Oxidized and reduced Azotobacter vinelandii ferredoxin I at 1.4 angstrom resolution: Conformational change of surface residues without significant change in the [3Fe-4S](+/0) cluster

The refined structure of reduced Azotobacter vinelandii 7Fe ferredoxin FdI at 100 K and 1.4 Angstrom resolution is reported, permitting comparison of [3Fe-4S](+) and [3Fe-4S](0) clusters in the same protein at near atomic res olution. The reduced state of the [3Fe-4S](0) cluster is established by sin gle-crystal EPR following data collection. Redundant structures are refined to establish the reproducibility and accuracy of the results for both oxid ation states. The structure of the [4Fe-4S](2+) cluster in four independent ly determined FdI structures is the same within the range of derived standa rd uncertainties, providing an internal control on the experimental methods and the refinement results. The structures of the [3Fe-4S](+) and [3Fe-4S] (0) clusters are also the same within experimental error, indicating that t he protein may be enforcing an entatic state upon this cluster, facilitatin g electron-transfer reactions. The structure of the FdI [3Fe-4S](0) cluster allows direct comparison with the structure of a well-characterized [Fe3S4 ](0) synthetic analogue compound. The [3Fe-4S](0) cluster displays signific ant distortions with respect to the [Fe3S4](0) analogue, further suggesting that the observed [3Fe-4S](+/0) geometry in FdI may represent an entatic s tate. Comparison of oxidized and reduced FdI reveals conformational changes at the protein surface in response to reduction of the [3Fe-4S](+/0) clust er. The carboxyl group of Asp(15) rotates similar to 90 degrees, Lys(84), a residue hydrogen bonded to Asp(15), adopts a single conformation, and addi tional H2O molecules become ordered, These structural changes imply a mecha nism for H+ transfer to the [3Fe-4S](0) cluster in agreement with electroch emical and spectroscopic results.