Diferric iron clusters

Methylococcus capsulatus, diferric iron cluster, 43 362-363 mixed-valent state, 43 389 s derived from reaction mechanism, 43 391-393... 

Methylococcus capsulatus, methane monooxygenase, diferric iron cluster, 43 362-363 2-(l-Methylpyridinium-4-yl)-4,4,5,5,-... 

Fig. 2. Diferric iron clusters from ribonucleotide reductase R2 subunit and methane monooxygenase hydroxylase. The drawings are based on (18, 19) for RNR-R2 and (15) for MMOH.

Fig. 1. Diferric iron clusters form hemer3fthrin, ribonucleotide reductase R2 subunit, and methane monooxygenase hydroxylase. The figure was made with the RasMol 2.0 program, and the protein coordinates as PDB files were obtained from Brookhaven Protein Data Bank. Only the amino acids (histidines, green carboxylates, black oxygen, red nitrogen, yellow acetate, blue iron, violet) coordinated to the iron cluster are shown, coordinated waters are not indicated. The first subunit containing the cluster is shown. Diferric Hr is from sipunculid worm Themiste dyscritra). The RNR-R2 is from E. coli. The MMOH is from Methvlococcus caosulatus (Bath).

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. 

FIGURE 8. Postulated mechanism for MMO. The inner cycle are postulated intermediates in the catalytic cycle (only the binuclear iron cluster of the MMOH component is shown). The outer cycle represents the intermediates detected during a single turnover beginning with diferrous MMOH and ending with diferric MMOH. The rate constants shown are for 4 C and pH 7.7. The rate shown for the substrate reaction RH with Q is that for methane. The alignment of the two cycles shows the postulated structures for the intermediates. 

Davydov, A., Davydov, R., Gr%oslund, A., Lipscomb, J. D., and Andersson, K. K., 1997, Radiolytic reduction of methane monooxygenase dinuclear iron cluster At 77K6EPR evidence for conformational change upon reduction or binding of component B to the diferric state, J. Biol. Chem. 272 702267026. 

Structure of the Iron Center Formation of the Iron Center and Tyrosyl Radical Spectroscopy of the Diferric Iron Center Spectroscopy of the Tyrosyl Radical Redox Properties of the Iron Center Mixed-Valent Form of the Iron Center Diferrous Form of the Iron Center Inhibitors to Iron-Containing Ribonucleotide Reductase Methane Monooxygenase A. Spectroscopy of the MMOH Cluster X-Ray Structure of MMOH... 

The main characteristics of the diferric iron-oxygen clusters in these proteins can be discussed in terms of the following ... 

E. coli R2 protein have been reported (68). Parallel-mode EPR experiments showed a signal originating from a ferromagnetically coupled high-spin diferrous cluster, i.e., with S = 4. Azide was shown to bind to the iron cluster, and high concentrations of azide caused a shift as well as an increase in intensity of the EPR signal (68). 

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. 

---