Nitrogenase composition

The biological nitrogen fixation process is Introduced. Discussion focusses on the Dominant Hypothesis of nitrogenase composition and functioning. The enzyme system catalyzes the six-electron reduction of N2 to 2 NH3 concomitant with the evolution of H2. ATP hydrolysis drives the process. The two protein components of the enzyme,... 

A comprehensive description of the mechanism of molybdenum nitrogenase has been provided by the Lowe-Thorneley scheme 102) (Figs. 8 and 9). In this scheme the Fe protein (with MgATP) functions as a single electron donor to the MoFe protein in the Fe protein cycle (Fig. 8), which is broken down into four discrete steps, each of which may be a composite of several reactions ... 

The elucidation of the crystal structures of two high-spin EPR proteins has shown that the proposals for novel Fe-S clusters are not without substance. Two, rather than one novel Fe-S cluster, were shown to be present in nitrogenase, the key enzyme in the biotic fixation of molecular nitrogen 4, 5). Thus the FeMoco-cofactor comprises two metal clusters of composition [4Fe-3S] and [lMo-3Fe-3S] bridged by three inorganic sulfur atoms, and this is some 14 A distant from the P-cluster, which is essentially two [4Fe-4S] cubane moieties sharing a corner. The elucidation of the crystal structure of the Fepr protein (6) provides the second example of a high-spin EPR protein that contains yet another unprecedented Fe-S cluster. 

Recently, a second or alternative nitrogenase has been isolated from Azotobacter vinelandii (21) and Azotobacter chroococcum (22) that contains vanadium as opposed to molybdenum. The MoFe and VFe nitrogenase proteins from A. vinelandii (called Av and. 4vl , respectively) are known to have different polypeptide structures and it obviously of interest to know to what extent the cluster composition is conserved. Variable temperature MCD studies of the as isolated and thionine oxidized proteins provided a convenient means of addressing this question. 

Nitrogenase, which catalyzes the reduction of N2 to two molecules of NH3, has a different molybdenum -iron cofactor (FeMo-co). It can be obtained by acid denaturation of the very oxygen-labile iron-molybdenum protein of nitrogenase followed by extraction with d i methyl formamide.655,656 The coenzyme is a complex Fe-S-Mo cluster also containing homocitrate with a composition MoFe7S9-homocitrate (see Fig. 24-3). Nitrogenase and this coenzyme are considered further in Chapter 24. 

Harris TV, Szilagyi RK. Comparative Assessment of the Composition and Charge State of Nitrogenase FeMo-Cofactor. Inorg Chem. 2011 50 4811-24. 

Unlike the nitrogenases, which do not vary much in composition and physical properties with the source, nitrate reductases vary considerably from one organism to the next. The only feature which they all seem to have in common is an absolute requirement for the presence of molybdenum. 

The Fe protein of the vanadium nitrogenase, encoded by vnfH, shares a high degree of homology with the niJH-encoded Fe protein of the molybdenum nitrogenase. Like its counterpart in the molybdenum enzyme, the Fe protein of the vanadium nitrogenase has an U2 homodimeric composition and contains an [4Fe-4S] cluster. 

Benthic cyanobacteria also show strong diel patterns which depend on the composition of the cyanobacterial population. Mats dominated by heterocyctous cyanobacteria generally show a daytime maximum while those dominated by non-heterocystous forms exhibit maximum nitrogenase activity at night (e.g., Paerl et al., 1988 Stal and Krumbein, 1987 Steppe and Paerl, 2005). 

As we have already seen, FeMo cofactor of nitrogenase is a polynuclear complex of composition Fc7MoS9 (homocitrate), and all the available evidence implies it is the active site at which dinitrogen and other nitrogenase substrates are activated and reduced. Yet despite its first isolation in 1977 the catalytic activity of FeMoco was detected only in 1997, i.e. 20 years later. 

As discussed below, the consensus view of the redox centers present in MoFe proteins is that there are two types (see Ref. 28 for discussion). There are two FeMoco centers (which have the approximate composition one Mo, six to eight Fe, four to nine S, and one homocitrate this is unique to nitrogenase and different from the Mo cofactors of other Mo-containing enzymes), and four [4Fe-4S] centers (the P clusters). If these assignments are correct, then because only 16 Cys residues are invariant among MoFe proteins, conventional Cys ligation to the P ... 

---