Molybdenum-iron proteins

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... 

The electrons transferred from the Fe4S4 cluster of the iron-protein to the molybdenum-iron-protein are believed to shuttle through the P cluster before eventually reaching FeMo-co where N 2 binding, reduction, and protonation occurs. 

Each P-cluster is actually a joined pair of cubane-type clusters, one Fe4S4 and one Fe4S3 with two bridging cysteine -SH groups and one iron atom bonded to three sulfide sulfur atoms (Fig. 24-3).17/23 The FeMo-coenzyme can be released from the MoFe-protein by acid denaturation followed by extraction with dimethylformamide.24 While homocitrate was identified as a component of the isolated coenzyme, the three-dimensional structure of FeMo-co was deduced from X-ray crystallography of the intact molybdenum-iron protein.14/17/18... 

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. 

The destructive effect of dioxygen on the iron and molybdenum-iron proteins is thought to result from the oxidation of the clusters, and the formation of superoxide and peroxide which oxidize the proteins irreversibly. In view of this, it is remarkable that aerobic and facultative organisms can fix dinitrogen. The cyanobacteria evolve dioxygen photosynthetically and simultaneously fix dinitrogen 1447... 

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.

Kim, J. Rees, D.C. (1992). Crystallographic structure and functional implications of the nitrogenase molybdenum-iron protein from Azotobacter vinelandii. Nature (London) 360,553-560. 

Nitrogenase (Clostridium or Klebsiella) molybdenum iron protein... 

General Considerations. The nitrogenase enzyme consists of two separately isolable proteins—the molybdenum-iron protein (Component I, Fraction I, molybdoferredoxin) and the iron protein (Component II, Fraction II, azoferredoxin). The most recent work on nitrogenase com-... 

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-... 

Figure 1. EPR spectra of the molybdenum-iron protein of Azotobacter vine-landii, enriched as indicated by isolation of the protein from bacteria grown on isotopically enriched media.

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. 

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