Molybdenum-Nitrogenase: Structure and Function

Abstract

Molybdenum (Mo) was believed, as early as 1930, to be absolutely essential for nitrogen fixation, even though vanadium (V) was found to be almost as stimulatory to bacterial growth on N₂ (1). Recently, however, it has been shown that biological nitrogen fixation can occur without Mo. Nitrogenase is now known to exist in three genetically distinct forms; the conventional Mo-based system (Mo-nitrogenase) and the two alternative systems, V-nitrogenase and nitrogenase-3 (2–4). In general, the enzymes from all N₂-fixing bacteria are very similar, comprising two component proteins. For Mo-nitrogenase, these are the MoFe protein and the Fe protein (5), neither of which has activity alone. The deduced amino-acid sequences of the subunits of the MoFe, VFe and FeFe proteins show significant identity with one another and the conservation of both the domain structures around the eight strictly conserved Cys residues and the spacing between them indicates that all three protein types will have the same general structural features. All N₂-fixing species examined so far have Mo-nitrogenase. In some species, it occurs alone, while in others it is found in all permutations with the V-based and third systems. It is not at all clear why each species has its own combination of these nitrogenases (6). This paper reviews only studies of the Mo-based nitrogenase system.