Dinuclear iron center

Under conditions of copper deficiency, some methanotrophs can express a cytosolic, soluble form of MMO (sMMO) (20-23), the properties of which form the focus of the present review. The sMMO system comprises three separate protein components which have all been purified to homogeneity (24,25). The hydroxylase component, a 251 kD protein, contains two copies each of three subunits in an a 82y2 configuration. The a subunit of the hydroxylase houses the dinuclear iron center (26) responsible for dioxygen activation and for substrate hydroxylation (27). The 38.6 kD reductase contains flavin adenine dinucleotide (FAD) and Fe2S2 cofactors (28), which enable it to relay electrons from reduced nicotinamide adenine dinucleotide (NADH) to the diiron center in the... 

FIGURE 13.2 An EPR-monitored redox titration of an Fe-O-Fe cluster with three stable oxidation states. The dinuclear iron center (= +210 mV and = +50 mV) in Pyrococcus furio-sus ferritin was titrated in the presence of a mediator mix. The fit is based on Equation 13.14. (Data from Tatur and Hagen 2005.)... 

The ferrooxidase site is a dinuclear iron center (see Section 8) in which two iron ions (probably Fe2+) are bound as in Fig. 16-4. They are then converted to Fe3+ ions by 02, which may bind initially to the Fe2+, forming a transient blue intermediate that is thought to have a peroxodiferric structure, perhaps of the following type.71 73 Reaction of this intermediate with H20... 

Figure 16-4 The dinuclear iron center or ferroxidase center of human ferritin based on the structure of a terbium(III) derivative.73 Courtesy of Pauline Harrison.

Reactions with Dioxygen. Despite the tremendous interest in the atom transfer chemistry of synthetic dinuclear iron centers, little is known about their interactions with dioxygen. This chemistry is of extreme interest owing to the demonstrated or postulated interactions of... 

Sanders-Loehr, J., Wheeler, W. D., Shiemke, A. K., Averill, B. A., and Loehr, T. M., 1989, Electronic and Raman spectroscopic properties of oxo-bridge dinuclear iron centers in proteins and model compounds, J. Am. Chem. Soc. 111 808498093. 

Vincent, J. B., G. L. Olivier-Lilley, and B. A. Averill. Proteins containing oxo-bridged dinuclear iron centers A bioinorganic perspective. Chem. Rev. 90 (1990), 1447-1467. 

Comparisons of dinuclear iron centers in ferritins and otHer proteins... 

Figure 17-6 Proposed model for redox-dependent activity of the Fe-Zn and Fe-Fe forms of calcineurin. Native calcineurin isolated from bovine brain contains a dinuclear Fe +-Zn + cluster which is catalytically active. The presence of Zn + in the M2 site precludes further oxidation of the cluster, a property which may prevent oxidative damage to this form of the enzyme. Reduction of the Fe ion by one electron produces the inactive Fe +-Zn + cluster [14]. Calcineurin can also be assembled to contain a dinuclear iron center. Three oxidation states are accessible in this form, of which only the mixed valence oxidation state is catalytically active [35]. The dependence of the activity on the redox state of the Fe-Fe center of calcineurin parallels observations made of calcineurin activity in crude brain extract [48].

In the Desulfovibrio species, a large variety of metalloproteins has been isolated and the rubredoxin- and desulforedoxin-like centers can be found in association with other types of mononuclear or dinuclear iron centers as shown in Section 20.5. 

The crystal structure of the enzyme contains another workhorse dinuclear iron center, although here it is bridged by oxygen donor ligands such as glutamate in 18. 

Studies of ligand binding to NOR and the kinetics of these processes benefit from comparative studies on heme copper oxidases of known structure. Spectroscopic studies suggest that the spin-coupled dinuclear iron center, comprised of the heme 3-FeB entity, is the active site of NOR, and the low-spin hemes b and c have a role in electron transfer to the active site. 

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