Nitrate dissimilatory

In bacteria, with Pichinoty type A dissimilatory nitrate reductase, induction of the enzyme occurs only under anaerobic conditions, and occasionally anaerobiosis is sufficient to trigger formation in the absence of nitrate (Schulp Md Stouthamer, 1970 Sinclair and White, 1970). However, in these cases of induction by anaerobiosis, the level of enzyme production is greatly enhanced by the presence of nitrate. Dissimilatory nitrate reductase may also be induced under anaerobic conditions by nitrite and azide. 

DiChristina TJ (1992) Effects of nitrate and nitrite on dissimilatory iron reduction by Shewanella putrefaciens 200. J Bacteriol 174 1891-1896. 

Others oxotransferases) (2 pyranopterins bonded to Mo) (8-10 members) Nitrate reduction dissimilatory terminal respiratory oxidase Pyridoxal oxidase Xanthine dehydrogenases Pyrogallol transhydrolase Nitrate to nitrite... 

Figure 3. The general nitrogen model for illustrating the bio geochemical cycling in Forest ecosystems. Explanations for the fluxes 1, ammonia volatilization 2, forest fertilization 3, N2-fixation 4, denitrification 5, nitrate respiration 6, nitrification 7, immobilization 8, mineralization 9, assimilatory and dissimilatory nitrate reduction to ammonium 10, leaching 11, plant uptake 12, deposition N input 13, residue composition, exudation 14, soil erosion 15, ammonium fixation and release by clay minerals 16, biomass combustion 17, forest harvesting 18, litterfall (Bashkin, 2002).

Denitrification, a dissimilatory pathway of nitrate reduction (see Section 3.3 also) into nitrogen oxides, N2O, and dinitrogen, N2, is performed by a wide variety of microorganisms in the forest ecosystems. Measurable rates of N20 production have been observed in many forest soils. The values from 2.1 to 4.0 kg/ha/yr are typical for forest soils in various places of Boreal and Sub-Boreal Forest ecosystems. All in situ studies (field monitoring) of denitrification in forest soils have shown large spatial and temporal variability in response to varying soils characteristics such as acidity, temperature, moisture, oxygen, ambient nitrate and available carbon. 

At this site in the eastern tropical North Pacific, denitrification is responsible fiar the midwater loss of nitrate and production of nitrite. The size of the secondary nitrite maximum is dependent on the relative rates of its production from NO3 and its loss via dissimilatory reduction to N2. The amount of nitrate lost to denitrification is shown as the difference between the measured nitrate and the calculated nitrate. The latter was estimated by multiplying the observed phosphate concentrations by the average nitrate-to-phosphate ratio in the three deepest samples (11.9 1.6pmolN/L). Note that the zone of denitrification is restricted to mid-depths, i.e., the depths of the OMZ at this site. 

The fact that the cytochrome P-450 was induced even in the presence of NH3, which is the end product of assimilatory N-oxide reductions, suggested that it might funciton in dissimilatory N-oxide reductions. Anaerobic growth experiments with induced cells showed that reduction of nitrate to nitrite was energy yielding in F. oxysporum but reduction of nitrite to N2O was probably not (Shoun and Tanimoto, 1991). 

Alefounder, P. R., and Ferguson, S. J. (1980). The location of dissimilatory nitrite reductase and the control of dissimilatory nitrate reductase by oxygen in Paracoccus denitri-ficans. Biochem.J. 192, 231-240. 

Carlson, C. A., Ferguson, L. P., and Ingraham, J. L. (1982). Properties of dissimilatory nitrate reductase purified from the denitrifier Pseudomonas aeruginosa. J. Bacteriol. 151, 162-171. 

Tiedje, ]. M. (1988). Ecology of denitrification and dissimilatory nitrate reduction to ammonia. In Biology of Anaerobic Microorganisms (A. J. B. Zehnder, ed.). pp. 179-244. Wiley, New York. 

A number of other reductases and dehydrogenases, including dissimilatory nitrate reductases of E. coli and of denitrifying bacteria (Chapter 18), belong to the DMSO reductase family. Other members are reductases for biotin S-oxide,649 trimethylamine N-oxide, and polysulfides as well as formate dehydrogenases (Eq. 16-63), formylmethanofuran dehydrogenase (Fig. 15-22,... 

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. 

Generally, the assimilatory nitrate and nitrite reductases are soluble enzymes that utilize reduced pyridine nucleotides or reduced ferrodoxin. In contrast, the dissimilatory nitrate reductases are membrane-bound terminal electron acceptors that are tightly linked to cytochrome by pigments. Such complexes allow one or more sites of energy conservation (ATP generation) coupled with electron transport. 

Nitrate reductase (dissimilatory) Escherichia coli 200000 1 4Fe4S4... 

Pseudomonas aeruginosa can synthesize both types of nitrate reductase, depending upon the environmental conditions, the dissimilatory enzyme being repressed by dioxygen.1049... 

Nitrate reductase from Chlorella, an assimilatory enzyme, is a homotetramer of molecular weight 360 000 and contains one each of Mo, heme and FAD per subunit. The nitrate reductase from E. coli is a dissimilatory enzyme. EXAFS data are available on the molybdenum sites in both enzymes (Table 24).1050 The environment of the molybdenum in the assimilatory enzyme is similar to that found for sulfite oxidase, with at least two sulfur ligands near the molybdenum and a shuttle between monoxo and dioxo forms with redox change in the enzyme. This allows a similar mechanism to be put forward for the assimilatory nitrate reductase,1051 shown in equation (57), where an oxo group is transferred from nitrate to MoIV with production of nitrite and MoVI. 

The structure of the reduced dissimilatory reductase from E. coli shows noticeable differences. In particular, there is a long-range interaction with an unidentified neighbour. It is tempting to suggest that this is an iron atom from an iron-sulfur centre, and to propose that there is a concerted two-electron transfer to the nitrate.1052 The different function of the two enzymes can then be related to the different mechanisms. 

E. coli uses nitrate as a terminal electron acceptor through a respiratory, dissimilatory nitrate reductase whose synthesis is induced when nitrate is provided, and which is repressed by oxygen. Nitrate reductase is discussed with other molybdoenzymes in Section 62.1.9, and catalyzes the reduction of nitrate to nitrite. The enzyme is isolated from the cytoplasmic membrane of E. coli, and contains three subunits (a, j8 and y) although the y-subunit may be absent in some preparations. The -y-subunit is a b-type cytochrome, and the a-subunit is reported to be the catalytic subunit. The enzyme contains a number of iron-sulfur clusters, including a HiPIP and at least two ferredoxins.1054,1437... 

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