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?