Crystal structure of the all-ferrous [4Fe-4S](0) form of the nitrogenase iron protein from Azotobacter vinelandii

The structure of the nitrogenase iron protein from Azotobacter vinelandii i n the all-ferrous [4Fe-4S](0) form has been determined to 2.25 Angstrom res olution by using the multiwavelength anomalous diffraction (MAD) phasing te chnique. The structure demonstrates that major conformational changes are n ot necessary either in the iron protein or in the cluster to accommodate cl uster reduction to the [4Fe-4S](0) oxidation state. A survey of [4Fe-4S] cl usters coordinated by four cysteine ligands in proteins of known structure reveals that the [4Fe-4S] cluster of the iron protein has the largest acces sible surface area, suggesting that solvent exposure may be relevant to the ability of the iron protein to exist in three oxidation states.