Kinetics of the oxidation of ascorbic acid, ferrocyanide and p‐phenolsulfonic acid by chloroperoxidase compounds I and II

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Abstract
For the first time elementary reactions involving chloroperoxidase compounds I and II have been investigated. A multi‐mixing stopped‐flow apparatus was used to study the kinetics of the reactions of compounds I and II with ascorbic acid, ferrocyanide and p‐phenolsulfonic acid. The second‐order rate constants of the reactions of both compounds with all three substrates were determined between pH 3 and pH 7. In all cases the rate constants decrease with increasing pH. The reactions of p‐phenolsulfonic acid are influenced by a catalytically important group on both compounds I and II with a pKa of 3.7 ± 0.2. With ascorbic acid and ferrocyanide as substrates, a decrease in rate was observed upon ionization of the substrate. Comparisons with horseradish peroxidase show that chloroperoxidase is a much less efficient peroxidatic enzyme. The kinetic data were used to calculate the percentage composition of the mixture of chloroperoxidase species which contribute to the spectra measured during the turnover with ascorbate as substrate.