Metal and sulfur composition of iron-molybdenum cofactor of nitrogenase.

Abstract
The S content of N-methylformamide solutions of cofactor from Clostridium pasteurianum nitrogenase was determined to be 11.9 (.+-. 0.9) mol/mol Mo. This number was determined radiochemically, using Fe Mo cofactor isolated from Mo-Fe protein from bacteria grown on 35SO4. A total of 3.2 (.+-. 0.2) mol S/mol Mo was present in cysteine and methionine, probably arising from contaminating proteins not intrinsic to the cofactor. Combined with accumulated evidence that is discussed, these results lead to an updated stoichiometry of MoFe6S8or9, not MoFe6S4 as previously thought for this cluster.