EXAFS and NRVS reveal a conformational distortion of the FeMo-cofactor in the MoFe nitrogenase propargyl alcohol complex.

We have used EXAFS and NRVS spectroscopies to examine the structural changes in the FeMo-cofactor active site of the α-70(Ala) variant of Azotobacter vinelandii nitrogenase on binding and reduction of propargyl alcohol (PA). The Mo K-edge near-edge and EXAFS spectra are very similar in the presence and absence of PA, suggesting PA does not bind at Mo. By contrast, Fe EXAFS spectra show a clear and reproducible change in the long Fe-Fe interaction at ~3.7 Å on PA binding with the apparent appearance of a new Fe-Fe interaction at 3.99 Å. An analogous change in the long Mo-Fe 5.1 Å interaction is not seen. The NRVS spectra exclude the possibility of large-scale structural change of the FeMo-cofactor involving breaking the μ(2) Fe-S-Fe bonds of the Fe(6)S(9)X core. The simplest chemically consistent structural change is that the bound form of PA is coordinated at Fe atoms (Fe6 or Fe7) adjacent to the Mo terminus, with a concomitant movement of the Fe away from the central atom X and along the Fe-X bond by about 0.35 Å. This study comprises the first experimental evidence of the conformational changes of the FeMo-cofactor active site on binding a substrate or product.