Denitrification nitrite reductase

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. 

Nitrite reductase Denitrification Denitrifying bacteria Liu etal. (1986)... 

Nitrite reductases and nitrous oxide reductases are relatively newly found copper-containing proteins involved in bacterial denitrification. N2O reductase may bear a relationship to cytochrome oxidase and, indeed, parallels it somewhat in function, being the terminal electron acceptor in its pathway. 

O-exchange studies of Ye et al. (1991) support, we believe, the catalysis by nitrite reductase of redox reversibility between nitrite and NO as depicted in the first line of Eq. (3). They observed by analyzing the 0 content of product N2O that all eight strains of denitrifying bacteria studied could catalyze the exchange of 0 between water and nitrite or NO by way of an electrophilic (nitrosyl donor) species of NO. The rates and extent of these exchange reactions depended on whether the bacterium made use of a heme- or Cu-type nitrite reductase. Contrary to the conclusions of Ye et al. (1991), we do not believe that this study otherwise informs about the pathway of denitrification or whether NO is an intermediate. 

Hulse, C. L., and Averill, B. A. (1989). Evidence for a copper-nitrosyl intermediate in denitrification by the copper-containing nitrite reductase of Achromobacter cycloclastes. J. Am. Chem. Soc. Ill, 2322-2323. 

Jiingst, A., Braun, C., and Zumft, W. G. (1991a). Close linkage in Pseudomonas stutzeri of the structural genes for respiratory nitrite reductase and nitrous oxide reductase, and other essential genes for denitrification. Mol. Gen. Genet. 225, 241-248. 

Bacterial assimilatory nitrate reductases have similar properties.86/86a In addition, many bacteria, including E. coli, are able to use nitrate ions as an oxidant for nitrate respiration under anaerobic conditions (Chapter 18). Tire dissimilatory nitrate reductases involved also contain molybdenum as well as Fe-S centers.85 Tire E. coli enzyme receives electrons from reduced quinones in the plasma membrane, passing them through cytochrome b, Fe-S centers, and molybdopterin to nitrate. The three-subunit aPy enzyme contains cytochrome b in one subunit, an Fe3S4 center as well as three Fe4S4 clusters in another, and the molybdenum cofactor in the third.87 Nitrate reduction to nitrite is also on the pathway of denitrification, which can lead to release of nitrogen as NO, NzO, and N2 by the action of dissimi-latory nitrite reductases. These enzymes873 have been discussed in Chapters 16 and 18. 

This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 

There are four different types of nitrite reductases the copper-containing protein Copper Enzymes in Denitrification and cytochrome cd perform a one-electron rednetion of nitrite to nitric oxide, and are involved in denitrification " the siroheme-containing protein and the cytochrome c ititrite reductase (cNiR) both perform the complete, six-electron reduction, of nitrite to ammonia. The cNiR is present in the y, 5 or e-subclasses of proteobacteria, and is encoded by the nrf operon (nitrite reduction with /ormate), which has different gene composition in the different classes of bacteria, having in common only the gene for the catalytic subunit, ntfA. 

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...

It is also possible to use antibody based approaches to assay the accumulation of enzymes involved in denitrification pathways in environmental samples. Ward and Cockroft (1993) developed antisera to nitrite reductase from a marine Pseudomonas stutzeri isolate, and assayed sediment, water column and microbial mat samples. The antibody identified nitrite reductase in mat samples and was more specific than a DNA probe designed against the same species (Ward and Cockroft, 1993). JVIore recently, other strains of denitrifiers have been isolated from both sediment and water column samples. Sequence information regarding the denitrifying genes should prove invaluable for future refinement of both sequence based approaches and the development of additional antibodies to denitrifiers important in the marine environment. 

Inorganic nitrogen nitrate, nitrite, and ammonium ions These ions are relevant parameters in environmental analysis because they are involved in important biological processes such as nitrate reduction, nitrite reduction and denitrification. Nitrate and nitrite reductases play a key... 

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 ... 

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