Ferredoxin-nitrate reductase

First, ferredoxin-nitrate reductase (EC 1.7.7.2) is a molybdenum-iron-sulfur protein that converts nitrate to nitrite while undergoing oxidation. Its structure is not yet well-defined. Then, ferridoxin-nitrite reductase (EC 1.7.7.1), an iron (as heme and at least one iron sulfur complex, vide infra) can continue the reduction to ammonia (NH3). 

There is now strong evidence that iron is very important in floral development in Xanthium (Smith etal. 1957). When Fe was supplied iron deficient plants after six photoinductive cycles, normal staminate inflorescence development occurred but the pistillate inflorescences were abnormal and reduced in number. If Fe was not added to the — Fe plants following photoinduction, many of them died. It is not possible at this time to say if Fe is directly or indirectly involved. Most probably, however, the effects are indirect and reflective of the importance of Fe in cytochromes, ferredoxin, nitrate reductase, and sulfite reductase as well as several other enzymes that are essential in cellular function. 

Fig. 6. Representative EPR spectra displayed by trinuclear and tetranucleEir iron-sulfur centers, (a) and (b) [3Fe-4S] + center in the NarH subunit of Escherichia coli nitrate reductase and the Ni-Fe hydrogenase fromD. gigas, respectively, (c) [4Fe-4S] + center in D. desulfuricans Norway ferredoxin I. (d) [4Fe-4S] center in Thiobacillus ferrooxidans ferredoxin. Experimental conditions temperature, 15 K microwave frequency, 9.330 GHz microwave power, (a) 100 mW, (b) 0.04 mW, (c) smd (d) 0.5 mW modulation amplitude (a), (c), (d) 0.5 mT, (b) 0.1 mT.

E. coli uses nitrate as a terminal electron acceptor through a respiratory, dissimilatory nitrate reductase whose synthesis is induced when nitrate is provided, and which is repressed by oxygen. Nitrate reductase is discussed with other molybdoenzymes in Section 62.1.9, and catalyzes the reduction of nitrate to nitrite. The enzyme is isolated from the cytoplasmic membrane of E. coli, and contains three subunits (a, j8 and y) although the y-subunit may be absent in some preparations. The -y-subunit is a b-type cytochrome, and the a-subunit is reported to be the catalytic subunit. The enzyme contains a number of iron-sulfur clusters, including a HiPIP and at least two ferredoxins.1054,1437... 

Fig. 1. The nitrate assimilation pathway in higher plants. The pathway of nitrate assimilation in the tobacco leaf is illustrated. In some other species an additional cytosolic GS is found in the leaf. The pathway in plant roots is more poorly documented and more variable GS in roots is mostly cytosolic, and some enzymes such as GOGAT are found as isoforms utilising alternate reducing substrates. T, expected nitrate carrier NR, nitrate reductase NiR, nitrite reductase GS, glutamine synthetase GOGAT, glutamate synthase Fd, ferredoxin Gin, glutamine Glu, glutamate.

The laser photolysis results on the ET behavior of these mutants have been confirmed by steady-state kinetic measurements [58, 59]. Interestingly, the latter experiments have shown that nonconservative mutations at F65 and E94 not only severely inhibit reactivity with FNR, but also with two other ferredoxin-dependent enzymes, nitrite reductase and nitrate reductase [58]. Apparently, similar structural constraints in their interactions with Fd are also operative in these other enzymes. 

In all photoautotrophs, reduction of NOj" to NH4 is achieved in two distinct enzymatic steps (Campbell, 2001). First, assimilatory nitrate reductase (NR) catalyzes the two electron reduction from NOj" to NO2. NR is a large soluble cytoplasmic enzyme with FAD (flavin adinine dinucleotide), an iron-containing cytochrome and molybdopterin prosthetic groups, and requires NADH and/or NADPH as an electron donor (Guerrero et al, 1981). Functional NR is in the form of a homodimer and therefore requires two atoms of iron per enzyme. Following transport into the chloroplast, NO2 undergoes a 6 e reduction to NH4 via assimilatory nitrite reductase (NiR). NiR, a soluble chloroplastic enzyme, contains five iron atoms per active enzyme molecule, and requires photosynthetically reduced ferredoxin as an electron donor (Guerrero et al., 1981). 

See also The Nitrogen Cycle, Nitrogen Fixation, Nitrate Utilization, Utilization of Ammonia, Nitrate Reductase, Dimethylsulfoxide Reductase, Nitrite Reductase, Siroheme, Ferredoxin... 

Nitrate reduction in cyanobacteria is tightly coupled to photosynthesis by ferredoxin, which provides both nitrate reductase (NR) and nitrite reductase (NiR) with electrons. In contrast to higher plant NR, cyanobacterial NR consists of only a single polypeptide of about 80 kD with one [Fe S ] or two [Fe2S2l clusters and a molybdenum cofactor. 

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