Lecture 11 aa catabolism - BergTymoczkoStryer Biochemistry...

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Biochemistry Chapter 23: Protein Turnover and Amino Acid Catabolism Copyright ©  by W. H. Freeman and Company   Berg • Tymoczko • Stryer
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Three sources of amino acids: 1) Digestion of dietary proteins 2) Degradation of cellular proteins (misfolded or damaged; regulation) 3) Biosynthesis Amino acids building blocks for synthesis of proteins and nitrogenous compounds (nucleotide bases) Cannot be stored (in contrast to carbohydrates and fatty acids) Urea Major Metabolic intermediates Glucose TCA
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Diet is an important source of essential amino acids some amino acids cannot be synthesized and must be obtained from the diet Nine essential amino acids in humans:
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Digestion of dietary proteins The acidic environment of the stomach promotes protein denaturation Denaturation of proteins into random coils increases accessibility of peptide bond
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Action of digestive proteases Endopeptidases Exopeptidases NH3+ C R H C=O NH H C=O R C NH H R C COO- Aminopeptidase Carboxypeptidase Pepsin: R=aromatic Trypsin: R= Arg, Lys Chymotrypsin: R= aromatics NH H C=O R C Endopeptidases: hydrolyze peptide bonds within chains
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Proteolytic Enzymes are synthesized as inactive precursors, zymogens Zymogen (proenzyme) --------> active enzyme Pepsinogen --------> pepsin H + Trypsinogen --------> Trypsin Enteropeptidase Chymotrypsinogen --------> chymotrypsin Procarboxypeptidase --------> carboxypeptidase trypsin In stomach: In intestine: Autocatalytic activation
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Zymogens are activated by proteolytic cleavage Fig. 10.23 Protein digestion is more likely to be severely affected in an individual lacking the enteropeptidase. Why?
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Degradation of cellular proteins misfolded, damaged, regulation Ubiquitin tags proteins for destruction Ub is a small protein Ub is covalently linked to lysine residues on the protein targeted for destruction
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Ubiquitin conjugation E1: ubiquitin activating enzyme E2: ubiquitin conjugating enzyme E3: ubiquitin protein ligase
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Tagging proteins with ubiquitin requires ATP E1 adenylates and activates Ub using ATP then transfers the Ub to one of its own cysteine residues (thioester bond E1: ubiquitin activating enzyme
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Lecture 11 aa catabolism - BergTymoczkoStryer Biochemistry...

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