Biochemistry of Nitrogenase and the Physiology of Related Metabolism [and Discussion]

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$_2$H$_2$ to C$_2$H$_4$, but the vanadium enzyme is less efficient in the last two reactions. The MoFe protein is an $\alpha_2$$\beta_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.