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FIGURE 6-20 The pH dependence of chymotrypsin-catalyzed reactions. (a) The rates of chymotrypsin-mediated cleavage produce a bell-shaped pH-rate profile with an optimum at pH 8.0. The rate (v) being plotted is that at low substrate concentrations and thus reflects the term kcat/Km. The plot can be broken down to its components by using kinetic methods to determine the terms kcat and Km separately at each pH. When this is done (b and c), it becomes clear that the transition just above pH7 is due to changes in kcat, whereas the transition above pH 8.5 is due to changes in 1/Km. Kinetic and structural studies have shown that the transitions illustrated in (b) and (c) reflect the ionization states of the His57 side chain (when substrate is not bound) and the a-amino group of Ile16 (at the amino terminus of the B chain), respectively. For optimal activity, His57 must be unprotonated and Ile16 must be protonated.

FIGURE 6-20 The pH dependence of chymotrypsin-catalyzed reactions. (a) The rates of chymotrypsin-mediated cleavage produce a bell-shaped pH-rate profile with an optimum at pH 8.0. The rate (v) being plotted is that at low substrate concentrations and thus reflects the term kcat/Km. The plot can be broken down to its components by using kinetic methods to determine the terms kcat and Km separately at each pH. When this is done (b and c), it becomes clear that the transition just above pH7 is due to changes in kcat, whereas the transition above pH 8.5 is due to changes in 1/Km. Kinetic and structural studies have shown that the transitions illustrated in (b) and (c) reflect the ionization states of the His57 side chain (when substrate is not bound) and the a-amino group of Ile16 (at the amino terminus of the B chain), respectively. For optimal activity, His57 must be unprotonated and Ile16 must be protonated.

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