第二十章氨基酸代谢.pptx

Protein Degradation&AA MetabolismProtein degradationExtracellular degradationIntracellular degradation ATP-independent occurs in lysosomes ATP-dependent i)N-end rule and“PESTPEST”sequence ii)The role of Ubiquitin(泛素）泛素）iii)Proteasome（蛋白酶体）（蛋白酶体）iiii)Highly regulatedN-end ruleA proteins half-life correlates with its N-terminal residue.$Proteins with N-terminal Met,Ser,Ala,Thr,Val,or Gly have half lives greater than 20 hours.$Proteins with N-terminal Phe,Leu,Asp,Lys,or Arg have half lives of 3 min or lessPEST proteinsNIt has also been found that proteins rich in Pro(P),Glu(E),Ser(S)and Thr(T),called PEST proteins,are more rapidly degraded than other proteins.Ubiquitin$Ubiquitin is a highly-conserved,76 residue(8.5 kDa)protein found widely in eukaryotes$Proteins are tagged for selective destruction by ubiquitin$An isopeptide bond links the terminal carboxyl of ubiquitin to the e-amino group of a lysine residue of a condemned protein.$Three enzymes are involved,designated E1,E2&E3.Proteasome20S Large multifunctional protease complex in the cytosol that degrades intracellular ubiquitin-tagged proteins.(Just like a barrel)19S regulatory subunit(like a cap)This regulatory cap complex recognizes multi-ubiquitinated proteins,unfolds them,removes ubiquitin chains,and provides a passageway for threading unfolded proteins into the core complex.26SAA MetabolismAmino Acid Degradation 1)Removal of the -amino group Deamination i)L-Amino Acid Oxidase ii)D-Amino Acid Oxidase iii)Glutamate Dehydrogenase Transamination Combined Deamination 2)Fates of C skeleton ketogenic aa&glycogenic aa 3)Detoxification and Excretion of AmmoniaAmino Acid Synthesis L-Amino Acid Oxidase L-amino acid oxidase is an enzyme involved in amino acid catabolism.It is a flavoprotein-containing enzyme that catalyzes the reaction below,yielding a hydrogen peroxide intermediate.Glutamate dehydrogenaseATP,GTPpromoteADP,GDPactivateTransaminase Transaminase is a name for a category of enzymes involved in exchange of an oxygen from an -keto acid(such as -ketoglutarate)and an amine from an amino acid.Aminotransferases utilize a coenzyme pyridoxal phosphate GPT&GOT Not all amino acids undergo transamination(Thr,Pro,Lys)TransaminasesLysine side chainPyridoxal phosphateTransamination NH2|+HOOC-CH-R O|HOOC-C-R O|+HOOC-C-R NH2|HOOC-CH-RAn example-GOTAmino Acid“X”+-KetoglutarateGlutamate +-Keto Acid“X”-Ketoglutarate +NH4+NAD(P)HCombined Deamination TransaminationGlutamate Dehydrogenase+NAD(P)+Fates of C skeleton$Glucogenic$Ketogenic:Leu&Lys$Both glucogenic and ketogenic:Trp,Thr,Tyr,Ile,Phe(tttip)Metabolic fates of amino groupsDietaryproteinCellular proteinAminoacidsCarbonskeletonNH4+Fates of Ammonium Ion$Direct excretion$Asn(Asn synthetase)-in Plants$Gln(Gln synthetase)-in Animals$Urea or uric acidGlutamine synthetase -Generation of Biologically Active Amide Nitrogen Glutamine synthetase is a dodecamer In animals,the enzyme is a key participant in detoxifying ammonia,particularly in the brain,and in ammonia excretion in the kidney.Accumulation of glutamate and glutamine depletes -ketoglutarate,which would interfere with the citric acid cycle.Glutamine synthetase is tightly regulated.The amide nitrogen of glutamine is used for the synthesis of several amino acids,purine and pyrimidine nucleotides,and amino sugarsRegulation of glutamine synthetaseAllosteric regulation:Cumulative feedback Inhibition$Eight specific feedback inhibitors,which are either metabolic end products of glutamine(tryptophan,histidine,glucosamine-6-phosphate,carbamoyl phosphate,CTP,or AMP)or indicators of the general status of amino acid metabolism(alanine or glycine)$Bind to any of the subunits of the enzyme and at least partially inhibit it.The more inhibitors that bind,the greater the inhibition.Covalent modification:adenylylation$regulatory protein PII$UT:stimulated by ATP&-ketoglutarate;inhibited by glutamine Tyr397inactivatedRegulation of the activity of E.coli glutamine synthetase.GS:Glutamine synthetaseAT:Adenylyl transferaseUT:Uridylyl transferasePII:Regulatory proteinNitrogen excretionNH4+Uric acidUrea+uric acidRegulation of the urea cycleAmino Acid Biosynthesis$Plants and microorganisms can make all 20 amino acids and all other organisms need N metabolites$In these organisms,glutamate is the source of N,via transamination(aminotransferase)reactions$Mammals can make only 10 of the 20 amino acids$The others are classed as essential amino acids and must be obtained in the diet$All amino acids are grouped into families according to the intermediates that they are made fromEssential Amino AcidsAny amino acid for which the corresponding -keto acid is notavailable as an intermediate ofcarbohydrate metabolism.Essential vs Non-essential aa ThreonineThreonine IsoleucineIsoleucine PhenylalaninePhenylalanine MethionineMethionine TryptophanTryptophan ValineValine HistidineHistidine ArginineArginine LysineLysine LeucineLeucine AlanineAlanine AspartateAspartate AsparagineAsparagine CysteineCysteine GlutamateGlutamate GlutamineGlutamine GlycineGlycine ProlineProline SerineSerine TyrosineTyrosineTipMTVHall