Biochemistry of Nitrogenase and the Physiology of Related Metabolism

The properties of the newly discovered vanadium nitrogenase are compared with those of the better-known molybdenum nitrogenase and some aspects of the physiology of the latter are discussed. Both nitrogenases have dimeric Fe proteins of relative molecular mass (M_r) ca. 65 000 containing a single [4Fe-4S] cluster. These act as MgATP-activated electron transfer agents to the MoFe or VaFe proteins, which include the substrate binding and reducing site. Both enzymes reduce H^+ to H_2, N_2 to NH_3 and C_2H_2 to C_2H_4, but the vanadium enzyme is less efficient in the last two reactions. The MoFe protein is an α_2β_2 tetramer of M_r ca. 220 000 and containing 2 Mo atoms and about 30 Fe atoms and S^2- ions per molecule. The VaFe protein has a similar polypeptide structure and may also have an additional, small (M_r backsimeq 6000) ferredoxin-like subunit. Current preparations contain 2 Va atoms and about 20 Fe atoms and S^2- ions in a molecule of M_r ca. 210 000. The active site of the MoFe protein is an iron-molybdenum cofactor of unknown structure and complex biosynthesis. The Lowe-Thorneley model for nitrogenase function is summarized. Ferredoxins or flavodoxins are the physiological electron carriers to molybdenum nitrogenase. Many aerobic diazotrophs have an uptake hydrogenase to recycle the electrons and energy wasted by the obligate H_2 evolution that accompanies N_2 fixation. Both nitrogenases are damaged by O_2, but many diazotrophs are aerobes or generate O_2 from photosynthesis. Some of the complexities of the interactions between O_2 and N_2-fixation are discussed.