Lambeir_1983_J.Biol.Chem_258_13558

The chloroperoxidase-peracetic acid-chloride-2-chlorodimedone system is used as a model for the investigation of enzymatic halogenation reactions. Systematic variation of the concentrations of the three substrates under steady state conditions yields sets of kinetic parameters containing both kinetic and mechanistic information. Three distinct enzyme species are involved in the halogenation cycle: native enzyme, compound I, and a short-lived halogenating intermediate. Analysis of the kinetic data is complicated by the fact that chloride serves as a substrate in the second step and as an inhibitor of the first step of the overall halogenation reaction. The inhibitor binding site on the native enzyme must be protonated prior to the binding of chloride. Chloride appears to be a competitive inhibitor for both compound I formation and cyanide binding to chloroperoxidase. Only the latter reaction can be studied directly in stopped-flow experiments since compound I disappears rapidly by reacting with chloride present in solution. Rate constants are calculated for the individual steps of the reaction at four different pH values. The rate constant for compound I formation is independent of pH, but the actual rate is reduced at lower pH values due to pH dependence of the chloride inhibition. The rate constant for the oxidation of chloride by compound I decreases with increasing pH. The rate of the halogenation step appears to be independent of pH.