Thorneley

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. 

The Lowe-Thorneley scheme was devised using sodium dithionite as reductant, but the foregoing data imply that when other reductants are used, including the natural reductants, the same path is not necessarily followed. It seems clear that some reinvestigation of the Lowe-Thorneley scheme using alternative reductants is necessary. However, the detailed simulation of product formation provided by the scheme implies that much of it is likely to remain intact, although the exact nature of the rate-determining step may not be the same with all reductants. 

Early data on the substrate and inhibitor reactions of nitrogenase were interpreted in terms of five binding sites, with competitive, noncompetitive, unclassified, and negative inhibition being observed (127). This apparent complexity can be readily rationalized in terms of the Lowe—Thorneley scheme (Fig. 9) by assuming that different substrates bind at different oxidation states of the same site. 

Rodrfguez-Lopez, J.N., Gilabert, M.A., Tudela, J., Thorneley, R.N.R, and Garcfa-Canovas, R, Reactivity of horseradish peroxidase compound II toward substrates kinetic evidence for a two-step mechanism,... 

It is now believed that the MoFe-protein s P-cluster contains a [4Fe-3S] cuboid joined to a [4Fe-4S] cuboid, although, as discussed below, it was first reported crystallographically as two [4Fe-4S] clusters.8 Uncertainty existed for sometime as to exact nature of bridging disulfide or sulfide ligand joining the two Fe S clusters but it is now known that the P-cluster does NOT contain a disulfide bond. This is important because the all-ferrous structure [4Fe-4S]° proposed from Mossbauer studies then becomes more possible for the P-cluster s [4Fe-4S] cube. In 1993 Bolin et al.1 proposed a six-coordinate S for the P-cluster s center as in Figure la,b of Thorneley s article.8 This is now believed to be the correct conformation. A central six-coordinate S makes this cluster much harder to synthesize in the laboratory, and this feat has not been accomplished as of the date of this text s publication. Whatever its oxidation state or structure, the P-cluster mediates electron transfer from Fe-protein to the M center of MoFe-protein, and it must be reduced at some point to allow transfer of its electron(s). 

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.)...

One might inquire at this point about the addition of a secondary amine, which cannot yield a stable neutral product by dehydration as the primary amine can. Diebler and Thorneley measured rate constants of the addition step for reaction of piperazine (22) with pyridine-4-aldehyde in the pH range 5.8-10.8, a range in which the addition step is very fast so that, for primary amines, the kinetics would be determined by the rate-limiting dehydration.100 They were... 

---