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One of the two protein components of the nitrogenase enzyme system (the so-called MoFe protein) contains two Mo atoms and 24 to 32 Fe atoms per 220 to 270,000 molecular weight. Despite many hypotheses about the Mo site and its involvement in dinitrogen reduction, there has been no spectroscopic means of unambiguously probing the state of Mo. In this paper, the results of x-ray absorption spectroscopy studies are described and the analysis of these results provides the first direct evidence of the Mo coordination environment in the MoFe protein from Clostridium pasteurianum nitrogenase. The Mo is found to have primarily S ligation and in the resting state (''semireduced form'') of the protein there appears to be no Mo=O species present. On air oxidation, however, clear evidence is found for the presence of Mo=O. The interaction of the Mo with one other metal, most probably Fe and most certainly not Mo, is also seen and it is suggested that the Mo is present in a Mo--Fe--S cluster not yet adequately modeled by any synthetic inorganic system. 8 figures, 6 tables, 40 references.

The molybdenum site of nitrogenase. Preliminary structural evidence from x-ray absorption spectroscopy