Iron EXAFS studies of the ironmolybdenum cofactor of nitrogenase and the 3Fe ferredoxin II of desulfovibrio gigas

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Chemical and physical analyses indicate that the ironmolybdenum cofactor (FeMo(co)) of nitrogenase contains 6–8 mol or iron and 4–6 mol of sulfur per mol of molybdenum. The physical properties of this cofactor suggest that it contains a novel MoFeS cluster. Extended X-ray absorption fine structure (EXAFS) data taken at the Mo edge indicate that the molybdenum has two or three iron atoms and four or five sulfur atoms as nearest neighbors. Several models are consistent with these data. More information concerning the iron environment is needed to better define the structure of the FeMo(co). In this talk, we will present our recent results [1] on the iron edge EXAFS of the FeMo(co) from Azotobacter vinelandii and relate structural information about the iron sites in the cluster. In a related study, we have obtained the iron K-edge EXAFS of the 3Fe ferredoxin II of Desulfovibrio gigas in the oxidized and reduced states [2]. For both states, interpretation of the EXAFS data suggests that the FeS distance is near 2.25 Å, in agreement with crystallographic studies of model compounds and proteins containing 2Fe2S and 4Fe4S centers, as well as with a recent crystallographic study of Azotobacter vinelandii ferredoxin I (D. Ghosh, W. Furey, Jr., S. O'Donnell and C.D. Stout, J. Biol. Chem. 256, 4185 (1981).). The FeFe distance of 2.7 Å, however, agrees with similar distances observed in other FeS centers, but disagrees with the 3Fe cluster in the Azotobacter vinelandii ferredoxin I structure, for which an FeFe distance of 4.2 Å was reported. We conclude that the two 3Fe ferredoxins may have substantially different core dimensions, a possibility apparently unique to 3Fe centers among known FeS systems in proteins. The implication of such structural variation of 3Fe centers in the nitrogenase problem will be discussed.