Molybdenum-iron protein in nitrogenase

Kim, J. and D.C. Rees. Structural models for the metal centers in the nitrogenase molybdenum-iron protein. Science 257,1677-1682 (1992). 

MgATP hydrolysis and, 47 189-191 nitrogenase complex, 47 186-189 substrates, 47 192-202 molybdenum iron proteins, 47 161, 166-174, 176-183, 191-192 structure, 47 162-164, 166-170 nitrogen fixation role, 36 78 in nitrogen fixation systems, 27 265-266 noncomplementary reactions with Sn", 10 215... 

As noted earlier, nitrogenase is made up of two proteins, the iron protein, and the molybdenum-iron protein, and will be linked to an electron-transport chain. The iron protein accepts electrons from this chain (a ferredoxin or flavodoxin in vivo, or dithionite in vitro) and transfers them to the molybdenum-iron protein. The MoFe protein is then able to reduce a number of substrates in addition to dinitrogen. No replacement electron donor will function instead of the iron protein. 

Figure 2.6 Diffraction pattern from a crystal of the MoFe (molybdenum-iron) protein of the enzyme nitrogenase from Clostridium pasteurianum. Notice that the reflections lie in a regular pattern, but their intensities (darkness of spots) are highly variable. [The hole in the middle of the pattern results from a small metal disk (beam stop) used to prevent the direct X-ray beam, most of which passes straight through the crystal, from destroying the center of the film.] Photo courtesy of Professor Jeffery Bolin.

An input-output scheme for nitrogenase is shown in Figure 2. The material in the box represents the catalytic entities—the iron protein, the molybdenum-iron protein, and Mg2+ ions. Input consists of a reduc-... 

Bolen, J.T., Cambasso, N., Muchmore, S.W., Morgan, T.V., and Mortenson, L. E. (1993) Structure and Environment of metal clusters of the nitrogenase molybdenum iron protein from Clostridium pasterianum, in Stiefel, E.I., Coucouvanis, D., and ewton, W.E. (eds.), Molibdenum Enzymes, Cofactors, and Model Systems, Am. Chem. Soc., Wahington, DC. 

Lanzilotta, W.N and Seefeldt, L.C. (1997) Changes in the midpoint potential of the nitrogenase metal centers as a result of iron protein-molybdenum-iron protein complex formation, Biochemistry 36, 12976-12983. 

The sequential electron transfer path in nitrogenase is followed first models of the Fe4S4 cluster of the iron-protein are discussed, then mimics of the P-cluster in the molybdenum-iron protein, and finally structural and functional models of the FeMo-cofactor are summarized. 

These clusters are each composed of eight iron atoms and seven sulfide ions. In the reduced form, each cluster takes the form of two 4Fe-3S partial cubes linked by a central sulfide ion. Each cluster is linked to the protein through six cysteinate residues. Electrons flow from the P cluster to the FeMo cofactor, a very unusual redox center. Because molybdenum is present in this cluster, the nitrogenase component is also called the molybdenum-iron protein (MoFe protein). The FeMo cofactor consists of two M-3Fe-3S clusters, in which molybdenum occupies the M site in one cluster and iron occupies it in the other. The two clusters are joined by three sulfide ions. The FeMo cofactor is also coordinated to a homocitrate moiety and to the a subunit through one histidine residue and one cysteinate residue. This cofactor is distinct from the molybdenum-containing cofactor found in sulfite oxidase and apparently all other molybdenum-containing enzymes except nitrogenase. 

VU Fig. 28.18 The structures of the two types of cluster unit present in the nitrogenase molybdenum-iron protein isolated from Azotobacter vinelandii (a) the P-cluster in its reduced state and (b) the FeMo cofactor. Colour code Fe, green Mo, pale grey S, yellow C, grey N, blue O, red. Each non-terminated stick represents the connection of a coordinated amino acid to the protein backbone. 

T. V. Morgan and L. E. Mortenson, Structure and environment of metal clusters in the nitrogenase molybdenum-iron protein from Clostridium pasteurianum, in Molybdenum Enzymes, Cofactors and Model Systems , eds. E. I. Stiefel,... 

Figure 4.2. Schematic models of Fe-Mo cofactor with possible binding of dinitrogen substrate (a) and P cluster (b). Y indicates a bridging ligand. (Part a adapted, with permission, from Kim, J., and Rees, D. C. Structural models for metal centres in the nitrogenase molybdenum-iron protein. Science 257 1677-82. Copyright 1992, American Association for the Advancement of Science. Part b adapted, with permission, from Chan, M. K., Kim, J., and Rees, D. C. The nitrogenase Fe-Mo-cofactor and P-cluster pair 2.2A resolution structures. Science 260 792-4. Copyright 1993, American Association for the Advancement of Science.)...

In 1930, Hermann Bortels (1902-1979) recognised that nitrogen fixation is a molybdenum-dependent process. Obviously, the nitrogenases from Rhizobium meliloti, Azotobacter vinelandii and Clostridium pasteurianum have a similar constitution. In 1966, Leonard E. Mortenson identified for the first time an Fe- and a MoFe-protein as parts of the nitrogenase enzyme system. The exact structure of the nitrogenase-molybdenum-iron protein from Azotobacter vinelandii [28] was clarified in 1992, and that from Clostridium pasteurianum [29] in 1993, both by Douglas C. Rees. [30] The Fe-protein is a y2-dimer with a molar mass of some 60,000 Daltons, and the MoFe-protein is an ca. 

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