Nitrogenase oxidation states

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. 

One possible reaction scheme, an elaboration of the so-called Chatt cycle, results in the nitrogenase cycle shown in Figure 6.16.51 The cycle shown operates between molybdenum oxidation states Mo(0) and Mo(IV), but others have been proposed that operate between Mo(II) and Mo(VI) levels. 

Figure 3.11 (a) and (b) the P-cluster of nitrogenase in its reduced and oxidized state and (c) the FeMo-cofactor. The molecules are represented with C green, N blue, O red, S yellow, Fe orange and Mo pink. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.)... 

The P clusters of nitrogenase. The enzyme nitrogenase consists of two proteins the Fe protein (m.w. 55,000), which contains a single 4Fe-4S center, and the more complex MoFe protein (m.w. 220,000) (48,49). The minimum functional unit of the latter appears to be the half molecule, an asymmetric dimer containing 1 Mo, 14-16 Fe, and 16-18 sulfides. Application of a vast array of spectroscopic methods to the MoFe protein in a variety of oxidation states has led to the conclusion that it contains two types of metal-sulfur cluster in a 2 1 ratio unusual Fe S units termed P clusters, and the protein-bound form of the FeMo-cofactor (50). 

Fig. 3 Known structures of the P cluster of nitrogenase in two oxidation states.

Yet another enzyme able to release or transfer sulfur in the S° oxidation state is the PLP-dependent cysteine desulfurase that is encoded by the nifS gene of the nitrogenase gene cluster shown in Fig. 24-4. 

In this scheme the electrons are all supplied by the metal, which undergoes a change in oxidation state of six. In nitrogenase, electrons would be supplied from the iron protein and there would be no need for large changes in oxidation state in the molybdenum. 

A more specific operational definition excludes the unique clusters of the nitrogenase enzymes (which are treated in Chapter 7) from our considerations biological iron-sulfur clusters in this context only deal with Fe and do not contain any other metal. Under physiological conditions they can occur in two, and not more than two, oxidation states. All clusters are either dinuclear or one of the... 

Thus, the hybrid cluster is a putative iron-sulfur redox catalyst. It is, however, a very uncommon cluster (perhaps only comparable to the nitrogenase active site) in two aspects (1) it is a hybrid cluster i.e., it contains intrinsic building blocks that are distinctly strange to iron-sulfur clusters and (2) it can exist in more than two (in fact, four [63]) oxidation states. 

The potentially serious aspects of vanadium pollution, the function of biologically occurring enzyme systems, the role of vanadium on the function of numerous enzymes, and the associated role in the insulin-mimetic vanadium compounds are inextricably linked. The key to our understanding all such functionality relies on understanding the basic chemistry that underlies it. This chemistry is determined to a significant extent by the V(IV) and V(V) oxidation states but clearly is not restricted to these states. Indeed, the redox interplay between the vanadium oxidation states can be a critical aspect of the biological functionality of vanadium, particularly in enzymes such as the vanadium-dependent nitrogenases, where redox reactions are the basis of the enzyme functionality. 

---