Ammonia from nitrite

In various species of bacteria several different types of non-assimilatory nitrite reductases are found. Escherichia coli has a cytoplasmic NAD(P)H-dependent enzyme whose role seems to be detoxification of nitrite. This type of enzyme, coded for by the nirB gene, also contains siroheme as the redox active catalytic center (Cole, 1988). Additionally in E. coli, and expressed under different conditions to the cytoplasmic enzyme, is a periplasmic nitrite reductase that catalyses formation of ammonia from nitrite (Cole, 1988). This enzyme has five c-type (Figure 1) hemes per polypeptide chain one of these hemes, the catalytic site, has the unique CXXCK sequence as its attachment site (Einsle et al., 1999). Electrons reach this type of nitrite reductase, which is fairly widely distributed amongst the microbial world, from the cytoplasmic membrane electron transfer chain. The exact electron donor partner from such chains for this type of nitrite reductase is unknown (Berks et al., 1995). 

There is also considerable current environmental interest in hyponitrite oxidation because it is implicated in the oxidation of ammonia to nitrite, an important step in the nitrogen cycle (p. 410). Specifically, it seems likely that the oxidation proceeds from ammonia through hydroxylamine and hyponitrous acid to nitrite (or N2O). 

Commercially produced amines contain Impurities from synthesis, thus rigid specifications are necessary to avoid unwanted Impurities In final products. Modern-day analytical capability permits detection of minute quantities of Impurities In almost any compound. Detection In parts per million Is routine, parts per billion Is commonplace, and parts per trillion Is attainable. The significance of Impurities In products demands careful and realistic Interpretation. Nltrosatlng species, as well as natural amines, are ubiquitous In the environment. For example, Bassow (1976) cites that about 50 ppb of nitrous oxide and nitrogen dioxide are present In the atmosphere of the cities. Microorganisms In soil and natural water convert ammonia to nitrite. With the potential for nitrosamine formation almost ever-present In the envlronmeit, other approaches to prevention should Include the use of appropriate scavengers as additives In raw materials and finished products. 

The bis-hydroxylamine adduct [Fe (tpp)(NH20H)2] is stable at low temperatures, but decomposes to [Fe(tpp)(NO)] at room temperature. [Fe(porphyrin)(NO)] complexes can undergo one-and two-electron reduction the nature of the one-electron reduction product has been established by visible and resonance Raman spectroscopy. Reduction of [Fe(porphyrin)(NO)] complexes in the presence of phenols provides model systems for nitrite reductase conversion of coordinated nitrosyl to ammonia (assimilatory nitrite reduction), while further relevant information is available from the chemistry of [Fe (porphyrin)(N03)]. Iron porphyrin complexes with up to eight nitro substituents have been prepared and shown to catalyze oxidation of hydrocarbons by hydrogen peroxide and the hydroxylation of alkoxybenzenes. ... 

Nitrification seems limited to a number of autotrophic bacteria. The dominant genus that is capable of oxidizing ammonia to nitrite in soils is Nitmsomonas, and the dominant genus capable of oxidizing nitrite to nitrate is Nitrobacter. Normally, the two processes are closely connected and nitrite accumulation does not occur. Nitrifying bacteria are chemolithotrophs that utilize the energy derived from nitrification to assimilate C02. 

Unlike DOC, which can be easily differentiated from dissolved inorganic C (DIC), DON is calculated by subtracting the concentrations of nitrate, nitrite, and ammonia from total dissolved N (TDN). This technique introduces several errors to estimates of DON concentration, and robust marine DON concentrations are a relatively recent phenomenon (Sharp et al., 2004). Methods for quantifying each of these N pools are discussed in McCarthy and Bronk (this volume). Contemporary studies often omit the filtration step that differentiates dissolved organic matter (DOM) from particulate organic matter (POM) because DOM is far more abundant even in surface waters. Therefore, reported concentrations often represent total organic N (TON) rather than simply the dissolved species. 

Summers D. P. and Chang S. (1993) Prebiotic ammonia from reduction of nitrite by iron(II) on the early Earth. Nature 365, 630-633. 

Oxidation of ammonia to nitrite, N02, and nitrate, N03, is called nitrification the reverse reaction is ammonification. Reduction from nitrite to nitrogen is called denitrification. All these reactions, and more, occur in enzyme systems, many of which include transition metals. A molybdenum enzyme, nitrate reductase, reduces nitrate to nitrite. Further reduction to ammonia seems to proceed by 2-electron steps, through an uncertain intermediate with a -fl oxidation state (possibly hyponitrite, N202 ) and hydroxylamine ... 

Diatomic nitrogen makes up about 79 percent of Earth s atmosphere. Nitrogen is an essential element for life, yet only a few organisms can use atmospheric nitrogen directly. A few species of soil bacteria can produce ammonia, NH, from atmospheric nitrogen. Other species of bacteria can then convert the ammonia into nitrite and nitrate ions, which can be absorbed and used by plants. 

As mentioned above, ammonia is oxidized to nitrous acid via hydroxylamine in N. europaea first ammonia is oxidized to hydroxylamine by the catalysis of ammonia monooxygenase, and hydroxylamine formed is oxidized to nitrous acid by the catalysis of hydroxylamine oxidoreductase. Molecular oxygen is not necessary to the reaction itself of NH2OH — HN02 (Yamanaka and Sakano, 1980) but it is required for the consumption of electrons liberated from the reaction, NH2OH + H20 —> HN02 + 4H+ + 4c. Electrons thus liberated are transferred first to cytochrome c-554, then to cytochrome c-552, and finally oxidized with molecular oxygen by the catalysis of cytochrome c oxidase. Based on the results described above, the electron transfer pathway in the oxidation of ammonia to nitrite or nitrous acid by N. europaea will be presented as shown in Fig. 3.3. 

Although the oxidation mechanism of nitrite to nitrate in the heterotrophic nitrifiers has not been known at all on the enzyme level, the oxidation mechanism of ammonia to nitrite has been partially clarified. Ammonia is oxidized to nitrite through hydroxylamine also in the heterotrophic bacteria. The oxidation of ammonia to hydroxylamine is catalyzed by ammonia monooxygenase as in the enzyme of Nitrosomonas europaea. The enzyme purified from Paracoccus pantotropha GB17 (formerly Thiosphaera pantotropha GB17 or Paracoccus denitrificans GB17) catalyzes the oxidation of ammonia to hydroxylamine and contains copper, but its activity is not inhibited by acetylene (Moir et al., 1996), unlike the enzyme of Nitrosomonas europaea. 

The nitrifying bacteria, universally found in aerobic soil and aquatic environments, derive energy from the oxidation of reduced inorganic nitrogen compounds (ammonia and nitrite). As do autotrophic bacteria, they obtain carbon from carbon dioxide in the atmosphere. 

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