ATP‐Independent Turnover of Dinitrogen Intermediates Captured on the Nitrogenase Cofactor.

Nitrogenase reduces N2 to NH3 at its active‐site cofactor. Previous studies of an N2‐bound Mo‐nitrogenase from Azotobacter vinelandii suggest binding of three N2 species via asymmetric belt‐sulfur displacements in the two cofactors of its catalytic component (designated Av1*), leading to the proposal of stepwise N2 reduction involving all cofactor belt‐sulfur sites; yet, the evidence for the existence of multiple N2 species on Av1* remains elusive. Here we report a study of ATP‐independent, EuII/SO32−‐driven turnover of Av1* using GC‐MS and frequency‐selective pulse NMR techniques. Our data demonstrate incorporation of D2‐derived D by Av1* into the products of C2H2‐ and H+‐reduction, and decreased formation of NH3 by Av1* concomitant with the release of N2 under H2; moreover, they reveal a strict dependence of these activities on SO32−. These observations point to the presence of distinct N2 species on Av1*, thereby providing strong support for our proposed mechanism of stepwise reduction of N2 via belt‐sulfur mobilization. [ABSTRACT FROM AUTHOR]