Cofactor 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. 

The MoFe proteins are all a2 2 tetramers of 220-240 kDa, the a and (3 subunits being encoded by the nifD and K genes, respectively. The proteins can be described as dimers of a(3 dimers. They contain two unique metallosulfur clusters the MoFeTSg homocitrate, FeMo-cofactors (FeMoco), and the FesSy, P clusters. Neither of these two types of cluster has been observed elsewhere in biology, nor have they been synthesized chemically. Each molecule of fully active MoFe protein contains two of each type of cluster 2-7). 

Fig. 4. Structure of the iron molybdenum cofactor, FeMoco (after Chan, Kim, and Rees, (4) Bolin et al. (5) and Mayer et al. (7)). The FeMoco is ligated, within the a subunits of the a2j82 tetrameric structure, by residues Hisa442 and Cysa275 (Avl residue numbers).

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.

Actual electron transfer to the dinitrogen substrate at the MoFe-protein, with electrons first passing through the MoFe-protein s P-cluster. During this process, dinitrogen is most probably bound to the iron-molybdenum cofactor (FeMoco) of the MoFe-protein.6... 

The MoFe-protein s environment is tuning the cofactor FeMoco to maximize N2 reduction and minimize H2 production. 

Nitrogenases from various nitrogen fixing organisms seem to contain the same Fe/Mo/S structural unit that occurs as an extractable cofactor (FeMoco) (2). Extracts of the Fe-Mo component protein from inactive mutant strains of different microorganisms that do not contain the Fe/Mo/S center are activated upon addition of the FeMoco. 

X-ray crystallography has revealed that each a2p2 tetrameric MoFe protein contains two each of two types of cluster [13-16], These are the iron molybdenum cofactor (FeMoco) centers and the Fe8S7 P clusters. The two FeMoco centers are bound within the a subunits about 1 nm below the surface of the protein and are separated by about 7 nm. The P clusters are situated at the interface of the a and P subunits, and each is approximately 1.9 nm from one of the FeMoco centers. 

The conversion of dinitrogen to ammonia is one of the important processes of chemistry. Whereas the technical ammonia synthesis requires high temperature and pressure (1), this reaction proceeds at room temperature and ambient pressure in nature, mediated by the enzyme nitrogenase (2). There is evidence that N2 is bound and reduced at the iron-molybdenum cofactor (FeMoco), a unique Fe/Mo/S cluster present in the MoFe protein of nitrogenase. Although detailed structural information on nitrogenase has been available for some time (3), the mechanism of N2 reduction by this enzyme is still unclear at the molecular level. Nevertheless, it is possible to bind and reduce dinitrogen at simple mono- and binuclear transition-metal systems which allow to obtain mechanistic information on elemental steps involved... 

Nitrogen Fixation in Nature The nitrogenase enzyme is a two-component protein that consists of an electron-transfer Fe protein and a catalytic protein [85]. Three different nitrogenase enzymes are known, which differ primarily in the nature of the putative active site within the catalytic protein. The most common form is the MoFe protein, in which the active site for nitrogen reduction, the so-called FeMo cofactor (FeMoco), is composed of seven irons, one molybdenum, and nine sulfides... 

Enzymes containing molybdenum are of two types (1) Nitrogenases, which are required for converting atmospheric nitrogen to nitrogen compounds (NH3, for example) nitrogenases contain a characteristic polymetal atom cluster species called the iron-molybdenum cofactor, FeMoco (Section 17-E-10). (2) The other Mo enzymes, all of which have some variant of a characteristic molybdenum cofactor, Moco. 

Iron molybdenum cofactor, FeMoco was isolated from MoFe protein for the first time in 1977 by Shah and Brill [11]. Since then the cofactor, which naturally attracted much interest as the probable active site of the enzyme, has been thoroughly studied. Although the structure of isolated cofactor remains unknown X-ray studies by Rees et al. have enabled construction of the internal structure of the protein. 

Fig. 2. The fe -weighted EXAFS data associated with the iron K-edge of the iron-molybdenum cofactor (FeMoco) extracted from the FeMo-protein of the nitrogenase of Klebsiella pneumoniae, and its Fourier transform (19). 

S-adenosylmethionine (SAM or AdoMet) superfamily, aconitase, and others), enzymes that contain Fe-S heteroatomic clusters (nitrogenase iron-molybdenum cofactor (FeMoco), carbon monoxide dehydrogenase (CODH), and acetyl CoA synthase (ACS)), and enzymes that contain unique ligation sets around specialized iron centers ([NiFe] and [FeFe] hydrogenases) (Figure 1). ... 

The enzyme system responsible for N2 reduction, called the nitrogenase complex, consists of two separate proteins. As outlined in Figure 20.4, one protein-called component I, nitrogenase, or molybdenumiron protein-catalyzes the reduction of N2, and the other-called component II, nitrogenase reductase, or iron protein-transfers electrons from ferredoxin or flavodoxin to component I. Both component I and component II contain Fe4S4 iron-sulfur clusters, and component I also contains molybdenum, in the form of a tightly bound iron-molybdenum cofactor (FeMoCo). 

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