Nitrogenase MoFe protein models

Fig. 6. View of the nitrogenase MoFe protein P-cluster pair where ( ) represents Fe, (O) S, and (Q) C as modeled (153). The side chain of one of the...

Bolin JT, Campobasso N, Muchmoee SW, Morgan TV and Moetenson LE (1993) The structure and environment of the metal clusters in the nitrogenase MoFe protein from Clostridium pasteurianum. In Stiefel El, Coucouvanis D and Newton WE, eds. Molybdenum enzymes, cofactors and model systems, pp. 186-195. American Chemical Society, Washington, D. C. 

Fig. 9. The MoFe protein cycle of molybdenum nitrogenase. This cycle depicts a plausible sequence of events in the reduction of N2 to 2NH3 + H2. The scheme is based on well-characterized model chemistry (15, 105) and on the pre-steady-state kinetics of product formation by nitrogenase (102). The enzymic process has not been chsiracter-ized beyond M5 because the chemicals used to quench the reactions hydrolyze metal nitrides. As in Fig. 8, M represents an aji half of the MoFe protein. Subscripts 0-7 indicate the number of electrons trsmsferred to M from the Fe protein via the cycle of Fig. 8.

However, when the X-ray crystal structure of the MoFe protein was examined, it was clear that homocitrate could not directly hydrogen bond to the histidine, since the carboxylate group and imidazole are stacked parallel to each other in the crystal. Nevertheless, as noted in the previous section, studies on model complexes have suggested that homocitrate can become monodentate during nitrogenase turnover, with the molybdenum carboxylate bond breaking to open up a vacant site at molybdenum suitable for binding N2. 

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 17.8 Ribbon diagrams of the heterodimeric MoFe-protein (left) and homodimeric Fe-proteins of nitrogenase (right). The a- and P-subunits to the left of the MoFe-protein are shown in light and dark shading, respectively, while the metalloclusters are shown as dark space-filling models on the right side. The two subunits of the Fe-protein are shown in light and dark shading, with the 4Fe-4S cluster at the dimer interface as a space-filling model. (From Rees et al., 2005. Reproduced with permission of the Royal Society.)...

Of particular interest are those complexes which serve as model compounds for biological systems such as the active center of the MoFe protein in nitrogenase.107 These complexes will be discussed in detail in the relevant chapters. It should be noted here, however, that WS2- is a non-innocent ligand, and thus complexes such as [Co(WS4)2]" (n = 2,3)108>109 and [Fe(WS4)2]" (n = 2, 3) can easily be prepared.110,111,112 In addition, some unusual complexes have been synthesized with WS2- as ligands. One such example is [Fe3W3S12]4 which contains the Fe3(/r3-S)2 center as depicted in Figure 11. 

The most comprehensive model for the function of molybdenum nitrogenase in the reduction of N2 is that of Lowe and Thomeley, which was developed almost two decades ago. This model describes two aspects of nitrogenase catalysis, the Fe protein cycle and the MoFe protein cycle. 

Pre-steady-state stopped-flow and rapid quench techniques applied to Mo nitrogenase have provided powerful approaches to the study of this complex enzyme. These studies of Klebsiella pneumoniae Mo nitrogenase showed that a pre-steady-state burst in ATP hydrolysis accompanied electron transfer from the Fe protein to the MoFe protein, and that during the reduction of N2 an enzyme-bound dinitrogen hydride was formed, which under denaturing conditions could be trapped as hydrazine. A comprehensive model developed from a computer simulation of the kinetics of these reactions and the kinetics of the pre-steady-state rates of product formation (H2, NH3) led to the formulation of Scheme 1, the Thorneley and Lowe scheme (50) for nitrogenase function. 

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