)toNH3,whichistheultimatesourceofnitrogenforallnitrogen-containingbiomolecules.

N2+8H++8e-2NH3+H2

TheHabermethod:N2+3H22NH3

G`o=-33.5kJ/molwithironcatalyst,500oC,300atmospheres.Thenitrogenasecomplexconsistsofdinitrogenaseanddinitrogenaseredutase

bothbeingiron-sulfurproteins.Dinitrogenase(a2b2)orFeMoproteinReductase:adimeroftwoIdenticalsubunitsbridgedbya4Fe-4S.ATPhydrolysisiscoupledtoproteinconformatinalchanges.Dinitrogenasereductase(dimer)orFeproteinADPADP4Fe-4S8Fe-7S(P-cluster)Fe-Mocofactor

e-Fe-Mocofactor8Fe-7S(P-cluster)4Fe-4SADPADPMolybdenum(orvanadium)N2isbelievedtobereducedbytheFe-MocofactorN2FeFeFeFeFeFeFeSSSSSSSSSMo高柠檬酸ElectronsaretransferredthroughaseriesofcarrierstoN2foritsreductiononthenitrogenasecomplex.ElectronsaretransferredtoN2boundintheactivesiteofdinitrogenaseviaferredoxin/flavodoxinanddinitrogenaseReductase.N2+8H++8e-+16ATP+16H2O

2NH3+H2+16ADP+16Pi(orphotophosphorylation)Conformationalchangereducese-affinityTheoxidizeddinitrogenasereductasedissociatesfromthedinitrogenaseReduceddinitrogenasereductaseassociateswiththedinitrogenaseThenitrogenasecomplexisextremelylabiletoO2andvariousprotectivemechanismshaveevolved:livinganaerobically,formingthickwalls,uncouplinge-transportfromATPsynthesis(enteringO2isusedinmediately)orbeingprotectedbyO2-bindingproteins.Genesencodingtheproteincomponentsofthenitrogenasecomplexarebeingtransferredintonon-nitrogen-fixingbacteriaandplants.ReducednitrogenintheformofNH4+isassimilatedintoaminoacidsmainlyviaatwo-enzymepathway:glutaminesynthetaseandglutamatesynthase

(anenzymeonlypresentinbacteriaandplants).GlnsynthetaseGluSynthase(+NADPH+ATP)GlnsynthetaseThepathwaysforammoniatoenterorganiccompounds.GluDehydrogenaseVeryminor)

Asnsynthetase

CarbamoylPhosphateSynthetase

Transamination(orNH4+)