Inhibitor binding to the binuclear active site of tyrosinase: temperature, pH, and solvent deuterium isotope effects

Competitive inhibition of the monophenolase reaction of tyrosinase by a variety of compounds was investigated with respect to temperature and pH. Derivatives of benzoic acid as well as p-nitrophenol were the best inhibitors. Toluene and several N-heterocycles were all weak inhibitors. Thermodynamic parameters of toluene inhibition were qualitatively different from those of aromatic acids and were consistent with a hydrophobic binding site for toluene. Inhibition by both aromatic acids and p-nitrophenol was strongly pH-dependent over the range 5.1-8.0, with inhibitor binding favored at lower pH. In contrast, toluene binding and indazole binding were pH-independent while benzimidazole binding was favored at higher pH. For both carboxylic acids and p-nitrophenol, inhibitor binding was quantitatively accounted for by assuming the protonated, electrically neutral form of the inhibitor is the reactive species. A solvent deuterium isotope effect on the binding of benzoic acid was observed. A proton inventory study indicated that a single proton transferred from benzoic acid to a site with a fractionation factor of 0.64 +/- 0.02 accounts for the isotope effect. The identity of the proton acceptor and possible mechanistic implications are discussed.