Valence MMOH

ENDOR studies revealed at least one nitrogen from histidine residues is present in the ligation sphere (Hendrich et al., 1992) of mixed valence 

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Table 16-2 Mossbauer parameters for the valence localized Fe(III)Fe(IV) centers in R2, MMOH and a model compound... 

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. 

With respect to MMOH (Figure 5) there has been one report dealing with product binding to the Fe(III)Fe(III) as well as to the mixed-valence diiron sites of the enzyme and probing the structural aspects by proton, deuteron and F ENDOR spectroscopy of the nuclei of the products ethanol and trifluoro-ethanol. The results were taken to show that the structures determined for single crystals into which alcohols were introduced by diffusion were indeed the same as the solution structures determined in the ENDOR study. ... 

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