<p>This entry represents a structural domain with a closed beta-barrel fold with greek-key topology. Domains with this structure can be found in the following proteins:</p><p> <ul> <li>Riboflavin synthase, which contains two homologous domains of this structure [<cite idref="PUB00025941"/>].</li><li>The FAD-binding (N-terminal) domain of ferredoxin reductase (flavodoxin reductase), where the FAD-binding domain is coupled with a NADP-binding domain of the alpha/beta class [<cite idref="PUB00036699"/>].</li><li>The FAD-binding domain of NADPH-cytochrome p450 reductase; however, this domain has an additional alpha-helical domain inserted into it [<cite idref="PUB00004914"/>].</li> </ul> </p><p>Riboflavin synthase (<db_xref db="EC" dbkey="2.5.1.9"/>) catalyses the final step in the biosynthesis of vitamin B2, namely the dismutation of two molecules of 6,7-dimethyl-8-ribityllumazine to yield riboflavin and 4-(1-D-ribitylamino)-5-amino-2,6-dihydroxypyrimidine (which is recycled) [<cite idref="PUB00043427"/>].</p><p> Flavins can act as primary and secondary emitters in bacterial luminescence. Lumazine proteins are involved in the bioluminescence of certain marine bacteria. These proteins are catalytically inactive, but they resemble riboflavin synthase [<cite idref="PUB00043814"/>]. Lumazine is non-covalently bound to the fluorophore 6,7-dimethyl-8-ribityllumazine, which is the substrate of riboflavin synthase.</p><p>Ferredoxin reductase is a FAD-containing oxidoreductase that transports electrons between flavodoxin or ferredoxin and NADPH. In <taxon tax_id="562">Escherichia coli</taxon>, ferredoxin reductase together with flavodoxin is involved in the reductive activation of three enzymes: cobalamin-dependent methionine synthase, pyruvate formate lyase and anaerobic ribonucleotide reductase [<cite idref="PUB00036699"/>]. An additional function for the oxidoreductase appears to be to protect the bacteria against oxygen radicals. The beta-barrel domain found in ferredoxin reductase is similar to that found in: NAD(P)H:flavin oxidoreductase [<cite idref="PUB00030235"/>], the core domain of nitrate reductase [<cite idref="PUB00036526"/>], cytochrome b5 reductase [<cite idref="PUB00022784"/>], phthalate dioxygenase reductase (which contains an additional 2Fe-2S ferredoxin domain) [<cite idref="PUB00005157"/>], benzoate dioxygenase reductase [<cite idref="PUB00022011"/>], the PyrK subunit of dihydroorotate dehydrogenase B [<cite idref="PUB00024613"/>], the central domain of flavohaemoglobin (which contains an additional globin domain) [<cite idref="PUB00036827"/>], and methane monooxygenase component C (MmoC) [<cite idref="PUB00016237"/>]. </p><p>Microsomal NADPH-cytochrome P450 reductase (<db_xref db="EC" dbkey="1.6.2.4"/>) (CPR) (NADPH-haemoprotein reductase) is a membrane-bound protein that contains both FAD and FMN. CPR catalyses electron transfer from NADPH to all known microsomal cytochromes P450. The beta-barrel domain found in NADPH-cytochrome p450 reductase is similar to that found in: sulphite reductase flavoprotein [<cite idref="PUB00014352"/>], and the FAD/NADP+ domain of neuronal nitric-oxide synthase [<cite idref="PUB00024750"/>].</p> Riboflavin synthase-like beta-barrel