SOLUTION STRUCT STUDIES OF NITROGENASE PROTEINS BY SMALL ANGLE XRAY SCATTERING

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The FeMo cofactor in the MoFe protein is believed to be the substrate (e.g. atmospheric N2) binding and reducing site in the nitrogenase system. The FeMo cluster, obtained by a different preparation method, is a subset of the FeMo cofactor. The cluster exhibits a very similar S=3/2 EPR signal, and inhibits the insertion of the cofactor back into the cofactor-deficient MoFe protein. However, the cluster does not contain homocitrate, and has one less Fe atom in the first coordination shell and has a change in the long range order. The cluster cannot activate the FeMo-cofactor-deficient form of the MoFe protein. A previous SAXS study shows that the FeMo cofactor solution is not homogenous; the cofactor exists in large aggregates (upto 25 E in Rg) as well as in small oligomeric forms with an average Rg of 6.96 E. We are interested in comparing the aggregation state of the cluster with that of the cofactor by using solution small-angle x-ray scattering.