Evidence for a Catalytic‐Centre Heterogeneity of Molybdoferredoxin from Clostridium pasteurianum

Molybdoferredoxin, the high‐molecular‐weight component of clostridial nitrogenase has been separated into two components by gradient chromatography on DEAE‐cellulose. One component when combined with azoferredoxin, the low‐molecular‐weight component of nitrogenase, does not reduce acetylene, evolve hydrogen from dithionite or hydrolyse ATP; the other does. Compared with the active component, the inactive species lacks molybdenum and approximately two thirds of the iron. Both species can be identified by their electron paramagnetic resonance spectra. Active molybdoferredoxin has resonances at g = 4.27, 3.78 and 2.01, whereas the inactive species has signals at g = 2.05 and 1.92. Apart from the difference in their catalytic centre, the protein moiety of both species seems to be identical as based on subunit composition, sedimentation and electrophoretic behaviour, amino acid composition and immunological reaction with a molybdoferredoxin antiserum. The method currently employed for purification of nitrogenase proteins and the fractionation of molybdoferredoxin is described. The consequence of this finding influences previously reported physico‐chemical data of molybdoferredoxin.