<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"/>]; NADH:cytochrome B5 reductases; NADPH:P450 reductases; NADPH:sulphite reductases; nitric oxide synthases; phthalate dioxygenase reductase [<cite idref="PUB00005157"/>]; and various other flavoproteins. Phthalate dioxygenase reductase (PDR) is a member of a family of FPNCR/[2Fe-2S] ferredoxin fusion proteins, found in flavobacteria, that participate in oxidative metabolism of a variety of substrates. Other family members are vanillate demethylase reductase [<cite idref="PUB00002070"/>]; phenoxybenzoate dioxygenase; and pentachlorophenol 4-monooxygenase. All of contain a [2Fe-2S] ferredoxin domain fused to the C terminus of an FPNCR domain. The direction of electron flow is from flavin to the [2Fe-2S] centre. By contrast with most FPNCRs, PDR binds FMN instead of FAD. The 3D-structure of PDR has been solved [<cite idref="PUB00005157"/>]. The overall fold of the FPNCR module is similar to that of ferredoxin:NADP+ reductase [<cite idref="PUB00002370"/>]; the FMN-binding domain (N-terminal) has the topology of an anti-parallel beta-barrel; and the NAD(P)-binding domain (C-terminal) has the topology of a classical pyridine dinucleotide-binding fold, a central parallel beta-sheet flanked by 2 helices on each side.</p>