Nitrogenase FeMoco

As well as donating electrons to the MoFe protein, the Fe protein has at least two and possibly three other functions (see Section IV,C) It is involved in the biosynthesis of the iron molybdenum cofactor, FeMoco it is required for insertion of the FeMoco into the MoFe protein polypeptides and it has been implicated in the regulation of the biosynthesis of the alternative nitrogenases. 

In general there are few reproducible data on binding of reducible substrates to the isolated MoFe proteins. However, the S = EPR signal from the FeMoco centers of Kpl is pH dependent, the g values changing with a pKa of 8.7 (50). Of course, the proton is a substrate of nitrogenase however, there is no direct evidence for the proton associated with the pKa being bound directly to FeMoco. Nevertheless, this pKa can be perturbed by addition of the analog substrate acety-... 

A preparation of the third nitrogenase from A. vinelandii, isolated from a molybdenum-tolerant strain but lacking the structural genes for the molybdenum and vanadium nitrogenases, was discovered to contain FeMoco 194). The 8 subunit encoded by anfG was identified in this preparation, which contained 24 Fe atoms and 1 Mo atom per mol. EPR spectroscopy and extraction of the cofactor identified it as FeMoco. The hybrid enzyme could reduce N2 to ammonia and reduced acetylene to ethylene and ethane. The rate of formation of ethane was nonlinear and the ethane ethylene ratio was strongly dependent on the ratio of nitrogenase components. 

A great deal has been learned about the biosynthesis of nitrogenases, but at the moment the process is understood only in broad outline. The detailed roles of the individual gene products require much further investigation, which may once more indicate fresh approaches to some of the problems identified herein. In particular, if the biosynthetic steps can be emulated chemically, then it may be possible to synthesize FeMoco in large quantities in order to allow its detailed analysis at the atomic level. 

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. 

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.

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.

Concurrently with the X-ray crystallographic studies, extended X-ray absorption fine structure (EXAFS) studies confirmed many of the bond distances proposed for nitrogenase s FeMoco cluster. The EXAFS data of reference 25 indicate short Fe-Fe distances of 2.61, 2.58, and 2.54 A for M+, M (resting state), and M forms, respectively. The authors believe that the short M center bond lengths indicate Fe-Fe bonds in this cluster. In another study using dithionite-reduced MoFe-protein Fe-S, Fe-Fe, Fe-Mo distances of 2.32, 2.64, and 2.73 A, respectively, were found in the 1 to 3 A region and Fe-Fe, Fe-S and Fe-Fe distances of 3.8, 4.3, and 4.7 A, respectively, were found in the 3 to 5 A region.30... 

Many researchers have considered models for possible intermediates in the nitrogenase reaction. Two possible dinitrogen attachments to the FeMoco factor of MoFe-protein have been put forward. Symmetric, edge- or side-on modes discussed by Dance48 would lead to a reaction sequence such as is shown in reaction 6.11. In contrast, the asymmetric end-on terminal mode discussed in the work of Nicolai Lehnert50 may be favored thermodynamically and by molecular orbital calculations. Reaction sequence 6.13 below illustrates one scenario for the asymmetric model. 

Despite the availability of the molecular structures of the different active sites of the FeMo-nitrogenases, the mechanism of nitrogen fixation remains obscure. The interest of our discussion, however, is centred on the various modes proposed to describe how molecular nitrogen might coordinate the FeMoco. Some of the reported schemes are inspired by the type of coordination found in model compounds. 

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