7-Iron ferredoxin revisited.

The crystal structure of the 7Fe ferredoxin from Azotobacter vinelandii has been redetermined using area detector data to 2.7-A resolution and a new derivative. Tetragonal crystals of the protein were maintained at pH 8.0. The results show that the structure previously reported was in error and confirms a recent independent report of the structure (Stout, G.H., Turley, S., Sieker, L. C., and Jensen, L. H. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, in press). The protein fold is similar to the homologous 8Fe ferredoxin structure for the N-terminal half of the protein; the C-terminal residues wrap around this structure. The structure contains a 3Fe cluster coordinated by cysteines 8, 16, and 49 and a 4Fe cluster coordinated by cysteines 20, 39, 42, and 45. However, there are two free sulfhydryls, cysteines 11 and 24, in the new model. Cysteine 24 is in contact with the [4Fe-4S] cluster. Cysteine 11 is shielded from solvent by residues 86-90.