The Molybdenum Site of Nitrogenase. 2. A Comparative Study of Mo-Fe Proteins and the Iron-Molybdenum Cofactor by X-Ray Absorption Spectroscopy

Recently, the Mo environment in lyophilized MoFe protein from Closfridiuni pasteurianum has been structurally characterized by x-ray absorption spectroscopy measurements. As a further step in elucidating the structure and relationship of the Mo environment in other species, the MoFe component from Azotobacter cinelandii has been studied. The K-edge M o x-ray absorption spectra for the intact crystalline MoFe component and for the cofactor (FeMo-co) derived from this component have been recorded. There exists a striking similarity in both the edge and EXAFS regions of the spectra between the Clostridiurn and Azotobacter data. Detailed analysis leads to the conclusion that the Mo environment is not significantly perturbed by the lyophilization process and that the M o environment in the protein from Azofobarfer has the same environment a t the Mo site-an as yet chemically unknown Mo-Fe-S cluster. Finally. the studies shon that the basic featurcs of tlic Mo environment in the cofactor are preserved during the extraction process and that the intact protein and the cofactor share quite similar Mo sites. These data lend quantitative support to the idea of a common Mo site i n the nitrogena5e MoFe proteins.