第二十章氨基酸代谢课件

ProteinDegradation&AAMetabolismProteindegradationExtracellulardegradationIntracellulardegradation

ATP-independentoccursinlysosomes

ATP-dependent

i)N-endruleand“PEST”sequenceii)TheroleofUbiquitin(泛素）

iii)Proteasome（蛋白酶体）

iiii)HighlyregulatedN-endruleAprotein's

half-life

correlateswithitsN-terminalresidue.ProteinswithN-terminalMet,Ser,Ala,Thr,Val,

or

Glyhavehalflivesgreaterthan20hours.ProteinswithN-terminalPhe,Leu,Asp,Lys,orArghavehalflivesof3minorlessPESTproteinsIthasalsobeenfoundthatproteinsrichin

Pro(P),Glu(E),Ser(S)andThr(T),

called

PESTproteins,aremorerapidlydegradedthanotherproteins.UbiquitinUbiquitinisahighly-conserved,76residue(8.5kDa)proteinfoundwidelyineukaryotesProteinsaretaggedforselectivedestructionbyubiquitinAnisopeptidebondlinkstheterminalcarboxylofubiquitintothee-aminogroupofalysineresidueofa"condemned"protein.Threeenzymesareinvolved,designatedE1,E2&E3.Proteasome20S

Largemultifunctionalproteasecomplexinthecytosolthatdegradesintracellularubiquitin-taggedproteins.(Justlikeabarrel)19S

regulatorysubunit(likeacap)This

regulatorycapcomplexrecognizesmulti-ubiquitinatedproteins,unfoldsthem,removesubiquitinchains,andprovidesapassagewayforthreadingunfoldedproteinsintothecorecomplex.26SAAMetabolismAminoAcidDegradation1)Removalofthe-aminogroup

Deaminationi)L-AminoAcidOxidaseii)D-AminoAcidOxidaseiii)GlutamateDehydrogenase

TransaminationCombinedDeamination2)FatesofCskeletonketogenicaa&glycogenicaa3)DetoxificationandExcretionofAmmoniaAminoAcidSynthesisL-AminoAcidOxidase

L-aminoacidoxidase

isanenzymeinvolvedinaminoacidcatabolism.Itisaflavoprotein-containingenzymethatcatalyzesthereactionbelow,yieldingahydrogenperoxideintermediate.GlutamatedehydrogenaseATP,GTPpromoteADP,GDPactivateTransaminase

Transaminaseisanameforacategoryofenzymesinvolvedinexchangeofanoxygenfroman-ketoacid(suchas-ketoglutarate)andanaminefromanaminoacid.Aminotransferasesutilizeacoenzyme－pyridoxalphosphateGPT&GOT

Notallaminoacidsundergotransamination(Thr,Pro,Lys)TransaminasesLysinesidechainPyridoxalphosphateTransamination

NH2|+HOOC-CH-R

O||HOOC-C-R

O||+HOOC-C-R

NH2|HOOC-CH-RAnexample-GOTAminoAcid“X”

+-KetoglutarateGlutamate+-KetoAcid“X”-Ketoglutarate+NH4++NAD(P)HCombinedDeamination

TransaminationGlutamateDehydrogenase+NAD(P)+

FatesofC

skeletonGlucogenicKetogenic:Leu&LysBothglucogenicandketogenic:

Trp,Thr,Tyr,Ile,Phe(tttip)MetabolicfatesofaminogroupsDietaryproteinCellularproteinAminoacidsCarbonskeletonNH4+FatesofAmmoniumIonDirectexcretionAsn(Asn

synthetase)-inPlantsGln(Glnsynthetase)-inAnimalsUreaoruricacidGlutaminesynthetase

-GenerationofBiologicallyActiveAmideNitrogen

GlutaminesynthetaseisadodecamerInanimals,theenzymeisakeyparticipantindetoxifyingammonia,

particularlyinthebrain,andinammoniaexcretioninthekidney.Accumulationofglutamateandglutaminedepletesa-ketoglutarate,

whichwouldinterferewiththecitricacidcycle.Glutaminesynthetaseistightlyregulated.Theamidenitrogenofglutamineisusedforthesynthesisofseveral

aminoacids,purineandpyrimidinenucleotides,andaminosugarsRegulationofglutaminesynthetaseAllostericregulation:

CumulativefeedbackInhibition

Eightspecificfeedbackinhibitors,whichareeithermetabolicendproducts

ofglutamine(tryptophan,histidine,glucosamine-6-phosphate,carbamoyl

phosphate,CTP,orAMP)orindicatorsofthegeneralstatusofaminoacid

metabolism(alanineorglycine)Bindtoanyofthesubunitsoftheenzymeandatleastpartiallyinhibitit.

Themoreinhibitorsthatbind,thegreatertheinhibition.Covalentmodification:

adenylylationregulatoryproteinPIIUT:stimulatedbyATP&a-ketoglutarate;inhibitedbyglutamineTyr397inactivatedRegulationoftheactivityofE.coliglutaminesynthetase.GS:GlutaminesynthetaseAT:AdenylyltransferaseUT:UridylyltransferasePII:RegulatoryproteinNitrogenexcretionNH4+UricacidUrea+uricacidRegulationoftheureacycleAminoAcidBiosynthesisPlantsandmicroorganismscanmakeall20aminoacidsandallotherorganismsneedNmetabolitesIntheseorganisms,glutamateisthesourceofN,viatransamination(aminotransferase)reactionsMammalscanmakeonly10ofthe20aminoacidsTheothersareclassedas"essential"aminoacidsandmustbeobtainedinthedietAllaminoacidsaregroupedintofamiliesaccordingtotheintermediatesthattheyaremadefromEssentialAminoAcidsAnyaminoacidforwhichthecorresponding-ketoacidisnotavailableasanintermediateofcarbohydrate