Sandbox 35: Difference between revisions
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==Catalytic Mechanism== | ==Catalytic Mechanism== | ||
[[Image:Papainmech6.jpg|200px|left|thumb| General mechanism of papain catalysis<ref>[http://chemistry.umeche.maine.edu/CHY431/Peptidase10.html] University of Maine</ref>.]] Papain's catalytic mechanism is like serine proteases. Its catalytic triad of residues Cys 25- His159- Arg-175 appear to work with a fourth residue, Gln-19, suspected to be involved in oxyanion hole formation. When a peptide binds to the active site, His-159 deprotonates Cys-25 which in turn attacks the substrate carbonyl carbon. The oxyanion hole then stabilizes the resultant covalent, tetrahedral intermediate. Subsequently, nitrogen in the peptide bond in protonated by His-159 (acting as an acid). This action frees the C-terminal portion of the peptide so that it is released. The entrance of water into the active site then attacks the carbonyl carbon while it is deprotonated by His-159, resulting in another tetrahedral covalent intermediate stabilized through the oxyanion hole. At the end, carbonyl reformation and the Cys-25 sulfur action as the leaving group releases the N-terminal portion of the peptide and later renegerates the enzyme. <ref>[http://chemistry.umeche.maine.edu/CHY431/Peptidase10.html] University of Maine</ref> | [[Image:Papainmech6.jpg|200px|left|thumb| General mechanism of papain catalysis<ref>[http://chemistry.umeche.maine.edu/CHY431/Peptidase10.html] University of Maine</ref>.]] Papain's catalytic mechanism is like serine proteases. Its catalytic triad of residues Cys 25- His159- Arg-175 appear to work with a fourth residue, Gln-19, suspected to be involved in oxyanion hole formation. When a peptide binds to the active site, His-159 deprotonates Cys-25 which in turn attacks the substrate carbonyl carbon. The oxyanion hole then stabilizes the resultant covalent, tetrahedral intermediate. Subsequently, nitrogen in the peptide bond in protonated by His-159 (acting as an acid). This action frees the C-terminal portion of the peptide so that it is released. The entrance of water into the active site then attacks the carbonyl carbon while it is deprotonated by His-159, resulting in another tetrahedral covalent intermediate stabilized through the oxyanion hole. At the end, carbonyl reformation and the Cys-25 sulfur action as the leaving group releases the N-terminal portion of the peptide and later renegerates the enzyme. <ref>[http://chemistry.umeche.maine.edu/CHY431/Peptidase10.html] University of Maine</ref> | ||
