Table 2.

Maximal Values of Rate Constants for Hydration of CO₂ and Proton Transfer in Dehydration Catalyzed by HCA II and Variants.

Enzyme	kcatexch/KeffCO2 CO2 hydration (μM-1s-1)a	kB proton transfer (μs-1)b
Wild type	120	0.80
Y7Fc	120	3.9
N67Q	50	1.7
Y7F-N67Qd	80	9.0

^aMeasured from the exchange of ¹⁸O between CO₂ and water in the hydration direction (eq 5). Derived for each variant by a fit of the data (as in Figure 2) to a single ionization. The standard errors for these rate constants are generally 15% or less.

^bMeasured from the exchange of ¹⁸O between CO₂ and water (as in Figure 3) using eq 6 in the dehydration direction. The standard errors are 22% or less.

^cData at 10°C from Fisher et al. (11).

^dData at 10°C.