Fe-4S and Clusters

One of the most significant discoveries in Fe-S biochemistry in the last decade has been that of [3Fe-4S] clusters. Beinert and Thomson (114) have summarized the early work leading to the recognition of these clusters. This work has culminated in the proof by protein crystallography of the voided-cubane structure 3 (Fig. 1) for clusters in Azotobacter vinelandii (/4v) Fd I (which also contains one [4Fe-4S] cluster) (11, 13, 14), Desul-fovibrio gigas (Dg) Fd II (12, 15), and aconitase from pig heart (115). In these clusters, Fe-Fe separations occur in the range 2.M-2.77 A. As will be seen, aconitase and Dg Fd II assume particular significance in the context of site-specific properties. 

The formal stoichiometry for the oxidative removal of an Fe atom from a [4Fe-4S] cluster is represented by Reaction 7. It was initially thought 

Cluster ground spin states have been firmly established by magnetization measurements (119). 

Trinuclear clusters have been detected in over 20 proteins as well as a number of enzymes, among them aconitase, beef heart succinate-ubiquinone oxidoreductase (120), Escherichia coli nitrate reductase (121), E. coli fumarate reductase (122), and succinate dehydrogenase (123). Selected instances of the occurrence of [3Fe-4S] clusters are listed in Table II. Because of the paramagnetic ground states of both oxidation levels, these clusters can be uniquely identified by a number of spectroscopic techniques. Among these, Mossbauer spectroscopy in applied magnetic fields (124, 128, 132, 141-143) and low temperature MCD spectroscopy (127, 138, 144-146) are decisive. While there are small spectroscopic differences among certain [3Fe-4S] centers, the similarities dominate and support the essential structure 3 for all. In a number of the earlier papers on protein 

3-Fc sites, the clusters were written as [3Fe-3S] or [3Fe-xS]. There is now reason to believe that all such clusters are properly described as [3Fe-4S]. 

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