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Myoglobin (Mb), normally a carrier for molecular oxygen, can catalyze hydrogen peroxide supported peroxygenation of a variety of substrates, including olefin epoxidation and thioether sulfoxidation. 1 However, the turnover numbers for sulfoxidation by Mb are lower than the values obtained by the incubation with peroxidases. 2 In contrast to the oxidation of alkene mediated by cytochrome c peroxidase (CcP), horseradish peroxidase (HRP) mutants, and chloroperoxidase (CPO) from the fungus Caldariomyces fumago, olefin epoxidation by Mb results in low yield incorporation of peroxide oxygen due to the competitive molecular oxygen incorporation. 2, 3 Catalysis by Mb is presumably associated with an intermediate equivalent to compound I of peroxidase; however, the exact location of one of the two oxidation equivalents has not clearly been defined yet. 4 In order to identify active site residues controlling ferryl …