Fe and Mo EXAFS of Azotobacter vinelandii Nitrogenase in Partially Oxidized and Singly Reduced Forms

Fe and Mo K-edge EXAFS spectra of the nitrogenase MoFe protein in the indigo disulfonate (IDS) oxidized form and under slow turnover conditions have been recorded. The EXAFS of the one-electron reduced form E sub(1) was obtained as a difference spectrum between the slow turnover and resting (E sub(0)) spectra. Average Fe-S, Fe-Fe, and Fe-Mo distances of 2.33, 2.60, and 2.66 angstrom, respectively, along with a second Fe-Fe distance at 3.72 angstrom were found for E sub(1). The IDS-oxidized MoFe protein contains partially oxidized "P-clusters". For this sample, average Fe-S, Fe-Fe, and Fe-Mo interactions at 2.31, 2.65, and 2.71 angstrom, respectively, were found along with the long Fe-Fe interaction at 3.74 angstrom. Combination of the current results with previous data on resting and thionin-oxidized nitrogenase shows a general trend - a significant number of the metal-metal distances tend to contract as the enzyme becomes more reduced.