Resolution of two subunits from the molybdenum-iron protein of Azotobacter vinelandii nitrogenase

The Mo-Fe protein of Azotobacter vinelandii nitrogenase was fractionated on 9.5 M urea isoelectric focusing gels and gave three distinct bands (alpha', alpha", beta'). Protein focused on nondenaturing gels gave a single brown band, which when excised and refocused on a denaturing gel gave the three-band pattern. Partial trypsin digestion of the subunits and fractionation of the peptides by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the alpha' and alpha" polypeptide moieties were the same. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the alpha' and beta' proteins with appropriate molecular weight standards indicated Mr = 61,000 and 57,000, respectively. This is consistent with an overall alpha 2 beta 2 mass of 236,000 daltons.