FeMo-cofactor structure

The FeMo-cofactor structure can be viewed as the assembly of two incomplete... 

Finally, we must consider how protons are delivered to complete the reaction. Without definitive evidence, molecular modeling has identified three likely proton-transfer routes (71). One is the interstitial channel, filled with water molecules, that nms between the a- and )3-subunits from the surface of the MoFe protein to the pool of water molecules around the homocitrate of the FeMo-cofactor. This channel could deliver protons rapidly to boimd substrate and might also provide a pathway for N2 and NH4+ to enter and leave the reduction site (see also the section Substrate-Binding Site). Extensive theoretical studies of the hydrogen-related chemistry of the FeMo-cofactor use this same interstitial channel to deliver protons (75). These studies indicate that the delivery of electrons to the FeMo-cofactor causes its sulfur atoms to become more basic which, in turn, makes them attractive sites for protonation by water molecules in the interstitial pool. Once transferred, these protons become reduced to hydrogen atoms that can then migrate across the FeMo-cofactor structure to other Fe and S atoms and become involved in substrate reduction (76). 

One type of the constituent metallocenters in the MoFe protein has the properties of a somewhat independent structural entity. This component, referred to as the FeMo cofactor (FeMo-co), was first identified by Shah and Brill (1977) as the stable metallocluster extracted from acid-denatured MoFe protein. The FeMo-co was able to fully activate a defective protein in the extracts of mutant strain UW45, a protein which subsequently was shown to contain the P clusters but not the EPR-active center. The isolated cofactor accounted for the total S = t system observed by EPR and Mdssbauer spectroscopies of the holo-MoFe protein (Rawlings et al., 1978). Elemental analysis indicated a composition of Mo Fee-8 Se-g for the cofactor, which, if there are two FeMo-co s per a2 2> accounts for all the molybdenum and approximately half the iron in active enzyme (Nelson etai, 1983). Although FeMo-co has been extensively studied [reviewed in Burgess (1990)] the structure remains enigmatic. To date, all attempts to crystallize the cofactor have failed. This is possibly due to the instability and resultant heterogeneity of the cofactor when removed from the protein. Also, there is a paucity of appropriate models for spectral comparison (see Coucouvanis, 1991, for a recent discussion). Final resolution of this elusive structure may require its determination as a component of the holoprotein. 

Fig.4 Structures of the FeMo cofactor of nitrogenase (Y is C, N, or O) and limiting stoichiometry of the catalyzed reaction.

Fig. 1. New structure of the FeMo-cofactor found by Rees and collaborators in 2002 (5). (Top structure of the cluster Bottom FeMoco and surrounding ligands and amino acids in the protein pocket.)...

In autumn 2002, Rees and collaborators (5) reported the new X-ray structure for the FeMo-cofactor, which differed from the 1992 structure in that it possesses an interstitial atom (see Fig. 1), which is most likely a nitrogen atom (41). All quantum chemical calculations reported until the end of 2002 were thus based on the wrong geometrical structure of the active site with considerable implications for the electronic structure and, thus, for the chemical behavior. Not surprisingly, new (standard) DFT calculations for the new structure were published by Dance (42), by Hinnemann and Norskov (43), by Lovell et al. (44) and by Blochl and collaborators (45) shortly after the publication of the work of Rees et al. 

Figure 3.2. Structural models of FeMo cofactor (a), the dithionate reduced P-cluster (b), and oxidized P-cluster (c) (Rees and Howard, 2000). Reproduced with permission...

Sellmann, D., Utz, J., Blum, N., and Heinemann, F. W. (1999) On the function of nitrogenase FeMo cofactors and competitive catalysts chemical principles, structural blue-prints, and the relevance of iron sulfur complexes for N2 fixation, Coord. Chem. Rev. 190-192, 607-627. 

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