Effects of chloride on the kinetics and stereochemistry of chloroperoxidase catalyzed oxidation of sulfides

At pH 3, chloride dramatically influenced both the K(m) of chloroperoxidase (CPO) for methyl p-tolyl sulfide, which decreased, and its activity, which increased. The K(m) value changed from 75 μM in the absence of chloride to ≤ 1.2 μM in ≥ 0.9 mM chloride, and the k(cat) from 53 s-1 in 0 to 1750 s-1 in 50 mM halide. The k(cat)/K(m) value at 0.9 mM chloride was 414 μM-1 s-1 compared to 0.7 μM-1 s-1 in the absence of the halide. At pH 5, the activating effect was less pronounced. Chloride also acted as inhibitor versus hydrogen peroxide. The data are consistent with a reaction mechanism in which, on one hand, chloride competes with hydrogen peroxide for the native enzyme and, on the other hand, activates sulfide oxidation by binding to CPO Compound I to give a CPO-chlorinating intermediate (EOCl-). However, contrary to what happened in the absence of chloride, where the oxidation was enantioselective and an oxygen atom of H2O2 was incorporated in the sulfoxide (from experiments with 18O-labeled H2O2), in the presence of the halide the oxidation was not enantioselective and there was no incorporation of oxygen from H2O2. The data suggest that sulfide oxidation takes place through an enzyme-generated freely dissociable oxidized halogen intermediate formed by the interaction of EOCl- with Cl-.