Nitrogenase, reaction catalyzed

ParGnthGSGs indicate reactions catalyzed by the alternative nitrogenases. Data taken from reviews 13, 123, 124) and Ref. 125). 

There has been considerable interest in the chemistry of molybdenum in recent years, in part due to its occurrence in nitrogenase which catalyzes the reaction ... 

Although several of the reactions catalyzed by molybdoenzymes are classified as dehydrogenases, all of them except nitrogenase involve H20 as either a reactant or a product. The EXAFS spectra suggest that the Mo(VI)02 unit is converted to Mo(IV)0 during reaction with a substrate Sub (Eq. 16-65, step a). Reaction of the Mo(IV)0 with water (step b) completes the catalysis. 

Figure 1. The Nitrogenase Reaction. The electron transfer proteins ferredoxin (Fd) and flavodoxin (Fid) serve to couple the nitrogenase reaction to metabolically generated reducing equivalents. Ammonia synthesis requires 8 electrons 6 for the reduction of dinitrogen and 2 for the coupled, obligatory synthesis of H2. These reactions are catalyzed by the terminal component in the complex, the MoFe-protein. The electrons are transferred to the MoFe-protein from the Fe-protein in a process coupled to the hydrolysis of 2ATP/electron (Howard and Rees, 1994,1996).

The fundamental nitrogenase reaction is the reduction of molecular nitrogen to ammonia and it takes place in certain very primitive microorganisms. The enzyme nitrogenase that catalyzes the reduction... 

Nitrogenases represent a class of complex metalloen-zymes that catalyze the process of biological N2 fixation, during which dinitrogen is reduced to ammonia in a nucleotide-dependent process. The ammonia produced is subsequently used for cell growth or maintenance. The overall reaction catalyzed by nitrogenases is usually depicted as follows... 

In contrast, two of the best known aqueous systems that reduce N2 are based on vanadium (55, 56) but are difficult to characterize and mechanistic conclusions are often controversial. Of these, the V(II) catechol system, which functions at alkaline pH, provides a good analog for the reactions catalyzed by nitrogenase. In the absence of N2 this system reduces protons to H2. As with nitrogenase, this reaction, which is inhibited by N2 and the limiting stoichiometry that occurs at room temperature and pressure, is... 

The two-component Mo-nitrogenase enzyme catalyzes the reduction of N2 to 2NH4 + as its physiological reaction. Concomitant with the reduction of N2, H2 evolution occurs, with electrons supplied by the same reductants that reduce N2. The limiting stoichiometry appears to be... 

At the same time, this redox lability makes Mo well suited as a cofactor in enzymes that catalyze redox reactions. An example is the prominence of Mo in nitrogen fixation. This prokaryotic metabolism, the dominant pathway for conversion of atmospheric Nj to biologically-useful NH, utilizes Mo (along with Fe) in the active site of the nitrogenase enzyme that catalyzes Nj reduction. Alternative nitrogenases that do not incorporate Mo have been identified, but are markedly less efficient (Miller and Eady 1988 Eady 1996). 

In addition to the physiological reaction of N2 reduction, nitrogenase catalyzes a wide variety of reactions involving small unsaturated molecules(56). Table III lists key reactants and products for FeMo nltrogenases. All substrate reductions involve minimally the transfer of two electrons. Multielectron substrate reductions may involve the accretion of such two-electron... 

The prosthetic group associated with the molybdenum atom of the molybdenum cofactor found in most molybdenum-containing enzymes except nitrogenase (See Molybdenum Cofactor). Many of these enzymes catalyze two-electron redox reactions involving the net exchange of an oxygen atom between the substrate and water. In bacterial enzymes a nucleotide is linked to the phosphoryl group. 

Nitrogenase (ferredoxin) [EC 1.18.6.1] catalyzes the reaction of three reduced ferredoxin molecules with protons, N2, and n ATP molecules to produce three oxidized ferredoxin molecules, two ammonia molecules, n ADP molecules, and n orthophosphate molecules where n is between 12 and 18. This iron-sulfur system also uses either molybdenum or vanadium ions. (2) Nitrogenase (flavodoxin) [EC 1.19.6.1] catalyzes the reaction of six reduced flavodoxin molecules with protons, N2, and n ATP molecules to produce six oxidized flavodoxin molecules, two ammonia molecules, n ADP molecules, and n orthophosphate molecules. This system uses iron-sulfur and molybdenum ions. 

---