... observed rate, kA is the limiting rate at high pH, kH is the rate for singly protonated form, kH2 is the limiting rate at low pH(zerointhiscase),K2 is the dissociation constant for the first ... min)1 for A328W. This activity is titratable, with pKavalues of 6.0–6.6, suggestingthat the catalytic histidine is protonated at pH 5. The existence of activity when the catalytic histidine is ... 5.0. The pKavalues that we found for both kcatand bkcatare consistent with titration of the catalytic histidine, H438(440). The persistence of activity in the presence of the protonated form...