生物化学原理课件（英文）：Chapter31 AA Metabolism

1、杨荣武杨荣武 生物化生物化 学原理学原理 第二版第二版 Chapter31 AA Metabolism Outline Hydrolysis of Protein Breakdown of amino acids Synthesis of amino acids and their derivatives Protein degradation Extracellular degradation Intracellular degradation ATP-independent occurs in lysosomes ATP-dependent i) N-end rule and sequen

2、ce ii) The role of Ubiquitin iii) Proteasome iiii) Highly regulated N-end rule A 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

3、lives of 3 min or less PEST proteins N It 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 $Protein

4、s 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 inhibited by glutamine Tyr397 inactivated Regulation of the activity of E.

5、coli glutamine synthetase Nitrogen excretion NH4+ Uric acid Urea + uric acid How are ammonia fed into the urea cycle? The urea cycle The urea cycle Regulation of the urea cycle carbamoyl phosphate synthetase 1, is allosterically activated by N-acetylglutamate N-acetylglutamate is synthesized from gl

6、utamate and acetyl-CoA the remaining enzymes regulated by substrate levels substrate level buildup stimulates urea cycle enzymes product buildup inhibits urea cycle enzymes Amino 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 a