Nitrogenase reductase

Nitrogenase is a two-protein complex. One component, called Nitrogenase Reductase (NR) is an iron-containing protein that... 

Oxidized forms of DIN (NC>3 and NO2-) must also be converted into ammonia by either nitrate or nitrate reductases before being fixed into organic matter. Similarly, there is a nitrogenase reductase reaction involved in the fixation of N2. The range of fractionation observed with NC>3 assimilation is similar to that observed with NH4+, with more fractionation (by diatoms) occurring with higher ambient NO3 concentrations (Wada and Hattori, 1978). Active transport of NC>3 has been observed by marine diatoms however, details of the membrane-bound enzyme involved with this reaction remain unclear (Falkowski, 1975 Packard, 1979). 

Figure 10.3 Crystallographic structures of the most important iron-sulphur centres in proteins (a) dimeric centre of ferrodoxin from Spinacia oleracea [54], (b) trimeric centre of ferredoxin from Bacillus schlegelii [55], (c) cubane cluster of nitrogenase reductase from Azotobacter vinelandii [56], (d) nitrogenase octameric cluster from Azotobacter vinelandii [57], (e) nitrogenase octameric cluster from Clostridium pasteurianum [58], (f) MoFe cluster of nitrogenase from Azotobacter vinelandii [59], and (g) active centre of sulphite reductase from Escherichia coli [60]...

Hageman, R. V., and Burris, R. H. 1978. Nitrogenase and nitrogenase reductase associate and dissociate with each catalytic cycle. Proc. Natl. Acad. Sci. USA 75, 2699-2702. 

Ferredoxin is used by nitrogenase reductase (component II), an enzyme of nitrogen fixation, and also by nitrite reductase, which catalyzes reduction of nitrite to ammonia... 

The enzyme system responsible for N2 reduction, called the nitrogenase complex, consists of two separate proteins. As outlined in Figure 20.4, one protein-called component I, nitrogenase, or molybdenumiron protein-catalyzes the reduction of N2, and the other-called component II, nitrogenase reductase, or iron protein-transfers electrons from ferredoxin or flavodoxin to component I. Both component I and component II contain Fe4S4 iron-sulfur clusters, and component I also contains molybdenum, in the form of a tightly bound iron-molybdenum cofactor (FeMoCo). 

See also Metalloenzymes, Nitrogenase, Nitrogen Fixation, Figure 20.2, Ferredoxin, Nitrogen Fixation, Nitrogenase Reductase... 

See also Function of Coenzymes, Nitrogenase Reductase (vanadium-containing in Azotobacter)... 

Crystal structures of nitrogenase and nitrogenase reductase (Figure 20.5) are now available and may help scientists understand how the enzyme system for nitrogen fixation operates. 

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