Nitrate reductase classes

There are four different classes of nitrate reductases (234). The nitrate reductases from D. desulfuricans show a strong homology to the a-subunit of the class of periplasmic respiratory nitrate reductases, and also to some of the enzymes that are included on the class of cytoplasmic assimilatory nitrate reductases. Because of this fact, a proposal was made for a new class of monomeric NAP, which contains the minimal arrangement of metal centers to perform nitrate reduction one [4Fe-4S] cluster and a Mo bound to two MGD. 

The two-domain, structural motif in FNR represents a common structural feature in a large class of enzymes that catalyze electron transfer between a nicotinamide dinucleotide molecule and a one-electron carrier. Beside the photosynthetic electron-transfer enzyme, others non-photosynthetic ones include flavodoxin reductase, sulfite reductase, nitrate reductase, cytochrome reductase, and NADPH-cyto-chrome P450 reductase. FNR belongs to the group of so-called dehydrogenases-electron transferases, i.e., flavoproteins that catalyze electron transfer from two, one-electron donor molecules to a single two-electron acceptor molecule. 

Relative to other nutrients, iron is disproportionately utilized in cell metabolism over cell structure. As a result, cellular Fe C ratios tend to vary with Fe availability (and, by extension, metabolic rate), in contrast to the more or less fixed ratios of C N P measured in marine phytoplankton [2,3]. Fe C ratios measured in pelagic phytoplankton and marine particulate matter range from 1 3000 to 1 500,000 [3-7]. This variation in cellular Fe content is intrinsic to different classes of phytoplankton as has been shown in culture experiments [8]. In particular, because iron is present in the catalytic centers of nitrate- and nitrite-reductase enzymes, in reducing equivalent molecules (NADPH, ferredoxin) and in the N2-fixing enzyme nitrogenase, organisms assimilating... 

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