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Bacterial diheme peroxidases (bCCP) act in the detoxification of reactive oxygen species by reduction of peroxide to water. The substrate H2O2 is bound to the free axial position of a heme cofactor, and in a first step, one H2O molecule is released, while the remaining oxygen is stabilized as a metastable FeivO intermediate that awaits further reduction. Unlike other peroxidases, however, bCCPs employ two heme cofactors, each of which can provide one electron to form the oxoferryl species, so that the formation of a porphyrin radical cation as in the ‘Compound I’ of other peroxidases is not required. This functionality requires precise coordination of electron transfer and substrate binding, and indeed the bCCP family of enzymes in most cases undergo a complex set of redox-dependent structural changes to ensure that the active site heme group only becomes accessible for a substrate after the second heme group is reduced and able to provide an electron to the reaction. Within the family, unsurprisingly, exceptions to this general rule are found and, moreover, the basic structural core of bCCPs has been evolutionarily modified and adapted in several other enzymes to fulfill entirely different functional roles. |
