FMN-binding split barrel <p>The FMN-binding domain has a split beta-barrel structure with a Greek-key topology that is related in structure to the ferredoxin reductase-like FAD-binding domain. The FMN-binding split barrel domain is found in pyridoxine 5'-phoshate oxidase (PNP oxidase), FMN-binding protein, ferric reductase, and in phenol 2-hydroxylase component B (PheA2).</p><p>PNP oxidase (<db_xref db="EC" dbkey="1.4.3.5"/>) is an FMN flavoprotein that catalyses the oxidation of pyridoxamine-5-P (PMP) and pyridoxine-5-P (PNP) to pyridoxal-5-P (PLP). This reaction serves as the terminal step in the de novo biosynthesis of PLP in <taxon tax_id="562">Escherichia coli</taxon>, and as a part of the salvage pathway of this coenzyme in both E. coli and mammalian cells [<cite idref="PUB00016343"/>, <cite idref="PUB00016342"/>]. The binding sites for FMN and for substrate have been highly conserved throughout evolution. </p><p>The FMN-binding protein (FMN-bp) is one of the smallest proteins known to bind FMN. FMN-bp appears to participate in the electron-transfer pathway, and may have a structural relationship to the C-terminal domain of chymotrypsin [<cite idref="PUB00016344"/>, <cite idref="PUB00011837"/>].</p><p>Microbial ferric reductases are essential for generating more soluble ferrous iron to use in cellular proteins (assimilatory ferric reductases), and as terminal reductases of iron respiratory pathway of certain bacteria (dissimilatory iron reductases). Most assimilatory iron reductases are flavin enzymes [<cite idref="PUB00016340"/>].</p>