EXAFS STUDIES OF THE NITROGENASE IRON PROTEIN FROM AZOTOBACTER-VINELANDII
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We have recently shown that the [4Fe-4S]+ cluster of the iron protein can exist in S = xj2, S = 3/2, and probably S = s/2 spin states, depending on the solvent and on the occupancy of the protein's nucleotide binding sites. We have now examined the structure of the cluster in the reduced (1+) and oxidized (2+) electronic states, the S = l/2 and S = 3/2spin states, and in the MgATP-bound form by using the EXAFS technique. The average calculated Fe-S coordination number for the protein data is 4,0 (5) at a distance of 2.31 (1.5) A with a Debye-Waller factor of 0.06 (1) A. The cluster is probably bound to the protein via four sulfur (cysteine) ligands. Significant differences in the average Fe-S distance of oxidized synthetic clusters and reduced Fe protein clusters were observed. Neither the MgATP-induced conformational change nor the S ~ l/2 to S = 3/2spin-state conversion requires changes in the average Fe-S bond length of more than about 0.02 A. The average calculated Fe-Fe coordination number for the protein data is 2.4 (8), at a distance of 2.73 (3) A. The Debye-Waller factor for the Fe-Fe term of the Fe protein data (0.10 (2) A) is larger than that for the model cluster data (0.08 A), and this dissimilarity results in an underestimate of the Fe-Fe coordination number in the Fe protein cluster. 1987, American Chemical Society. All rights reserved.
