Preparation and Reconstitution of the Severely Metalloclusters-Deficient MoFe Protein of Nitrogenase from Azotobacter vinelandii Studies on the Reconstitution of P-Cluster-Deficient Molybdenum-Iron Protein The Effect of Urea on the Activity and Stability of Metallocluster of the MoFe Protein of Nitrogenase from Azotobacter vinelandii