Spectroscopic Characterization of an Eight-Iron Nitrogenase Cofactor Precursor that Lacks the "9^th Sulfur"

Nitrogenases catalyze the reduction of N₂ to NH₄⁺ at its cofactor site. Designated the M-cluster, this [MoFe₇ S₉ C(R-homocitrate)] cofactor is synthesized via the transformation of a [Fe₄ S₄ ] cluster pair into an [Fe₈ S₉ C] precursor (designated the L-cluster) prior to insertion of Mo and homocitrate. We report the characterization of an eight-iron cofactor precursor (designated the L*-cluster), which is proposed to have the composition [Fe₈ S₈ C] and lack the "9^th sulfur" in the belt region of the L-cluster. Our X-ray absorption and electron spin echo envelope modulation (ESEEM) analyses strongly suggest that the L*-cluster represents a structural homologue to the l-cluster except for the missing belt sulfur. The absence of a belt sulfur from the L*-cluster may prove beneficial for labeling the catalytically important belt region, which could in turn facilitate investigations into the reaction mechanism of nitrogenases.