Molybdenum-iron protein, nitrogenase

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...

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.

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... 

In this scheme the electrons are all supplied by the metal, which undergoes a change in oxidation state of six. In nitrogenase, electrons would be supplied from the iron protein and there would be no need for large changes in oxidation state in the molybdenum. 

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.

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. 

The iron-sulfur cubes that have been detected by x-ray analysis (49) as constituents of the enzymes ferridoxin and high-potential iron protein have been extruded from these enzymes by replacing the sulf-hydryl ligands of the enzymes with simple mercaptans, and these cubes identified with the corresponding synthetic compounds, II (50, 51, 52, 53). The latter have oxidation-reduction properties that closely mimic those of the enzymes. Similarly, an iron-sulfur-molybdenum double cube has been ejected from nitrogenase, and a similar double cube has been synthesized by Holm and his collaborators (54). It remains to be seen whether or not the iron-sulfur-molybdenum double cube can mimic the properties of nitrogenase (55). 

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-... 

Avaible experimental structural and kinetics data and energetic considerations indicate two plausible roles of ATP in the nitrogenase reduction a) the triggering of electron transfer from iron protein to iron-molybdenum protein (Howard and Rees, 1994 Rees and Howard, 2000) and the strengthening reducing power of the enzyme catalytic redox centers (Likhtenshtein and Shilov, 1977, Likhenshtein 1988a, Syrtsova and Timofeeva, 2001 see also Section 6.1.4). 

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. 

Dean, D.R., Setterquist, R.A., Brigle, K.E., Scott, D.J., Laird, N. F., Newton, W.E., (1990) Evidence that conserved residues Cys-62 and Cys-154 within the Azotobacter vinelandii nitrogenase iron-molybdenum protein a-subuflit are essential for nitrogenase activity but conserved residues His-83 and Cys-88 are not. Mol. Microbiol. 4(9),... 

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. 

Vanadium nitrogenase is produced by certain bacteria grown in molybdenum-deficient environments. It is effective in the reduction of N2 and other nitrogenase substrates, although with less activity than the Mo—Nase. The enzyme resembles the Mo analogue (see Sections 17-E-10 and 18-C-13) in the construction and structure of the prosthetic groups, as well as in its functions.101 It consists of a FeV protein, FeVco, and an iron protein (a 4Fe—4S ferredoxin). 

---