Nitrogenases FeMo protein

Figure 11 Cartoon showing CO binding to the FeMo-cofactor of nitrogenase FeMo protein. The S atoms are shown in yellow, the Fe atoms in red, and the Mo atom in blue-green. At lower CO partial pressure, the lo-CO form exists with a single, bound, bridging CO. At higher CO partial pressure, the hi-CO form exists, with two CO molecules bound to the cluster...

The two types of centers present in the nitrogenase FeMo protein are designated P clusters and FeMoco (or M) eenters. Both types of centers display unique spectroscopic properties, but only FeMoco continues to display most of those properties when it is extracted from the protein. 

M center in Clostridium pasteurianum nitrogenase FeMo protein ... 

Mossbauer spectrum of C. pasteurianum nitrogenase FeMo protein,indicating the various components (quadrupole doublets) and their assignments. The doublet labeled M is the cofactor signal those labeled D, S, and Fe + are attributed to the P-clusters. 

Fig. 7. View of the FeMo-cofactor prosthetic group of the nitrogenase MoFe protein with some of the surrounding amino acid residues where ( ) represents the molybdenum coordinated to a-His-442 and homocitrate (at the top), ( ) represents the iron, interspersed with the sulfur (O) and carbon...

These studies of protein-bound heterometallic cubanes have amply demonstrated that the heterometal site is redox active and able to bind small molecules. Although they have yet to be identified as intrinsic components of any protein or enzyme (except as part of the nitrogenase FeMo cofactor cluster (254)), they are clearly attractive candidates for the active sites of redox enzymes. 

Figure 32 X-Ray structure of the active site of FeMo-protein in the nitrogenase of Azotobacter vinelandii...

Additionally, Wang and Watt have shown that the FeMo protein alone can act as an uptake hvdrogenase(63). Specifically, H2 in the presence of [FeMo] causes the reduction of oxidizing dyes such as methylene blue or dichlorophenolindophenol in the absence of Fe protein. The hydrogen evolution and uptake behavior of nitrogenase proteins forces us to consider the ways in which hydrogen can interact with transition metal sulfur centers. This we discuss in the following section. 

Einsle O, Tezcan FA, Andrade SL, et al. Nitrogenase MoFe-protein at 1.16 A resolution A central ligand in the FeMo-cofactor. Science 2002 297 1696-700. 

The basic mechanism of nitrogenase with the use of dithionate as an electron donor for the iron protein involves the following steps (Thomeley and Lowe, 1985 Likhtenshtein, 1988a Burgess and Lowe, 1996 Smith, 1999 Seefeldt and Dean, 1997 Rees and Howard, 2000 Syrtsova and Timofeeva, 2001) 1) reduction of Fe-protein with flavodoxin or dithionate and attachment of two ATP molecules to the protein, 2) formation of a complex between the reduced FeP with two bound ATP molecules and FeMo-protein, 3) electron transfer between the reduced [Fe4S4] cluster of FeP to the P-cluster ofFeMoP coupled to the ATP hydrolysis, 4) electron transfer from P-cluster to... 

Figure 3.7. The energy profile of a nitrogenase reaction. Eo is the standard redox potential of the reactants, intermediates and products of the reaction Fd = ferredoxin FeP = Fe protein FeMo = FeMo protein. The arrow indicates the increase of the reduction potential upon ATP hydrolysis (Likhtenshtein 1988a). Reproduced with permission.

Kenneth O. Hodgson, Richard H. Hohn, et al. EXAFS (extended X-ray absorption fine structure) study of structure of FeMo protein component of nitrogenase... 

The assembly of the molybdenum nitrogenase Fe protein involves the nifH gene and at least the nifS, nifU, and niJM gene prodncts that are implicated in the [4Fe 4S] cluster assembly, while the assembly of MoFe protein requires at least 15 nif gene products and involves the biosynthesis of FeMo cofactor and FeMo cofactor deficient MoFe protein in separate pathways and the insertion of FeMo cofactor into the MoFe protein. 

Figure 12 35 GHz Fe and H CW ENDOR spectra of the hi-CO form of nitrogenase MoFe protein with varying types of Fe-enrichment. The H signals are included as intensity standards since the natural ahundance samples contain ohservahle amounts of Fe. The spectra were recorded at gobs = 2.06 (gj ). Cartoons on the left indicate the specific sample, wherein the FeMo-cofactor is indicated hy the diamond shape (in green), and the P-cluster is indicated by the two cubes (in red). These shapes roughly represent the clusters actual strucmres. Fe-enriched sites are indicated as colored blocks. The abbreviations used are as follows M(56)P(56) is natural abundance FeMo-cofactor and P-cluster M(56)P(57) is natural abundance FeMo-cofactor and enriched P-cluster M(57)P(56) is enriched FeMo-cofactor and natural abundance P-cluster M(57)P(57) is enriched FeMo-cofactor and P-cluster (i.e., globally enriched MoFe protein). (Adapted from Figure 1 in Christie, Lee, Cameron, Hales, Orme-Johnson and Hoffman. Reprinted with permission, 1996 American Chemical Society)...

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). 

Figure 23,13 Metal centres in the FeMo protein of nitrogenase. (a) P-cluster pair. Each of the four outer Fe atoms is further coordinated to the S of a cysteine group, (b) FeMo cofactor. (Y is probably S, 0 or N.) Fe-Fe bridge distances are in the range 240-260pm, suggesting weak Fe-Fe interactions. The Mo achieves 6-coordination by further bonds to N (of histidine) and two O atoms (of a chelating homocitrate), while the Fe at the opposite end of the cofactor is letrahedrally coordinated by attachment of a cysteine, (c) Possible intermediate in the interaction of Nj with FeMo cofactor.

The action of the Mo-nitrogenase enzyme involves the functioning of two separately isolatable component proteins, as sketched in Figure 1.21k. The larger of the two proteins, sometimes incorrectlydesignateddinitrogenase has, in the past, been called molybdoferredoxin, azofermo, or component I. More often this protein is called the MoFe or FeMo protein ([MoFe] or [FeMo]). The smaller protein, formerly called azoferredoxin or component II, is sometimes incorrectly referred to " as dinitrogenase reductase. This protein is properly... 

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