Nitric oxide reductase, denitrification

In summary, a considerable body of enzymatic, genetic, and analytical data supports the view that the major, if not sole, pathway of denitrification involves NO as an obligatory intermediate and requires the action of nitric oxide reductase. On the other hand, the ability of nitrite to modify nitrosyl transfer ratios and the N isotope fractionation factor during its reduction, are consistent with the reductive scheme of Averill and Tiedje (1982). It was suggested (Goretski... 

IV. NITRIC OXIDE REDUCTASE, NITRIC OXIDE-CONSUMING ENZYME OF DENITRIFICATION PATHWAY... 

Braun, C., and Zumft, W. G. (1991). Marker exchange of the structural genes for nitric oxide reductase blocks the denitrification pathway of Pseudomonas stutzeri at nitric oxide. . Biol. Chem. 266, 22785-22788. 

Miyata, M., Matsubara, T., and Mori, T. (1969). Studies on denitrification XL Some properties of nitric oxide reductase. J. Biochem. (Tokyo) 66, 759-763. 

Figure 2 The dissimilatory denitrification pathway. NO3 is reduced to NO2 by a membrane-bound or periplasmic nitrate reductase(NaR). N02 is reduced to NO by either a cytochrome cdi or copper nitrite reductase (NiR). NO is reduced to N2O by nitric oxide reductase (NOR). N2O is reduced to N2 by nitrous oxide reductase (N2OR). Electron transport from uhiquinol (UQH2) at NaR and the cyt hcj complex is coupled to generation of a proton gradient...

Nitric oxide reductase (Nor) forms part of the denitrification pathway found in Bacteria and Archaea. Denitrification is the five-step conversion of nitrate to dinitrogen according to NOs N02 NO N2O N2. Nor catalyzes the reaction,... 

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 ... 

Biochemically, in denitrification electrons are transported via cytochromes to nitrogen oxide reductases. These reductases are indicated in the scheme below as follows (1) nitrate reductase, (2) nitrite reductase, (3) nitric oxide reductase, and (4) nitrous oxide reductase. 

Each step of the denitrification pathway is catalyzed by a distinct enzyme, nitrogen oxide reductase (nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase), that transfers electrons from the chain to the particular intermediate. Thermodynamically, in the absence of oxygen, nitrogen oxides are the most preferred electron acceptors by facultative bacterial groups. The role of nitrogen oxides in regulating organic matter decomposition has been discussed in earlier chapters (see Chapter 5). 

Denitrification enzyme activity Catabolic nitrate redaction involves several enzymes including dissimilatory nitrate rednctase (nitrate to nitrite), nitrite rednctase (nitrite to nitric oxide), nitric oxide reductase (nitric oxide to nitrons oxide), and nitrons oxide reductase (nitrous oxide to nitrogen gas). The combined effect of all these enzymes is operationally dehned as denitrification enzyme activity (DEA), which is now rontinely measnred under... 

Very likely CO2, CO, and N2 were among the main components by the time of the origin of life within the Earth s early atmosphere. In addition, NO, H2, H2O, and sulfuric gases could also have been present, and probably trace amounts of O2 [23]. Thus, it was not too surprising that several molecular links do exist between the evolution of denitrification enzymes and cytochrome c oxidase, the terminal oxidase of aerobic respiration [24]. Perhaps the most important is the homology between nitric oxide reductase and heme/copper cytochrome oxidases, and the presence of the mixed-valence [Cu " (Scys)2Cu " CuA electron-transfer center in nitrous oxide reductases, the quinol-dependent NO reductase from the gram-positive Bacillus azotoformans, and heme/copper cytochrome oxidases [24-27]. 

The denitrification process could be described as a modular organization in which every biochemical reaction is catalyzed by specific reductase enzymes (Cuervo-Lopez et al., 2009). Four enzymatic reactions take place in the cell as follows (l) nitrate is reduced to nitrite by nitrate reductase (Nar) (ii) a subsequent reduction of nitrite to nitric oxide is carried out by nitrite reductase (Mr) (iu) afterwards, nitric oxide is reduced to nitrous oxide by the enzyme nitric oxide reductase (Nor) (iz ) finally, nitrous oxide is reduced to N2 by the enzyme nitrous oxide reductase (Nos) (Lalucat et al., 2006) (Table 9). These reactions take place when environmental conditions become anaerobic (Berks et al., 1995 Hochstein Tomlinson, 1988). The enzymatic reactions, which are thermodynamically favored, are carried out in the cell membrane and periplasmic space. Small half saturation constant values (Km) have been reported for different nitrogen substrates for some denitrifying bacteria, indicating that denitrifying enzymes have a high affinity for their substrate. However, several factors have to be considered, as the presence of small quantities of molybdenum, cooper and hem to ensure the successful enzymatic activity, as they are known cofactors for denitrifying enzymes. 

CYP55 Nitric oxide reductase Denitrification F. oxysporum, Cylindmcatpon tonkinese, A. oryzae, T. cuianeum [516,611,613,831]... 

Shoun H, Fushinobu S, Jiang L, Kim S-W, Wakagi T (2012) Fungal denitrification and nitric oxide reductase cytochrome P450nor. Philos Trans R Soc B Biol Sci 367 1186-1194... 

Reactions of Nitric Oxide Reductases The bacterial nitric oxide reductase (NOR) is able to reduce NO to N2O during bacterial denitrification [275-278]. A review on the topic has recently been published [274] and only the electron-transfer mechanism of the enzyme is discussed in the present review. 

Nitrite Reductase. Bacterial denitrification in anaerobic microorganisms involves a four-step process, with the overall reduction of nitrate (N03 ) to dinitrogen (N2). Each reaction in the reduction process is catalyzed by one or more different metalloenzymes, which have various transition metals found in diverse ligand environments nitrate reductase (NOR, molybdenum), nitrite reductase (NiR, iron or copper), nitric oxide reductase (NOR, heme and nonheme iron), and nitrous oxide reductase (N2OR, copper) (9). 

The organization of the enzymes of denitrification in gram-negative bacteria, as determined by antibody labelling and electron microscopy studies, is shown in Figure 5. The first enzyme, nitrate reductase (NaR), resides in the cytoplasmic membrane with its active site accessed from the cytoplasmic side, necessitating transport of nitrate across both the periplasmic and cytoplasmic membranes. The product nitrite is transported back into the periplasmic space, where it is reduced by the nitrite reductase (NiR). Most NiR s appear to be soluble enzymes, although there have been reports of preparations in which the activity was associated with membrane fractions. The nitric oxide reductase (NoR) is also localized in the cytoplasmic membrane, and releases its product N2O back into the cytoplasmic space, where the soluble enzyme nitrous oxide reductase (NoS) converts it to N2. 

Figure 5. The arrangement of the enzymes of denitrification in gram-negative bacteria NaR, nitrate reductase NiR, nitrite reductase NoR, nitric oxide reductase NoS, nitrous oxide reductase (35).

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