Oxygen exchange with water in heme‐oxo intermediates during H₂O₂‐driven oxygen incorporation in aromatic hydrocarbons catalyzed by microperoxidase‐8

Abstract

The present paper describes the oxygen incorporation into naphthalene and anthracene by H2O2-driven microperoxidase-8, forming A-naphthol and anthraquinone, respectively. Microperoxidase-8 is a minienzyme containing a histidinyl-coordinated Fe 3ϩ -protoporphyrin IX cofactor covalently attached to an eight-amino-acid peptide.Additional experiments were performed to investigate whether the reaction mechanism involved is like that of peroxidase and/or cytochrome P-450. A reaction pathway like that of cytochrome P-450 implies oxygen transfer to the substrate from the as yet uncharacterized iron-oxo species formed in the reaction of the heme cofactor with H 2 O 2 . In contrast, a peroxidase-type reaction chemistry involves reaction pathways proceeding by initial one-electron oxidation of, or H-abstraction from, the substrate, followed by incorporation of oxygen from sources other than the iron-oxo species, i.e. from other than H 2 O 2 .The results of the present study exclude Fenton-type chemistry and prove that the minicatalyst is able to catalyze the oxygen incorporation by both peroxidase and cytochrome P-450 types of reaction pathways, while exchange occurs between the high-valency iron-oxo species and H 2 O. The mechanistic implications of this exchange for cytochrome P-450 are discussed.

Investigation of the Mechanism of Action of Microperoxidase-11, (MP11), a Potential Anti-cataract Agent, with Hydrogen Peroxide and Ascorbate

“…In enzymatic (peroxidase) systems, in addition to these mechanisms, ascorbate may function as a substrate speeding up peroxide decomposition. Consistent with these conclusions are recent observations that ascorbate effectively blocks microperoxidase-8 catalysed oxidation reactions (Osman et al, 1996(Osman et al, , 1997Dorovska-Taran et al, 1998). In contrast to other peroxidase substrates, which produce free radicals and react with oxygen, oxidation of ascorbate produces a relatively unreactive ascorbate radical.…”

“…The likely mechanism for the formation of this species is self-oxygenation, since it is known that microperoxidases utilize H 2 O 2 to oxygenate many organic substrates (Mashino, Nakamura and Hirobe, 1990;Osman et al, 1996Osman et al, , 1997Dorovska-Taran, 1998). It has been reported that ascorbate does not inhibit oxygenation reactions catalysed by MP8, although it blocks MP8 peroxidase activity (Osman et al, 1996).…”

“…In MP11 the ®fth coordination position is occupied by the His-18 imidazole ligated axially to the iron and the sixth coordination position is occupied by H 2 O. In a number of studies it has been shown that in the presence of H 2 O 2 microperoxidases catalyse oxidation and oxygenation reactions (Osman et al, 1996(Osman et al, , 1997Dorovska-Taran et al, 1998). It is because of such reactions and because of their uncomplicated structure, that MP11 and related hemepeptides are useful in modeling different hemoprotein functions, in the study of the role of heme environment in enzymatic activity, and as potential anti-cataract agents.…”

Preparation of human drug metabolites using fungal peroxygenases

“…Recent studies, however, have demonstrated that this approach needs further optimization [10]. Alternatively, modified hemoproteins such as microperoxidases might be used to catalyze hydroxylations by a P450-like oxygen transfer mechanism, but so far these catalysts do not exhibit the necessary performance and selectivity [11][12][13][14][15][16].…”

Enzymatic hydroxylation of aromatic compounds

“…Upon reaction with peroxides, it forms high-valency, oxo-ferryl intermediates that are analogous to Compounds I and II and can take effect in peroxidase or P450 modes [181,182]. With respect to the latter mode, MP-8 was shown to hydroxylate aniline, phenol, and anthracene, as well as to some extent naphthalene, but not benzene [183,184]. Hydroxylation was favored over peroxidation after addition of ascorbate to the reaction mixture preventing the coupling of phenoxyl radicals [183].…”