Nitrogenase reaction

Non-enzvmatic simulation of nitrogenase reactions and the mechanism of biological nitrogen fixation. G. N. Schrauzer, Angew. Chem., Int. Ed. Engl., 1975, 14, 514-522 (36). 

Fe-protein interacts with MoFe-protein. Correct docking of Fe-protein to MoFe-protein is associated with conformational changes taking place during step 1. Steps 1 and 2 are prerequisites for all following nitrogenase reactions and for substrate reduction. 

Many researchers have considered models for possible intermediates in the nitrogenase reaction. Two possible dinitrogen attachments to the FeMoco factor of MoFe-protein have been put forward. Symmetric, edge- or side-on modes discussed by Dance48 would lead to a reaction sequence such as is shown in reaction 6.11. In contrast, the asymmetric end-on terminal mode discussed in the work of Nicolai Lehnert50 may be favored thermodynamically and by molecular orbital calculations. Reaction sequence 6.13 below illustrates one scenario for the asymmetric model. 

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).

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

Table 9.6 Apparent Equilibrium Constants K for Nitrogenase Reactions at 298.15 K...

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. 

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... 

An important structural feature, common to both metal-sulfur enzyme centers and (hypothetical) low molecular weight competitive catalysts, certainly is the sulfur coordination of the metals. The type of donor atom represents the most basic structural feature of any metal complex. Functional features indispensable for competitive catalysts are determined by Eq. 44. It expresses the hydrogenase, CO dehydrogenase, and nitrogenase reactions in their most general form. 

The experimentally established electron balance for the nitrogenase reaction is given by Eq. 60. The model reaction of Eq. 62a, rewritten as Eq. 63, does not require any electrons, and it uses only one-half of the D2 for HD formation. The other one-half remains bound in the N2D2 complex. 

At present, much attention is devoted to enzymes that utilize the energy of ATP hydrolysis for realization of energy-rich mechanics (myosin), transport (Na+,K+-ATPase, Ca2+-ATPase, chemical processes (nitrogenase), polymerases, topoisomerases, GTPases, and for creation of electrochemical gradients in biomembranes (H+-ATPase, ATP synthase ). In this section we focus on the latter process. The coupling mechanism in the nitrogenase reaction is discussed in Section 3.1. 

E0 = -0.042 V (Pierick et al., 1993). The reduced P-cluster of Av2 transfers at least one electron to FeMocoN to form the ESR silent "super-reduced" FeMoco, (FeMoco) 1. (Ohrme-Johnson et al., 1972 Miinck et al., 1975). A combination of cyclic voltampermetry, potentiometry and ESR spectroscopy has allowed the observation in isolated FeMoco of two redox transitions with essentially different potentials, E0 = - 0.3 eV and E0 = -1.0 eV (Newton et al., 1996). The first transition of FeMoco is from oxidize state to semireduced state of FeMoco and the second transition is related to the substrate-reducing state in FeMoco during the nitrogenase reaction turnover. Independently, for the redox pair FeMocoN ( FeMoco)2", the Eo was estimated as... 

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