Structure of the molybdoferredoxin complex from Clostridium pasteurianum and isolation of its subunits

Highly purified molybdoferredoxin, with a specific activity of 2.6 mumoles of acetylene reduced per min per mg of protein, was obtained from Clostridium pasteurianum. The protein at concentrations above 5 mg/ml exists in solution as a tetrameric complex with two subunits each of about 60,000 and 50,000 daltons. Two atoms of molybdenum are present per protein molecule of 220,000 daltons. The S(0) (20, w) was found to be 10.5. The tetramer dissociates into a dimer as demonstrated by a decreasing sedimentation coefficient with decreasing protein concentration. At low pH and ionic strength, further dissociation into the monomers is achieved. A method for the isolation of the protein subunits is described.