Thorneley-Lowe cycle

A comprehensive description of the mechanism of molybdenum nitrogenase has been provided by the Lowe-Thorneley scheme 102) (Figs. 8 and 9). In this scheme the Fe protein (with MgATP) functions as a single electron donor to the MoFe protein in the Fe protein cycle (Fig. 8), which is broken down into four discrete steps, each of which may be a composite of several reactions ... 

Figure 6.5 MoFe-protein cycle for reduction and protonation of N2. (Adapted from Figure 3 of Thorneley, R. N. F. Lowe, D. J. J. Biol. Inorg. Chem., 1996, 1, 576-580. Copyright 1996, Society of Biological Inorganic Chemistry.)...

Rodrigues-Lopez, J.N., D.J. Lowe, J. Hernandez-Ruiz, A.N.P. Hiner, F. Garcia-Canovas, and R.N.F. Thorneley (2001). Mechanism of reaction of hydrogen peroxide with horseradish peroxidase Identification of intermediates in the catalytic cycle. J. Am. Chem. Soc. 123, 11838-11847. 

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

Figure 6.2 Kinetics of the nitrogenase reaction as established by Thorneley and Lowe. Each dotted arrow represents one Fe-protein cycle.

A detailed discussion of the distribution of electrons over the Fe-S and Mo centres of Kpl under various conditions is beyond the scope of this short article and the reader should consult Lowe, Eady and Thorneley (1978) for the most recent data and a literature review. However, at some stage in the catalytic cycle, electrons and protons are accepted by N2 to give NH3. The chemistry of certain Mo-complexes suggests an enzymic mechanism involving the stepwise addition of electrons and protons to N2 coordinated to Mo to yield NH3 via partially-reduced dinitrogen-hydride species (Thorneley et al. 1979). 

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