Iron EXAFS of Azotobacter vinelandii Nitrogenase Mo-Fe and V-Fe Proteins

The structure of the iron sites of nitrogenase in dithionite-reduced and thionine-oxidized forms of the Mo-Fe and V-Fe proteins has been investigated using Fe K-edge X-ray absorption spectroscopy. For the dithionite-reduced Azotobacter uinelandii Mo-Fe protein, the dominant EXAFS Fourier transform peaks are assigned to F e 4 and Fe-Fe interactions at -2.32 and 2.64 A, as expected for F e S clusters. An additional Fe-Mo component at 2.73 A is required to completely fit the EXAFS in the 1-3-A region. In the 3-5-A region, a 3.8-A Fe-Fe component is identified, with an amplitude corresponding to almost one long Fe-Fe interaction, averaged over all of the iron in the sample. Features that can be explained as F e S and Fe-Fe interactions at 4.3 and 4.7 8, are also observed. A similar pattern of Fe interactions is observed for the reduced A. uinelandii V-Fe protein, except that the short Fe-Mo interaction is no longer required. In both Mo-Fe and V-Fe proteins, the first coordination sphere Fe-S distances contract slightly upon thionineoxidation. The long-range Fe-S and F e F e interactions arevery close (within 0.1 A) to corresponding distances in Fe&6 prismane clusters. If the amplitudes are adjusted by assuming that only 14 of 30 nitrogenase irons participate in the M center, then they are consistent with recently proposed crystallographic models.