Prokaryotes, nitrate reductases

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. 

Nitrate reductases (14) are found in a wide range of eukaryotes and prokaryotes and have a crucial role in nitrogen assimilation (33, 34) and dissimilation (35). These reductases catalyze the reduction of NO3 to N02. For the assimilatory nitrate reductases this reaction is followed by... 

The enzymes are subdivided into three families based on structural and sequence comparisons (Figure 1, Table 1). Oxotransferases isolated from prokaryotes (see Prokaryote) belong to the DMSO reductase family. These enzymes include DMSO reductase, biotin X-oxide reductase, trimethylamine A-oxide reductase, dissimilatory nitrate reductase, formate... 

This family includes the sulfite oxidases and dehydrogenases of prokaryotes Thiobacilli sp.), plants, birds, and animals, and the assimilatory nitrate reductases from bacteria, algae, fungi, and plants. The sulfite oxidases of higher eukaryotes are 100-110kDa homodimers (Table 1) they are located in the mitochrondrial intermembrane space and catalyze the oxidation of toxic sulfite to innocuous sulfate (equation 7). Human sulfite oxidase deficiency leads to major neurological abnormalities, mental retardation, dislocation of the ocular lenses, and early death. ... 

In addition to N assimilation, nitrogen compounds in nature can also be used as electron sources or sinks, especially by prokaryotes (but not exclusively, see Zvyagil skaya et al., 1996). The enzymes that mediate these dissimilatory reactions have been studied extensively in the context of inputs and losses of fixed N in the marine N budget. Many of these measurements have been based on enzyme activities (e.g., dissimilatory nitrate reductase). Since dissimilatory reactions have been reviewed elsewhere in this volume (Chapter 5 by Ward and Chapter 6 by Devol, this volume), these wiU not be discussed further and we refer the reader to these chapters. 

Assimilatory nitrate reductases from prokaryotic organisms have not been extensively characterized. Generally reduced pyridine nucleotides are not eflfective electron donors but reduction appears to be catalyzed by reduced flavins (Hattori, 1970 Alef and Klemme, 1977) and reduced ferrodoxin (Malofeeva et al., 1975 Manzano et al., 1976 Ortega et al., 1976). 

Although there are reports of inhibition of nitrate reductase by chelating agents which might chelate nonheme iron, analyses of the purified enzymes from eukaryotes have failed to reveal the Fe-sulfur complexes detected in the dissimilatory nitrate reductases from prokaryotes (Garrett and Nason, 1969 Ahmed and Spiller, 1976). The extensively purified nitrate reductases should be amenable for study with epr in order to determine the involvement of molybdenum in the reduction process. 

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]. 

In the global environment, nitrogen in the form nitrate or nitrite is assimilated (immobilized) into biomass in the form of NH3. This reduction is catalyzed by two assimilatory enzymes (nitrite reductase and nitrate reductase) and can be carried out by plants, fungi, and prokaryotes. This process is likely to dominate when reduced nitrogen is in low supply (e.g., during aerobic conditions). 

Nitrate reduction. According to Kaspar (1982), P. acidipropionici, P. freudenreichii, P. jensenii, P. shermanii and P. thoenii can reduce nitrate to nitrite and further to N2O. Formation of N2O from nitrite in prokaryotes may represent a mechanism of detoxification rather than transformation of energy. N2O was not further reduced oxygen inhibited nitrate reduction by P. acidipropionici and P. thoenii. The enzymes of nitrate and nitrite reduction were either constitutive or derepressed in anaerobiosis only P. pentosaceum contained a constitutive nitrate reductase. Nitrate stimulated the synthesis of nitrate reductase in P. acidipropionici, specific growth rates and biomass yields were increased by the addition of nitrate. Nitrite at a concentration of 10 mM was not inhibitory. 

Nitrate reductases are widespread in both eukaryotes and prokaryotes. They are broadly classified into assimilatory and dissimilatory NaR, based on their important role in nitrogen assimilation and dissimilation (Campbell, 1999). Eukaryotic NaR is part of the sulfite... 

Nitrate reductase, the first enzyme in the nitrate assimilation pathway, catalyzes the reduction of nitrate to nitrite. This requires two electrons which are donated by either NADH or NADPH in the eukaryotic NRs. The NADH-spe-cific NRs (EC 1.6.6.1) are found in most higher plants and numerous eukaryotic algae while the NADPH-specific NRs (EC 1.6.6.3) are found in the fungi. The NAD(P)H-bispecific NRs (EC 1.6.6.2) occur in some higher plant and algae species. The prokaryotic NRs which utilize a variety of electron donors, including ferredoxin, reduced pyridine nucleotides, and respiratory intermediates, will not be considered in this article (see Stewart, 1988). 

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