Nitrite Reductases EC

Nitrate reduction and assimilation is a fundamental biological process in plants and various microorganisms. In this process nitrate is reduced ultimately to ammonia. Thus, as shown in Eq. (5), the reduction of nitrate to ammonia requires eight electron or hydrogen equivalents. 

The first reduction product of nitrate is nitrite. This reaction is catalyzed 

Lafferty and R. H. Garrett, Abstr. 73rd. Annu. Meet Amer. Soc. Microbiol.-g. 194 (1973). 

Walker and Nicholas (338) have reported the isolation and 600-fold purification of an enzyme from P. aeruginosa, which reduces nitrite to nitric oxide. The preparation contained 1.5 nmoles of FAD per mg protein, a c-type cytochrome and an absorption band at 630-635 nm, suggestive of copper. As electron donors, reduced FMN, FAD, riboflavin, pyo-cyanine, and methylene blue were effective, but not NADH, NADPH, or reduced cytochrome c. The preparation required phosphate or sulfate for maximal activity. The cytochrome and the 630-635-nm band were reduced under anaerobic conditions with a suitable electron donor and readily oxidized by nitrite. The K for NaNOj is reported to be 3.1 X 10  

The presence of an active thiol in the enzyme is indicated by p-mercu-ribenzoate inhibition and glutathione reactivation. 

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Two-component system for nitrate Nitrate Nitrite reductase [EC 1.6.6.4] ... 

This redox protein participates in several enzyme-catalyzed reactions, including glutamate synthase (ferredoxin) [EC 1.4.7.1] ferredoxin-nitrite reductase [EC... 

Nitrite reductase (NAD(P)H) [EC 1.6.6.4] catalyzes the reaction of three NAD(P)H with nitrite to yield three NAD(P)+, NH4OH, and water. Cofactors for this enzyme include FAD, non-heme iron, and siroheme. (2) Nitrite reductase (cytochrome) [EC 1.7.2.1] is a copper-depen-dent system that catalyzes the reaction of nitric oxide with two ferricytochrome c and water to produce nitrite and two ferrocytochrome c. (3) Ferredoxin-nitrite reductase [EC 1.7.7.1], a heme- and iron-dependent enzyme, catalyzes the reaction of ammonia with three oxidized ferredoxin to produce nitrite and three reduced ferredoxin. (4) Nitrite reductase [EC 1.7.99.3] is a copper- and FAD-dependent enzyme that catalyzes the reaction of two nitric oxide with an acceptor substrate and two water to produce two nitrite and the reduced acceptor. 

Enzymatic assay of nitrite and nitrate is based on the following reactions catalyzed by nitrate reductase (EC 1.9.6.1) and nitrite reductase (EC 1.6.6.4) ... 

Nitrite reductase (EC 1.7.99.3) is one of the two copper enzymes in the dis-similatory pathway of denitrifying bacteria (for example Achromobacer, Pseudomonas, Rhodobacter) ... 

Although there is another nitrite reductase (EC 1.9.3.2), which contains iron and utilizes the redox-active cytochromes c and dx, the following will only treat the copper enzyme [90]. 

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). 

This two-electron reduction uses NADH or NADPH as electron donor depending on the particular nitrate reductase (EC 1.7.1.1 or EC 1.7.1.2, respectively). Nitrite is then reduced to ammonium in a six-electron process that involves the transfer of three electron pairs from NAD(P)H ... 

In contrast, many gram-negative bacteria contain a nitrate reductase (EC 1.7.99.4 and/or 1.9.6.1) that also reduces nitrate to nitrite although under anaerobic conditions. The dissimilatory nitrite reduction leading to denitrification encompasses then the reduction of nitrite to nitric oxide by dissimilatory nitrite reductases (NiR, EC 1.7.2.1) that, in combination with nitric oxide reductases (NOR) and nitrous oxide reductases (N2OR), transform nitrite into nitrogen ... 

Nitrate reductase (NADH) [EC 1.6.6.1], also known as assimilatory nitrate reduetase, eatalyzes the reaction of NADH with nitrate to produee NAD+, nitrite, and water. This enzyme uses FAD or FMN, heme, and a molybdenum ion as eofaetors. (2) Nitrate reductase (NAD(P)H) [EC 1.6.6.2], also known as assimilatory nitrate reduetase, eatalyzes the reaetion of NAD(P)H with nitrate to produee NAD(P)+, nitrite, and water. This enzyme uses FAD or FMN, heme, and a molybdenum ion as eofaetors. (3) Nitrate reductase (NADPH) [EC 1.6.6.3] eatalyzes the reaetion of NADPH with nitrate to produee NADP+, nitrite, and water. This enzyme uses FAD, heme, and a molybdenum ion as cofactors. (4) Nitrate reduetase (eytoehrome) [EC 1.9.6.1] catalyzes the reaetion of nitrate with ferroeytochrome to produce nitrite and ferrieytoehrome. (5) Nitrate reductase (ac-eeptor) [EC 1.7.99.4], also known as respiratory nitrate... 

The reduction of NADP by ferredoxiniNADP reductase (FNR, EC 1.18.1.2) is the terminal step in the photosynthetic generation of strong reductant in chloroplasts (1). FNR, an FAD containing enzyme, also catalyzes electron flow from NADPH to ferredoxin (Fd), which supplies electrons for reduction of nitrite and sulfite (2), and may facilitate the redox poising of cyclic electron transport around PSl (3). 

Nitrate reductase enzymes which catalyse the reduction of nitrate to nitrite. All N.r. studied so far contain iron and molybdenum. In the sequence of electron transfer, molybdenum appears to be the ultimate acceptor, which then transfers electrons to nitrate during this process the molybdenum alternates between Mo(V) and Mo(VI). Dissimilatory N.r. from E. coii (also called respiratory N.r.) (EC 1.7.99.4) is a transmembrane protein, containing Mo, inorganic sulfur and nonheme iron, approximate M,... 

Nitrate reductase, the first enzyme in the nitrate assimilation pathway, catalyzes the reduction of nitrate to nitrite. This requires two electrons which are donated by either NADH or NADPH in the eukaryotic NRs. The NADH-spe-cific NRs (EC 1.6.6.1) are found in most higher plants and numerous eukaryotic algae while the NADPH-specific NRs (EC 1.6.6.3) are found in the fungi. The NAD(P)H-bispecific NRs (EC 1.6.6.2) occur in some higher plant and algae species. The prokaryotic NRs which utilize a variety of electron donors, including ferredoxin, reduced pyridine nucleotides, and respiratory intermediates, will not be considered in this article (see Stewart, 1988). 

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