Chloroperoxidase

Abstract The substrates for chloroperoxidase can be divided into three groups. Group 1 substrates react with chloroperoxidase in the presence of peroxide and a halogen anion (chloride, bromide, or iodide) to form stable halogenated products. Group 2 substrates are oxidized in a halogen anion-dependent reaction. The pH optima for the reactions of chloroperoxidase with both Groups 1 and 2 substrates are below 4. Group 3 substrates are oxidized in the absence of a halogen anion. This category includes classical peroxidase substrates such as guaiacol, pyrogallol, and leucomalachite green. The pH optima for Group 3 reactions extend over a broad range from pH 4 to 7, depending on the particular substrate being tested. Chloroperoxidase also catalyzes reactions previously thought to be peculiar to catalase. In the absence of donors, chloroperoxidase reacts catalatically with hydrogen peroxide to form oxygen. At pH 4.5, the second order rate constant for this reaction is 2 x 105 m-1 sec-1. Although this represents only about 2% of the rate observed with catalases, it is several hundred times more effective than horseradish peroxidase. The enzyme also oxidizes ethanol to form acetaldehyde, a reaction not catalyzed by horseradish peroxidase. Evidently the four-subunit structure of catalase is not required for these activities since chloroperoxidase exists as a monomeric protein with one heme group per molecule. Chloroperoxidase also has the ability to form oxygen from ethyl hydrogen peroxide and from m-chloroperbenzoic acid. Neither catalase nor horseradish peroxidase will catalyze these reactions. By analogy with organic mechanisms for the decomposition of peracids, these results indicate that Compound I of chloroperoxidase may contain a reactive oxygen atom.