Lowe-Thorneley mechanism

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 ... 

However, some data have been more difficult to incorporate into the mechanism shown in Figs. 8 and 9. As reported 21) in Section II,B the Fe protein can be reduced by two electrons to the [Fe4S4]° redox state. In this state the protein is apparently capable of passing two electrons to the MoFe protein during turnover, although it is not clear whether dissociation was required between electron transfers. More critically, it has been shown that the natural reductant flavodoxin hydroquinone 107) and the artificial reductant photoexcited eosin with NADH 108) are both capable of passing electrons to the complex between the oxidized Fe protein and the reduced MoFe protein, that is, with these reductants there appears to be no necessity for the complex to dissociate. Since complex dissociation is the rate-limiting step in the Lowe-Thorneley scheme, these observations could indicate a major flaw in the scheme. 

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. 

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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