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Lecture 13

BCH2011: Textbook summary - Lecture 13

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LECTURE 13 Structure of Chymotrypsin: A representation of primary structure, showing disulfide bonds and the amino acid residues crucial to catalysis. The protein consists of 3 polypeptide chains linked by disulfide bonds. The active-site amino acid residues are grouped together in the 3D structure. Refer to ‘Example of an enzyme mechanism’ slide; picture on the left A depiction of the enzyme emphasizing its surface. The hydrophobic pocket in which the aromatic amino acid side chain of the substrate is bound is shown in blue. Key active site residues, including Ser 195, His , Asp102, are red. Refer to ‘Chymotrypsin is a protease’ slide; picture on the left The polypeptide backbone as a ribbon structure. Disulfide bonds are blue, the three chains are coloured. Refer to ‘Example of an enzyme mechanism’ slide; picture on the right A close up of the active site with a substrate (white and blue) bound. They 195 hydroxyl of Ser attacks the carbonyl group of the substrate (oxygen are red); the developing negative charge on the oxygen is stabilized by the oxyanion hole (amide nitrogens from Ser 195and Gly 193, in purple) The aromatic amino acid side chain of the substrate (yellow) sits in the hydrophobic pocket. The amid nitrogen of the peptide bond to be cleaved (protruding toward the viewer and projecting the path of the rest of the substrate polypeptide chain_ is shown in white. The Chymotrypsin Mechanism Involves Acylation and Deacylation of a Ser Residue: Bovine pancreatic chymotrypsin is a protease, an enzyme that catalyses the hydrolytic cleavage of peptide bonds. This protease is specific for peptide bonds adjacent to aromatic amino acid residues (Trp, Phe, Tyr). Chymotrypsin enhances the rate of peptide bond hydrolysis by a factor of at least 10 . It does not catalyse a direct attack of water on the peptide bond; instead, a transient covalent acyl-enzyme intermediate is formed. The reaction thus has two distinct phases. In the acylation phase, the peptide bond is cleaved and an ester linkage is formed between the peptide carbonyl carbon and the enzyme. In the deacylation phase, the ester linkage is hydrolysed and the nonacylated enzyme regenerated. Kinetic and structural analyses have revealed that the change in Kcat reflects the ionization state of His . The declin
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