FIGURE 4.

Values of the rate constant for catalysis by (▪) H64W HCA II; and (•) H64G HCA II as enhanced by the following exogenous proton donors. Values in parentheses are the solution pK_a obtained by titration at 0.2 M ionic strength: (a) 1,2-dimethylimidazole (pK_a 8.3), (b) 2-methylimidazole (8.2), (c) 2-ethylimidazole (8.0), (d) 4-methylimidazole (7.9), (e) 1-methylimidazole (7.3), (f) imidazole (7.0), (g) 3,4-dimethylpyridine (6.7), (h) 4-methylpyridine (6.1), and (i) 3-methylpyridine (5.8). These measurements were made a 25°C. The value of the pK_a of the zinc-bound water in H64W HCA II is 7.3 and for H64G, 7.2 under these conditions determined from its pH profile in k_cat/K_m for hydration of CO₂. Each measurement was made at a pH equivalent to the pK_a of the donor. Ionic strength in each measurement was maintained at a minimum of 0.2 M by addition of sodium sulfate. Solid lines in both plots are fits to the Marcus equation (9) with parameters given in Table 2.