Iron clusters, binuclear, oxidation states

Three oxidations states are potentially available in a binuclear iron center. Enzymes with octahedral fi-o o bridged iron clusters can be isolated in each of the three states the diferric and diferrous states appear to be the functional terminal oxidation states for most of the enzymes, while the mixed valence state may be an important intermediate or transition state for some reactions (Que and True, 1991). In these enzymes the cluster participates primarily as a two-electron partner in the redox of substrates, perhaps using sequential one-electron steps. Without additional coupled redox steps the enzyme is in a new oxidation state after one turnover. In contrast only the diferric and mixed valence oxidation states have been found for 2Fe 2S clusters. The diferrous state may not be obtainable because of the high negative charge on [2Fe 2S(4RS)] versus -1 or 0 net charge for the diferrous octahedral (i.e., non-Fe S) clusters. The 2Fe 2S proteins either are one-electron donor/acceptors or serve as transient electron transfer intermediates. 

Experimental studies [16] show that the best-characterized forms of the soluble MMO (sMMO) contain three protein components hydroxylase (MMOH), so-called B component (MMOB) and reductase (MMOR), each of which is required for efficient substrate hydroxylation coupled to NADH oxidation. The hydroxylase, MMOH, which binds O2 and substrate and catalyzes the oxidation, is a hydroxyl-bridged binuclear iron cluster. In the resting state of MMOH (MMOHqx), the diiron cluster is in the diferric state [Fe -Fe ], and can accept one or two electrons to generate the mixed-valence [Fe -Fe ] or diferrous state [Fe -Fe ], respectively. The diferrous state of hydroxylase (MMOHred) is the only one capable of reacting with dioxygen and initiating the catalytic cycle. 

Recombinant MIOX from Mus musculus kidney was recently subjected to a detailed spectroscopic and kinetic characterization. The nature of the iron active site in the presence and absence of substrate wyo-inositol was examined using both EPR and Mfissbauer methodologies [387]. The outcome of this study has not only confirmed that MIOX contains a binuclear non-heme iron center, but also that its diiron cluster is stable in its mixed-valent form and the fully oxidized state. Specifically, the low-temperature X-band EPR spectrum of fully reduced, diferrous MIOX reveals a geff 16 resonance that disappears upon sample exposure to O2, consistent with a conversion of the diferrous center to its mixed-valent form. The... 

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