1、Extracellular degradationIntracellular degradation ATP-independent occurs in lysosomes ATP-dependent i)N-end rule and sequence ii)The role of Ubiquitin iii)Proteasome iiii)Highly regulatedA proteins half-life correlates with its N-terminal residue.$Proteins with N-terminal Met,Ser,Ala,Thr,Val,or Gly
2、 have half lives greater than 20 hours.$Proteins with N-terminal Phe,Leu,Asp,Lys,or Arg have half lives of 3 min or lessNIt 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.$Ubiquitin is a highly-conserved,76
3、residue(8.5 kDa)protein found widely in eukaryotes$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&E3.20S Large multifun
4、ctional 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 unfold
5、ed proteins into the core complex.Amino 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 Ammonia
6、Amino Acid Synthesis 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,GDPactivate Transaminase is a name for a category of enzymes involved in exc
7、hange 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)Lysine side chainPyridoxal phosphate NH2|+HOOC-CH-R O|HOOC-C-R O|+HOOC-C-R NH2|HOOC-CH-
8、RAn example-GOTAmino Acid“X”+-KetoglutarateGlutamate +-Keto Acid“X”-Ketoglutarate +NH4+NAD(P)H TransaminationGlutamate Dehydrogenase+NAD(P)+Fates of Fates of skeletonskeleton$Glucogenic$Ketogenic:Leu&Lys$Both glucogenic and ketogenic:Trp,Thr,Tyr,Ile,Phe(tttip)Metabolic fates of aminoamino groupsDiet
9、aryproteinCellular proteinAminoacidsCarbonskeletonNH4+$Direct excretion$Asn(Asn synthetase)-in Plants$Gln(Gln synthetase)-in Animals$Urea or uric acid -Generation of Biologically Active Amide Nitrogen Glutamine synthetase is a dodecamer In animals,the enzyme is a key participant in detoxifying ammon
10、ia,particularly in the brain,and in ammonia excretion in the kidney.Accumulation of glutamate and glutamine depletes -ketoglutarate,which would interfere with the citric acid cycle.Glutamine synthetase is tightly regulated.The amide nitrogen of glutamine is used for the synthesis of several amino ac
11、ids,purine and pyrimidine nucleotides,and amino sugarsRegulation of glutamine synthetaseAllosteric rergulation:Cumulative feedback Inhibition$Eight specific feedback inhibitors,which are either metabolic end products of glutamine(tryptophan,histidine,glucosamine-6-phosphate,carbamoyl phosphate,CTP,o
12、r 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$UT:stimulated by ATP&
13、-ketoglutarate;inhibited by glutamine Tyr397inactivatedRegulation of the activity of E.coli glutamine synthetase.GS:Glutamine synthetaseAT:Adenylyl transferaseUT:Uridylyl transferasePII:Regulatory proteinNitrogen excretionNH4+Uric acidUrea+uric acid carbamoyl phosphate synthetase 1,is allosterically
14、 activated by N-acetylglutamate N-acetylglutamate is synthesized from glutamate and acetyl-CoA the remaining enzymes regulated by substrate levels substrate level buildup stimulates urea cycle enzymes product buildup inhibits urea cycle enzymes$Plants and microorganisms can make all 20 amino acids a
15、nd 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 fromAny amino acid for which the corresponding -keto acid is notavailable as an intermediate ofcarbohydrate metabolism.
