Bacteria nitric oxide production

Nitrite reductases (NiRs)—enzymes found in several strains of denitrifying bacteria— catalyze the one-electron reduction of nitrite anion to nitric oxide (Equation 1). - In addition to the importance of this process in the global nitrogen cycle (Figure 1), further incentive for the study of the denitrification process is provided by its environmental impact, ranging from the production of NO as a pollutant and NjO as a potent greenhouse gas, to lake eutrophication due to farm runoff that contains high concentrations of nitrates and nitrites. 

Ren T., Roy R., and Knowles R. (2000) Production and consumption of nitric oxide by three methanotrophic bacteria. Appl. Environ. Microbiol. 66, 3891—3897. 

Denitrification in the strict sense involves the dissimilatory reduction, by essentially aerobic bacteria, of nitrate or nitrite to the gaseous oxides (nitric oxide, NO, and nitrous oxide, N2O), which may themselves be further reduced to nitrogen (Nj). Denitrification does not include assimilation of these ions by plants or microorganisms or loss by leaching. The products of denitrification are not assimilable by higher plants and most microorganisms, and thus this process is deleterious to soil fertility [1, 27]. 

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. 

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