<p>Flavoprotein pyridine nucleotide cytochrome reductases [<cite idref="PUB00002674"/>] (FPNCR) catalyse the interchange of reducing equivalents between one-electron carriers and the two-electron-carrying nicotinamide dinucleotides. The enzymes include ferredoxin:NADP⁺reductases (FNR) [<cite idref="PUB00002370"/>], plant and fungal NAD(P)H:nitrate reductases [<cite idref="PUB00002674"/>, <cite idref="PUB00005356"/>], NADH:cytochrome b5 reductases [<cite idref="PUB00002328"/>], NADPH:P450 reductases [<cite idref="PUB00005367"/>], NADPH:sulphite reductases [<cite idref="PUB00002509"/>], nitric oxide synthases [<cite idref="PUB00004098"/>], phthalate dioxygenase reductase [<cite idref="PUB00005007"/>], and various other flavoproteins.</p><p>Despite functional similarities, FPNCRs show no sequence similarity to NADPH:adrenodoxin reductases [<cite idref="PUB00001372"/>], nor to bacterial ferredoxin:NAD⁺reductases and their homologues [<cite idref="PUB00003255"/>]. To date, 3D-structures of 4 members of the family have been solved: <taxon tax_id="3562">Spinacia oleracea</taxon> (Spinach) ferredoxin:NADP⁺ reductase [<cite idref="PUB00005135"/>]; <taxon tax_id="292">Burkholderia cepacia</taxon> (Pseudomonas cepacia) phthalate dioxygenase reductase [<cite idref="PUB00005007"/>]; the flavoprotein domain of <taxon tax_id="4577">Zea mays</taxon> (Maize) nitrate reductase [<cite idref="PUB00005247"/>]; and <taxon tax_id="9823">Sus scrofa</taxon> (Pig) NADH:cytochrome b5 reductase [<cite idref="PUB00001683"/>]. In all of them, the FAD-binding domain (N-terminal) has the topology of an anti-parallel beta-barrel, while the NAD(P)-binding domain (C-terminal) has the topology of a classical pyridine dinucleotide-binding fold (i.e. a central parallel beta-sheet with 2 helices on each side) [<cite idref="PUB00005007"/>]. In spite of such structural similarities, the level of amino acid identity between family members is at or below the limit of significance (e.g., nitrate reductase is only 15% identical to FNR) [<cite idref="PUB00005247"/>].</p> Flavoprotein pyridine nucleotide cytochrome reductase