Nitrogenase, P-clusters

Table 6.3 Oxidation and Spin States for Nitrogenase P-Cluster Iron Atoms...

An abundance of synthetic targets in iron-sulfur cluster remains, especially in unexplored areas such as pentanuclear clusters. Whether known clusters represent all energetically feasible topologies under ordinary conditions for nuclearities m = 4, 6, and 8, remains an open issue. Given that the nitrogenase P-cluster structure (5) (Figure 1) was not correctly predicted and is unprecedented, it is reasonable to assume that not all iron-sulfur cluster products of biosynthesis have been discovered. 

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...

The elucidation of the crystal structures of two high-spin EPR proteins has shown that the proposals for novel Fe-S clusters are not without substance. Two, rather than one novel Fe-S cluster, were shown to be present in nitrogenase, the key enzyme in the biotic fixation of molecular nitrogen 4, 5). Thus the FeMoco-cofactor comprises two metal clusters of composition [4Fe-3S] and [lMo-3Fe-3S] bridged by three inorganic sulfur atoms, and this is some 14 A distant from the P-cluster, which is essentially two [4Fe-4S] cubane moieties sharing a corner. The elucidation of the crystal structure of the Fepr protein (6) provides the second example of a high-spin EPR protein that contains yet another unprecedented Fe-S cluster. 

NIS measurements have been performed on the rubredoxin (FeSa) type mutant Rm 2-A from Pyrococcus abyssi [103], on Pyrococcus furiosus rubredoxin [104], on Fe2S2 - and Fe4S4 - proteins and model compounds [105, 106], and on the P-cluster and FeMo-cofactor of nitrogenase [105, 107]. 

Figure 2.10 Schematic structures of (a) sulfite reductase of Escherichia coli in which a 4Fe-4S cluster is linked via a cysteine to the iron in a sirohaem (b) P cluster of nitrogenase (c) FeMoCo cluster of nitrogenase (d) the binuclear site in Desulforibrio gigas hydrogenase.

FIGURE 12.4 S = 7/2 EPR of the [8Fe-7S] P-cluster in Azotobacter vinelandii nitrogenase. The experimental spectrum (trace A) has been simulated in the absence (trace B) and the presence (trace C) of D-strain modeled as a correlated distribution in the zero-field parameters D and E. 

On the other hand, such approaches to the metalloenzymes described above in Figures 1 and 2 are still under way. Thus, the model clusters reproducing precisely their complex metal-sulfur assemblies in the native form have not yet been isolated. In this section, the studies aiming at the syntheses of the model compounds of two clusters in nitrogenase, FeMo cofactor and P-cluster, will be surveyed. The choice of these clusters as the representatives of the metal-sulfur clusters in metalloenzymes arises from the fact that these are the largest and most complicated metal-sulfur clusters known at present among those observed in natural enzymes. 

Fig. 1. Schematic illustration of the enzyme nitrogenase being composed of the molybdenum-iron (MoFe) protein, an oc2p2 tetramer with two unique iron-sulfur clusters (P-cluster) and two iron-molybdenum cofactors (FeMoco), and the iron protein with one [4Fe-4S]-cluster and two ATP binding sites.

To successfully describe the structure and function of nitrogenase, it is important to understand the behavior of the metal-sulfur clusters that are a vital part of this complex enzyme. Metal-sulfur clusters are many, varied, and usually involved in redox processes carried out by the protein in which they constitute prosthetic centers. They may be characterized by the number of iron ions in the prosthetic center that is, rubredoxin (Rd) contains one Fe ion, ferredoxins (Fd) contain two or four Fe ions, and aconitase contains three Fe ions.7 In reference 18, Lippard and Berg present a more detailed description of iron-sulfur clusters only the [Fe4S4] cluster typical of that found in nitrogenase s Fe-protein is discussed in some detail here. The P-cluster and M center of MoFe-protein, which are more complex metal-sulfur complexes, are discussed in Sections 6.5.2. and 6.5.3. 

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.)... 

---