Table 3.

Values of Apparent pK_a Obtained by Kinetic Measurements of Catalysis by HCA II and Variants

Enzyme	pKa ZnH2Oa	pKa ZnH2Ob	pKa His64b
wild type	6.9	6.8	7.2
Y7Fc	7.1	7.0	6.0
N67Q	6.5	6.7	6.6
Y7F-N67Qd	6.9	6.3	6.2

^aMeasured from a fit of the data of Figure 2 to a single ionization. The standard errors in pK_a are mostly ± 0.1 and no greater than ± 0.2.

^bMeasured from the fits of eq 6 to the data of Figure 3. The values of pK_a have standard errors no greater than ± 0.2.

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

^dThese data at 10 °C.