Table 3.

Maximal values of rate constants for the hydration of CO₂ and dehydration of bicarbonate catalyzed by variants of cam, and related kinetic pK_a values.^a

Variant	kcatexch/KeffCO2 (µM−1 s−1)	pKa b	kB (ms−1)	pKZnH2O c	pK donor c
Wild-type	14 ± 1	5.7, 8.1	65 ± 7	6.0	8.6
W19N	1.2 ± 0.1	5.3, 8.4	4.9 ± 0.5	5.7	7.8
W19F	0.9 ± 0.1	5.7, 8.2	13 ± 4	6.0	7.2
Y200A	4.0 ± 0.4	5.2, 7.8	30 ± 3	5.7	8.3
Y200F	3.7 ± 0.4	5.6, 8.1	150 ± 60	6.5	6.8

^aObtained from ¹⁸O exchange data under the conditions of Figure 7 by published procedures (17–19).

^bThe standard errors are 0.3 pK units or less. These data were obtained from values of k_cat^exch/K_eff^CO2 using a fit to the following: (k/K)_obs = (k/K)₁/(1 + 10^pK1-pH) + (k/K)₂/(1 + 10^pK2-pH).

^cThe standard errors are 0.3 pK units or less. These data were obtained from values of the rate constant for release of ¹⁸O-labeled water from the enzyme R_H2O/[E] using a fit to the following: R_H2O/[E] = k_B / ([1 + (K_a)_His64 /[H⁺]][1 + [H⁺]/(K_a)_ZnH2O]).