南京農(nóng)業(yè)大學(xué)生化氨基酸代謝5-12-1課件

第12章氨基酸代謝第一節(jié)Thenitrogencycle

Nitrogenexistspredominantlyinanoxidizedstateintheenvironment,occurringprincipallyasN2intheatmosphereorasnitrateion(NO3-)inthesoilsandoceans.Itsacquisitionbybiologicalsystemsisaccompaniedbyitsreductiontoammoniumion(NH4+)andtheincorporationofNH4+intoorganiclinkageasaminogroup.ThereductionofNO3-toNH4+occursingreenplants,variousfungi,andcertainbacteriainatwo-stepmetabolicpathwayknownasnitrateassimilation.第一節(jié)Thenitrogencycle

TheformationofNH4+fromN2gasistermednitrogenfixation.N2fixationisanexclusivelyprokaryoticprocess.Noanimalsarecapableofeithernitrogenfixationornitrateassimilation.Animalsreleaseexcessnitrogeninareducedform,eitherasNH4+orasorganicnitrogenouscompoundssuchasurea.ThereleaseofNoccursbothduringlifeandasaconsequenceofmicrobialdecompositionfollowingdeath.第一節(jié)ThenitrogencycleDietaryproteinsaredigestedintoaminoacidsinthegastrointestinal(胃腸)tractviatheactionofpepsin,trypsin,chymotrypsin,carboxypeptidasesandaminopeptidases.SourcesofaminoacidsforanimalsProteins(butnotpepsin)unfoldedAbsorbedastri-&dipeptides,andaminoacidsDegradation&absorptionofdietaryproteinsPepsin:thefirstenzymediscovered(18thcentury).proteasesEssentialaminoacidsAminoacidscannotbestoredinanimals:excessbeingcompletelyoxidizedtoreleaseenergyorconvertedtostorablefuels(fattyacidsorcarbohydrates).

OverallfateofexcessaminoacidsGlu+NAD(P)+H2O

a-KG+NH4+

+NADH(P)

+H+

2.脫氫酶作用-GDH一.氨的去路

3.轉(zhuǎn)氨基作用谷丙轉(zhuǎn)氨酶催化的轉(zhuǎn)氨基作用機(jī)理

一.氨的去路

在氨基酸脫羧酶催化下進(jìn)行脫羧作用，生成一個伯胺類化合物和CO2，其反應(yīng)可以用下式表示二.

脫羧基作用

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

氨基酸脫羧酶

1.進(jìn)入TCA循環(huán)

Oxidationofthecarbonskeletonsofaminoacidsinmammals3.轉(zhuǎn)變?yōu)樘呛椭?/p>

當(dāng)體內(nèi)不需要將α-酮酸再合成氨基酸，并且體內(nèi)的能量供給充足時(shí)，α-酮酸可以轉(zhuǎn)變?yōu)樘腔蛑尽＠?，用氨基酸飼養(yǎng)患人工糖尿病的狗，大多數(shù)氨基酸可使尿中的葡萄糖的含量增加，少數(shù)幾種可使葡萄糖及酮體的含量同時(shí)增加。在體內(nèi)可以轉(zhuǎn)變?yōu)樘堑陌被岱Q為生糖氨基酸，按糖代謝途徑進(jìn)行代謝；能轉(zhuǎn)變?yōu)橥w的氨基酸稱為生酮氨基酸。

三.氨基酸碳架的分解

硝酸鹽還原分兩步進(jìn)行：第一步在硝酸還原酶（nitratereductase,NR）催化下，由NAD（P）H提供1對電子，硝酸鹽被還原為亞硝酸鹽，第二步是在亞硝酸還原酶（nitritereductase,NiR）下，由還原型鐵氧還蛋白（Fdred）提供3對電子，使亞硝酸鹽（NO2-）還原成氨。

第三節(jié)NitratereductionAmmoniumentersorganiclinkageviathreemajorreactionsthatarefoundinallcells.Theenzymesmediatingthesereactionsare：(1)Cabamoyl-phosphatesynthetaseI(氨甲酰磷酸合成酶)(2)Glutamatedehydrogenase（谷氨酸脫氫酶）,(3)Glutaminesynthetase（谷氨酰氨合成酶）.

第四節(jié)AmmoniumassimilationNH4+inhepatocytes(肝細(xì)胞)isconvertedintoureaforexcretionviatheureacycleinmostterrestrialvertebratesUreaisformedfromammonia,CO2(asbicarbonate)andAsp.ThepathwaywasalsodiscoveredbyHansKrebsin1932(fiveyearsbeforehediscoveredthecitricacidcycle).FourATPmoleculesareconsumedtoproduceeachurea.ThesynthesisofCarbamoyl（氨甲酰）

phosphaterequirestwoactivationsteps,consumingtwoATPmolecules:oneforactivatingHCO3-,theothertophosphorylatecarbamate.

ananhydride1.Carbamoyl-phosphatesynthetaseI

該反應(yīng)消耗2個ATP分子中的兩個高能磷酸鍵，其中1個是用于活化HCO3-，另1分子ATP則用于磷酸化氨甲酰基。第四節(jié)AmmoniumassimilationFumarateisconvertedbacktoAspviaapartialusageofthecitricacidcycle.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第四節(jié)AmmoniumassimilationTheglutamatedehydrogenasereaction第四節(jié)Ammoniumassimilation3.Glutaminesynthetase(GS)GlutaminesynthetasecatalysestheATP-dependentamindationofthe-carboxylgroupofglutamatetoformglutamine.GSactivitydependsonthepresenceofdivalentcationssuchasMg2+.GlutamineisamajordonorinthebiosynthesisofmanyorganicNcompoundsandGSactivityistightlyregulated.

GDHandGSareresponsibleformostoftheammoniumassimilatedintoorganiccompounds.第四節(jié)Ammoniumassimilation谷氨酰胺合成酶谷氨酰胺合成酶第四節(jié)AmmoniumassimilationTheGlutaminesynthetaseisaprimaryregulatorypointinnitrogenmetabolism:beingregulatedbyatleasteightallostericeffectorsandreversibleadenylylation.

Thebacterial

glutaminesynthetasehas12subunitsarrangedastworingsofhexamers.Activesites

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

Reductant+a-KG+Gln2Glu+oxidizedredctant

第四節(jié)AmmoniumassimilationTheglutamatesynthasereaction谷氨酸合酶第四節(jié)AmmoniumassimilationOnlycertainbacteriacanfixN2intoammoniaRhizobiaCyanobacteria藍(lán)細(xì)菌根瘤菌第5節(jié)NitrogenfixationThedinitrogenase(固氮酶)complexincertainbacteria(diazotrophs)catalyzestheconversionofN2(azote,“withoutlife”)toNH3,whichistheultimatesourceofnitrogenforallnitrogen-containingbiomolecules.

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

TheHabermethod:N2+3H22NH3G`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高檸檬酸ElectronsaretransferredthroughaseriesofcarrierstoN2foritsreductiononthenitrogenasecomplex.ElectronsaretransferredtoN2boundintheactivesiteofdinitrogenaseviaferredoxin/flavodoxinanddinitrogenaseReductase.N2+8H++8e-+16ATP+16H2O2NH3+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+)

SummaryAminoacidinexcesscanneitherbestored,norexcreted,butoxidizedorconverted.Theaminogroupsandcarbonskeletonsofaminoacidstakeseparatebutinterconnectedpathways.