Catalytic Sites of Hemoprotein Peroxidases

The structures of the active sites of horseradish and cytochrome c peroxidase, prototypical peroxidases with an imidazole heme ligand, suggest that small substrates are generally oxidized by peroxidases at the delta-meso edge of the heme group. This inference is supported by experimental results on the Coprinus macrorhizus peroxidase (52), manganese peroxidase (51), lignin peroxidase (50) and, less definitively, lactoperoxidase (90). Macromolecular substrates, exemplified by the cytochrome c peroxidase-cytochrome c interaction, are likely to be oxidized at peroxidase surface sites bearing no specific relationship to the delta-meso heme edge. The second oxidation equivalent in the two-electron Compound I states of the peroxidases is stored either as a porphyrin radical or as a protein radical, although some peroxidases have both types of compound I. The factors that control the location of the second oxidation equivalent remain unclear. Classical peroxidases do not generally catalyze olefin epoxidation and other monooxygenations but do catalyze sulfoxidation reactions. This is best rationalized by physical separation of the substrate from the ferryl oxygen, possibly by a protein barrier, because results with cytochrome c peroxidase show that there is no inherent mechanistic reason for the inability of peroxidases to epoxidize olefins. It is not yet clear why the barrier to oxygen transfer reactions is circumvented during sulfur oxidation reactions, although one possibility is that the relatively stable sulfur cation radical that is initially formed disrupts the barrier. Chloroperoxidase, the principal nonclassical hemoprotein peroxidase so far examined, has an open active site that readily catalyzes P450-like monooxygenation reactions. The active site of chloroperoxidase is a potentially useful model for that of myeloperoxidase, but caution must be used in extrapolating from one to the other because myeloperoxidase has a histidine rather than thiolate fifth heme ligand and therefore is a classical rather than nonclassical peroxidase.