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Chymotrypsin Catalytic Triad Mechanism


The serine protease chymotrypsin catalyzes peptide bond cleavage on the carboxyl side of aromatic amino acid residues as seen in the schematic below. The enzyme functions in the intestinal lumen (pH ~8) where it is highly pH dependent.

The catalytic mechanism involves general acid-base chemistry, nucleophilic attack, and the formation of an acyl-enzyme intermediate. Key to this catalytic mechanism is the concerted action of the enzyme’s catalytic triad of Ser195, His57 and Asp102.

When initial velocity (v) is plotted vs. pH, a typical bell-shaped curve is observed with a pH optimum at pH 8 and activity reaching almost zero at pH 6.

Which of the following statements best explains the inhibition of enzyme activity at pH 6?

Plasser, Felix. "Chemical Quantum Images." Chemical Quantum Images. Wikimedia Commons, 9 Feb. 2007. Web. 01 Feb. 2016.


At pH 6, Asp102 cannot H-bond with His57 preventing the nucleophilic attack of His57 on the peptide carbonyl group.


At pH 6, the hydroxyl group of Ser195 is deprotonated preventing the amino acid from participating in general acid attack on His57.


At pH 6, the imidazole ring of His57 is protonated preventing it from participating in a general base attack on Ser195, extracting a proton, and generating a nucleophilic Ser195 alkoxide ion.


At pH 6, the His57 imidazole ring is deprotonated preventing it from participating in ​general acid attack on Asp102, extracting a proton, and generating a nucleophilic alkoxide ion.