Rescue of the catalytic activity of an H42A mutant of horseradish peroxidase by exogenous imidazoles.

His-42 plays a critical role in the H2O2-dependent catalytic turnover of horseradish peroxidase (HRP). This is clearly illustrated by the finding that an H42A mutation decreases the rate of Compound I formation by a factor of approximately10(6). As shown here, the addition of 2-substituted imidazoles partially rescues both the rate of formation of Compound I and the peroxidase activity of the H42A mutant. 2-Substituted imidazoles are the most effective because they do not coordinate to the iron. In contrast to native HRP, which exhibits a parabolic pH profile, and the H42A mutant, for which the activity increases linearly with increasing pH, the activity of the H42A mutant in the presence of 1,2-dimethylimidazole (pKa = 8.0) exhibits a sigmoidal pH dependence with a midpoint at pH 8.0 +/- 0.2. Similar results are obtained with 2-methylimidazole. These results establish that the free base forms of these imidazoles facilitate HRP turnover. The spectroscopic binding constants for 1, 2-dimethylimidazole and 2-methylimidazole are Kd = 2.9 +/- 1.3 and 2. 5 +/- 0.2 M, respectively. When cyanide is bound to the heme, the Kd for 1,2-dimethylimidazole is 0.17 M. This >10-fold decrease in Kd may reflect hydrogen bonding of the protonated imidazole to the iron-coordinated cyanide. The log of the rate of Compound I formation exhibits a linear dependence on the molecular volume of the imidazoles used to rescue the activity. If the rates are corrected for differences in the size of the imidazoles, the log of the rates is linearly related to the pKa of the imidazoles. This Brönsted analysis predicts that approximately 60% of a positive charge develops on the imidazole in the transition state of Compound I formation. The results confirm the acid-base role of the distal histidine, demonstrate that exogenous histidines can function as surrogates for the missing histidine in the H42A mutant, and provide a transition state model of relevance to the formation of Compound I in the native protein.