Chloroperoxidase halogenation reactions. Chemical versus enzymic halogenating intermediates

In the absence of organic substrates, chloroperoxidase catalyzes the peroxidation of chloride and bromide ion to molecular chlorine and bromine. However, these molecular species are not formed as intermediates in the enzymic halogenation of organic halogen-acceptor substrates. The rate of oxidation of chloride to its respective molecular species is considerably slower than the rate of enzymic chlorination of acceptor substrates. Furthermore, differences are observed in substrate specificities between enzymic and chemical halogenation reactions. Thiourea and methionine are substrates in chloride-dependent oxidation reactions catalyzed by chloroperoxidase and are preferred at least 50:1 and 30:1 respectively over 2- chlorodimedone. Corresponding nonenzymic reaction preferences for the oxidation of thiourea versus 2-chlorodimedone chlorination are only 2:1 with hypochlorite and 3:1 with molecular chlorine. Also, hypochlorite shows essentially no preference for methionine compared with 2-chlorodimedone. In the bromide-dependent reactions catalyzed by chloroperoxidase, bromine is formed at a rate equivalent to that of the bromination of acceptor substrates. However, the specificity of the bromide-dependent oxidation of methionine versus the bromination of 2-chlorodimedone by chloroperoxidase is 4:1. This value is significantly higher than the ratio in reactions of these two substrates with molecular bromine, which is essentially 1:1. A general reaction scheme for all reactions of chloroperoxidase with its halogen-acceptor substrates is proposed. This process involves the initial formation of Compound I and its subsequent conversion into an iron (III) hypohalite halogenating intermediate.