Kinetic Analysis and pH Dependence of the Phosphite Dehydrogenase Reaction
The pH-rate profiles for active site mutants Lys76Ala, Glu266G1n, and Arg237Lys are also bell-shaped and the higher pKa of each is 8.4, which is also consistent with that observed for the pH dependence of sulfite inhibition for the wild type enzyme. Interestingly, the acidic limb of the Glu266Gln kcat/Km,HPt profile reveals a pKa of 7.2, which has been attributed to His292. A second variant, G1u266A1a, has a pH profile for kcat which is defined by a single pKa at 6.85 +/- 0.05 which also appears to be due to His292. The decrease in pKa for these mutants compared with the wild type indicates that a hydrogen bond exists between His292 and Glu266, and that interaction has been removed by replacing Glu266 with a neutral residue.
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