A Model for the Halogenating Agent of Chloroperoxidase
DiNello, R.K., Rousseau, K., and David Dolphin
Ann. N.Y. Acad. Sci.
The ferrihemoproteins catalase, horseradish peroxidase, and chloroperoxidase are characterized by their reactions with hydrogen peroxide. In each case the reaction brings about a two-electron oxidation of the resting enzyme to a so-called primary compound (also known as compound I). In each case the primary compound is green in color and it has often been pointed out how closely related these enzymatic systems are. Nonetheless, the chemistry of the primary compounds of the above three enzymes is very different. Thus, the primary compound of catalase reacts with a further molecule of hydrogen peroxide, which it oxidizes to molecular oxygen. The primary compound of horseradish peroxidase is unreactive toward additional hydrogen peroxide and instead oxidizes a variety of organic substrates by two one-electron steps. Chloroperoxidase once again has different chemistry associated with its highest oxidation state. It carries out a two-electron oxidation of a halide ion to the corresponding halonium ion. Using model systems the authors have suggested that oxidation of a bromide ion by the Co(III)OEP π-cation radical is a facile reaction, and may well mimic that carried out by chloroperoxidase.