Oxidation, nitrite to nitrate

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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