Why R-Homocitrate Is Essential to the Reactivity of FeMo-Cofactor of Nitrogenase: Studies on NifV--Extracted FeMo-Cofactor

The dinitrogen-binding site in the Mo-based nitrogenase is FeMo-cofactor, a metallo-sulfur cluster of composition MoFe7S9·R-homocitrate. The NifV- mutant nitrogenase from Klebsiella pneumoniae contains an FeMo-cofactor in which homocitrate has been replaced by citrate (i.e., MoFe7S9·citrate). Both the wild type and mutant cofactors (in the S = 3/2 spin state) can be extracted into N-methylformamide. The extracted cofactors bind one molecule of PhS- at the tetrahedral Fe, and the rate of this reaction depends on what else is coordinated to the cluster. No differences were observed between the reactivities of wild-type and NifV- cofactors with PhS- when they were complexed with CN-, N3-, or H+. However, when imidazole is bound, the kinetics of the reactions of PhS- with the two cofactors are very different. Here we propose that R-homocitrate (but not citrate) can hydrogen bond to the imidazole ligand on Mo, and that this perturbs the electron distribution within the cluster core, and hence its reactivity wi...