Bacterial nitrate reductase

Enzymes that belong to the sulfite oxidase family are comprised of the assim-ilatory eukaryotic nitrate reductases, bacterial YedY and the sulfite oxidizing enzymes. The latter are found in bacteria, plants, animals and humans, and are the primary focus of this section. A similar protein fold of the Mo domain, the so-called SUOX fold, characterizes these enzymes. The nature of the protein fold is one key factor that distinguishes SO family enzymes from the MOSC family proteins, which possess a very similar... 

Moreno-Vivian C, P Cabello, M Martmez-Luque, R Blasco, F Castillo (1999) Prokaryotic nitrate reduction molecular properties and functional distinction among bacterial nitrate reductases. J Bacteriol 181 6573-6584. 

McEwan AG, Benson N, BonnetTC, et al. 1991. Bacterial dimethyl sulfoxide reductases and nitrate reductases. Biochem Soc Trans 19 605-8. 

Two [Fe4S4] clusters have been identified and analyzed in bacterial and archaeal adenylylsulfate reductases. These enzymes are of importance in the sulfur cycle. The role of the iron-sulfur centres in relation to the third cofactor, FAD, has been studied. It was shown that the [Fe4S4] clusters function were electron transport guiding two electrons to the FAD catalytic site.234 A novel [Fe4S4] cluster with a high spin ground state (S = 3/2) was observed in the catalytic site of E. coli nitrate reductase A.235... 

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. 

As noted above, gastric acid serves as an important barrier to bacterial colonization of the stomach and small intestine. Increases in gastric bacterial concentrations are detected in patients taking proton pump inhibitors. An increase in nitrate-reductase positive strains could theoretically increase carcinogenic nitrites and N-nitrosamines. However, most studies do not demonstrate this. 

Nitrate reductase (dissimila- Bacterial a2, a(3, (MPTpG)Mo(O) Unknown 4 Fe4S4 107... 

Many species of bacteria also have an assimilatory nitrite reductase which is located in the cytoplasm. There is relatively little known about such enzymes but the electron donor is throught to be NADPH and the active site again has siroheme (Cole, 1988). The assimilatory nitrite reductases of both plants and bacteria use nitrite that is provided as the product of the assimilatory nitrate reductases. Nitrate is a very common natural N source for plant and bacterial growth. 

In the presence of nitrate, formate dehydrogenase-N (FDH-N) and nitrate reductase are induced (8). As a result, the formate is consumed preferentially by FDH-N due to its inherent high affinity for formate versus FDH-H. The Km value for formate of FDH-N is 0.12 mM, while that of FDH-H is 26 mM. The FHL system is induced by the presence of formate, however, formate consumption by an FDH-N - nitrate reductase system depresses the expression level of the FHL system. Consequently, both expression of FHL and hydrogen production are repressed by the presence of nitrate in the medium. The removal of nitrate from waste water is not a practical process therefore a bacterial strain capable of hydrogen production even in the presence of nitrate is favorable. The subunits of nitrate reductase are encoded in the fdn operon. The a-subunit of nitrate reductase is encoded in the... 

To date, only the role of vanadium in biogenic nitrogen fixation has been established unambiguously. In addition, there are a few reports on a putative importance of vanadium in (bacterial) nitrate reductases, otherwise a domain of molybdenum. These reports are based on investigations which remain to be backed up. They are nonetheless briefly addressed, with a question mark, in Section 4.4.2. 

The name cytochrome fei has been used for the bacterial type b cytochromes having the a band near 560 nm since its designation by Keilin (193). However, this name is now usually applied in a limited sense to the cytochromes with a band at 558 or 559 nm in the nitrate reductase (nitrate respiration) system, which is a more primitive form of respiration than the mitochondrial respiration system. 

The second category includes enzymes that typically catalyze proper oxygen atom transfer reactions to or from an available electron lone pair of a substrate, and can be further subdivided into two families. The first family includes sulfite oxidase and assimilatory nitrate reductase, the physiological functions of which are to reduce nitrate to nitrite in the first stage of its reduction to ammonia for use by the plant cell. The second family comprises bacterial enzymes such as dimethylsulfoxide... 

DMSO) reductase and biotin-S-oxidoreduc-tase, as well as the bacterial dissimilatory (or respiratory) nitrate reductases (Hille 1996) (Table 18.8). The mononuclear molybdenum enzymes possess a pterin cofactor and may be categorized based on the structure of their molybdenum center... 

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). 

Nitrate reductase activity has been demonstrated in particulate fractions of Nostoc muscorum (Ortegaal., 1976), Anacystis nidulans (Manzanoet al., 1976) a.nd Anabaena cylindrica (Hattori, 1970). The enzyme is particulate and although it has been released by detergent treatment of acetone precipitates (Hattori, 1970), there has been no description of molecular characteristics. The association of the assimilatory nitrate reductase with membranes in the cyanobacterium appears to be a distinct divergence from the classification for bacterial nitrate reductases proposed by Pichinoty. 

These are the eukaryotic assimilatory nitrate reductases and three distinct bacterial enzymes, comprising the cytoplasmic assimilatory (Nas), the membrane-bound (Nar), and the periplasmic dissimilatory (Nap) nitrate reductases [11], Nitrite oxidoreductase, a membrane-bound enzyme from nitrifying bacteria also exhibits nitrate reductase activity. This enzyme shows high sequence similarity to the membrane-bound Nar, and catalyzes the nitrite oxidation to nitrate, to allow chemoautotrophic growth [75]. Many bacteria have more than one of the four types of nitrate reductases [9]. The functional, biochemical, and structural properties of prokaryotic and eukaryotic nitrate reductases have been recently reviewed [3,4,76,77]. Protein sequence data have been used to determine phylogenetic relationships and to examine similarities in structure and function of nitrate reductases. Three distinct clades of nitrate reductase evolved the eukaryotic assimilatory Nas, the membrane-associated prokaryotic Nar, and a clade that included both the periplasmatic Nap and prokaryotic assimilatory Nas [78]. 

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