Protein azotobacter molybdenum-iron

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. 

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.

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. 

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. 

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),... 

Molybdenum has long been known to have a role in nitrogen fixation over 50 years ago Bortels showed that Mo stimulated the N2-dependent growth of Azotobacter (2). Subsequently, these observations were rationalized when all purified nitrogenases were shown to be separable into an Fe protein and a MoFe protein that contained Mo as part of an essential iron- and molybdenum-containing cofactor (Fe-Moco), the probable site at which N2 is reduced. 

Both proteins contain iron-sulfur clusters. Nitrogenase also contains a tightly bound iron-molybdenum cofactor. Some variations on this theme are known. Some bacteria, such as Azotobacter, contain more than one nitrogenase complex. Of the three systems in Azotobacter, one uses vanadium instead of molybdenum and another uses only iron. 

Fe Protein. Characteristics of the Fe protein from Azotobacter 101) and Clostridia (94, 108) are summarized in Table VII. Clostridial Fe protein is stated to be 90-95% pure and the absence of tryptophan (102) suggests that almost all of the contaminating proteins have been removed. Recent preparations of Azotobacter Fe protein show specific activities even higher than those of clostridial Fe protein (101). The clostridial protein has a molecular weight of 39,000 and contains 2-3 Fe and 2 S atoms (94, 108). Molybdenum is absent, no ESR is detectable in the native protein (102) and, surprisingly, a resonance at g value of 1.94 has not been observed on reduction. All other purified iron-sulfur proteins with the exception of the atypical HIPIP and rubredoxin exhibit resonance in this area on reduction. One may suggest that the Fe protein is an atypical iron-sulfur protein or requires additional examination at 4°K for a resonance at g = 1.94. The individual protein has no activity alone but has all N2ase activities in combination with the Mo-Fe protein. 

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