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 vinelandii Mo-Fe protein, the dominant EXAFS Fourier transform peaks are assigned to Fe-S and Fe-Fe interactions at approximately 2.32 and 2.64 angstrom, as expected for Fe-S clusters. An additional Fe-Mo component at 2.73 angstrom is required to completely fit the EXAFS in the 1-3-angstrom region. In the 3-5-angstrom region, a 3.8-angstrom 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 Fe-S and Fe-Fe interactions at 4.3 and 4.7 angstrom are also observed. A similar pattern of Fe interactions is observed for the reduced A. vinelandii 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 thionine oxidation. The long-range Fe-S and Fe-Fe interactions are very close (within 0.1 angstrom) to corresponding distances in Fe6S6 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.