Reduction potentials nitrogenase reaction

Table 9.5 Standard Apparent Reduction Potentials E n (in volts) at 298.15 K of Half-reactions in the Nitrogenase Reaction...

The effects of pH on the standard apparent reduction potentials of the half reactions involved in the nitrogenase reaction are shown in Table 9.5. The effects of pH on the apparent equilibrium constants of the reactions involved in the nitrogenase reaction as shown in Table 9.6. 

Figure 3.7. The energy profile of a nitrogenase reaction. Eo is the standard redox potential of the reactants, intermediates and products of the reaction Fd = ferredoxin FeP = Fe protein FeMo = FeMo protein. The arrow indicates the increase of the reduction potential upon ATP hydrolysis (Likhtenshtein 1988a). Reproduced with permission.

The so-called midpoint potential, Em, of protein-bound [Fe-S] clusters controls both the kinetics and thermodynamics of their reactions. Em may depend on the protein chain s polarity in the vicinity of the metal-sulfur cluster and also upon the bulk solvent accessibility at the site. It is known that nucleotide binding to nitrogenase s Fe-protein, for instance, results in a lowering of the redox potential of its [4Fe-4S] cluster by over 100 mV. This is thought to be essential for electron transfer to MoFe-protein for substrate reduction.11 3... 

Biological N2 fixation is catalyzed by Fe/Mo, Fe/V, or FeFe (the Fe-only) nitrogenases (150, 151). The extremely different reaction conditions of the biological and the Haber-Bosch processes of N2 reduction, that is, standard temperature and pressure and biological redox potentials on the one hand, red-hot temperatures and high pressures on the other hand, make the quest for low molecular weight competitive catalysts particularly challenging. 

While Fe(SCys)4, [2Fe-2S], [3Fe-4S] and [4Fe-4S] clusters all function as one-electron donors or acceptors, the more complex double-cubane [8Fe-7S] cluster that is found only in nitrogenases (see Nitrogenase Catalysis Assembly) has the potential to mediate two-electron transfer processes.Three methods have been employed to functionalize Fe-S centers for substrate binding and activation. The first involves having an accessible Fe coordination site as in the mononuclear Fe centers of nitrile hydratase and SOR, and the [4Fe-4S] clusters at the active sites of hydratases/dehydratases and radical-5-adenosylmethionine (SAM) enzymes.Indeed the recent recognition of the importance of the superfamily of radical-SAM enzymes in initiating radical reactions, via cluster-mediated reductive cleavage of SAM to yield a... 

Nitrogenase catalyzes the hydrolysis of ATP ADP + P in a reaction that is reductant-dependent 15,29, 30) and results in electron transfer and activation to provide a low-potential species capable of reducing a unique range of substrates see Reducible Substrate section). The ratedetermining step of ATP utilization may be bimolecular (25, 31). The product ADP is an inhibitor of ATP utilization 32), and thus an ATP-generating system is used in vitro to reconvert ADP to ATP (15, 33). 

Nitrogenase, as must now become clear, is a complex enzyme of two component proteins which requires ATP, a reductant, a reducible substrate, Mg " " as an activator, and an anaerobic environment to function. To this complexity must be added the difficulty that the component proteins have no enzymatic half reactions . There are, perhaps, four main questions to decide about the mechanism (1) the role(s) of the two component proteins (2) the role(s) of ATP (3) the nature of the active site(s) and (4) the mechanism of N2 reduction. Despite the complexities and difficulties mentioned above, progress in the last 15 years has partly answered all these questions. The Fe protein mediates an ATP-dependent electron transfer from the donor to the MoFe protein which contains the active site. MgATP binds and induces a conformational change in the Fe protein which lowers its redox potential. FeMoco, the molybdenum cofactor, which may be part of the active site of N2 reduction, has been isolated and partly characterized, while an intermediate in N2 reduction has recently been discovered (Thorneley ct al., 1978). The next part of this chapter describes the evidence for these claims. This evidence involves the noncatalytic reactions of the individual proteins, their... 

The Nl-Bl separation is consistent with an elongated single bond, while the N3-H2 and N3-N2 separations are indicative of hydrogen bonding. This complex also mediates Nj-centered redox reactivity, and in the presence of CoCp 2 and collidinium hydrochloride as a reductant and proton donor, respectively, ammonia was confirmed as a reduction product. The side-on binding mode and N-N bond scission reaction is of potential relevance to the Nj-fixation cycles since hydrazine is a known byproduct of N2 reduction to ammonia by V-nitrogenase [37]. Future work on this scaffold will target redox transformations of other small molecule substrates amenable to reductive activation, for instance, CO2, N2, and CO, and seek to elucidate the intimate role of each Lewis partner to effect substrate activation. 

The basic outlines of the electron flux through the nitrogenase system have been established, and involve the initial reduction of Fe-protein by low potential carriers such as ferredoxin or flavodoxin. The Fe-protein then transfers electrons to the MoFe-protein in a process that is coupled to the hydrolysis of ATP. After the appropriate numbers of electrons and protons are present in the MoFe-protein, substrate reduction can take place on the FeMo-cofactor. Despite extensive studies, die overall stoichiometry of the nitrogenase catalyzed reaction has still not been definitively established. The uncertainties are expressed in the following equation for the overall enzyme reaction (8) ... 

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