Eukaryotes, nitrate reductases

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]. 

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... 

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... 

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. ... 

Fischer, K., Barbierb, G. G., Hechtd, H. J., Mendela, R. R., Campbell, W. H., and Schwarza, G. (2005). Structural basis of eukaryotic nitrate reduction Crystal structures of the nitrate reductase... 

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. 

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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