The Biochemistry of N₂ Fixation

Abstract

The nature of the N2ase reaction is described with respect to electron donation, energy requirement, and reduction characteristics, with particular analysis of the seven classes of substrates reducible by N2ase, a complex of a Mo-Fe and Fe protein. Chemical and physical characteristics of Fe protein and crystalline Mo-Fe protein are summarized. The two-site mechanism of electron activation and substrate complexation is further developed. Reduction may occur at a biological dinuclear site of Mo and Fe in which N2 is reduced to NH3 via enzyme-bound diimide and hydrazine. Unsolved problems of electron donors, ATP function, H2 evolution and electron donation, substrate reduction, N2ase characteristics and mechanism, and metal roles are tabu lated. Potential utilities of N2 fixation research include in creased protein production and new chemistry of nitrogen.'"phe facile reduction of N2 to NH3 by the natural catalyst, nitrogenase, A is not understood and has never been equalled with synthetic cata lysts. Developments of the past decade may change this. A renaissance in N2 fixation research in both biochemistry and inorganic chemistry is providing the first opportunity for bioinorganic interaction between these previously independent approaches to catalytic N2 reduction. Remark able progress-from crude extracts to crystalline protein and from a single N2ase substrate to recognition of seven classes of substrates-in the elu cidation of the complex biochemistry of N2 fixation has been made during the past decade ( Figure 1) (1,2,3 , 4, 5, 6, 7, 8, 9, 10, 11). Moreover, advances in the inorganic chemistry of N2-fixation-synthesis of transition metal complexes of N2 and reduction of N2 under ambient conditionsare yielding the first examples of bioinorganic models.We will summarize the status of the biochemistry of N2 fixation, including nitrogenase (N2ase) reactions and characteristics, with exten-219

Nitrogenase reduction of carbon-containing compounds

“…Propane formed in this reaction is apparently not from the subsequent reduction of propene from the 6 e − /H + reaction. This is further supported by reduction of the 4C-containing nitrile analogues, for which the reaction patterns might be more complex [47]. However, reduction of up to 4C nitriles demonstrates that substantially larger substrates than N 2 and acetylene can be accommodated by the active site of nitrogenase, even though the branched versions were not reduced [47].…”

“…For example, both propyne (CH 3 -C≡CH) and 1-butyne (CH 3 CH 2 -C≡CH) were found to be reduced by 2 e − /H + to the corresponding alkenes at very slow rates [47–49]. The triple bonds in propargyl alcohol and propargyl amine are also reduced by nitrogenase to yield the corresponding double-bond compounds (Table 1), which have provided valuable probes for gaining insight into where and how alkyne substrates interact with FeMo-cofactor during catalysis [50–52].…”

“…However, very small amounts of C2 products, C 2 H 4 and C 2 H 6 , were observed, which might be derived from an insertion mechanism accounting for the isocyanide reduction and coupling [59,62]. Reduction of alkyl nitriles resulted in production of the corresponding alkanes and ammonia as products [47]. However, reduction of acrylonitrile, a better nitrile substrate, can be achieved by 6 e − /H + or 8 e − /H + , which resulted in the production of propene and ammonia, or propane and ammonia, respectively [63,64].…”

Electron Paramagnetic Resonance of Nitrogenase and Nitrogenase Components from Clostridium pasteurianum W5 and Azotobacter vinelandii OP

“…The principal difficulty has been that the proteins are rapidly inactivated by oxygen. We, as well as others, have described the electron paramagnetic resonance (EPR) signals obtained from the MoFe protein at low (<30'K) temperatures, both in vivo (10) and in vitro (10)(11)(12). Because these signals appear to arise from transition metal complexes, and because it has been generally anticipated that the transition metal complexes in nitrogenase are intimately involved in the six-electron reduction of N2 to NH3 (e.g., refs.…”

Catalytic Multielectron Reduction of Hydrazine to Ammonia and Acetylene to Ethylene with Clusters That Contain the MFe3S4 Cores (M = Mo, V)

“…(10) For some of the model reactions, (5,9,11,12) binuclear activation mechanisms also have been proposed. The latter are not as firmly supported as the single metal-atom mechanisms.…”