FAD-dependent monooxygenase phqK

UniProtKB accession:  L0E4H0

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UniProtKB description:  FAD-dependent monooxygenase; part of the gene cluster that mediates the biosynthesis of paraherquamide, a fungal indole alkaloid that belongs to a family of natural products containing a characteristic bicyclo[2.2.2]diazaoctane core (PubMed:23213353, PubMed:31904957). Within the pathway, phqK catalyzes spirocycle formation through two parallel pathways in the biosynthesis of paraherquamides A and G, using as substrates paraherquamides K and L, with paraherquamide L, bearing the dioxepin, being likely the favored substrate (PubMed:31904957). The first steps in the biosynthesis of paraherquamide is the production of the beta-methyl-proline precursor from L-isoleucine. They require oxidation of a terminally hydroxylated L-isoleucine to the corresponding aldehyde by enzymes which have still to be identified. Spontaneous cyclization and dehydration would yield the 4-methyl pyrolline-5-carboxylic acid, which is then reduced by the pyrroline-5-carboxylate reductase phqD leading to the beta-methyl-proline precursor. The next step of paraherquamide biosynthesis involves coupling of beta-methyl-proline and L-tryptophan by the bimodular NRPS phqB, to produce a monooxopiperazine intermediate. The reductase (R) domain of phqB utilizes NADPH for hydride transfer to reduce the thioester bond of the T domain-tethered linear dipeptide to a hemithioaminal intermediate, which spontaneously cleaves the C-S bond to release the aldehyde product. This compound undergoes spontaneous cyclization and dehydration to give a dienamine which is reverse prenylated at C-2 by the reverse prenyltransferase phqJ. The other prenyltransferase present in the cluster, phqI may be a redundant gene in the pathway. During biosynthetic assembly, the key step to produce the polycyclic core is catalyzed by the bifunctional reductase and intramolecular [4+2] Diels-Alderase, phqE, resulting in formation of the [2.2.2] diazaoctane intermediate preparaherquamide. Following formation of preparaherquamide, an indole 2,3-epoxidation-initiated pinacol-like rearrangement is catalyzed by the phqK FAD-dependent monooxygenase. The prenyltransferase phqA, the cytochrome P450 monooxygenase phqL, and the FAD-linked oxidoreductase phqH (or the cytochrome P450 monooxygenase phqM), are proposed to be involved in the formation of the pyran ring. The FAD-dependent monooxygenase phqK is likely responsible for generation of the spiro-oxindole, and the N-methylation is likely mediated by the phqN methyltransferase leading to the isolable natural product paraherquamide F. However, the order of these biosynthetic steps has still to be determined. In late-stage paraherquamide biosynthesis, the third P450 monooxygenase, phqO, is probably responsible for the C-14 hydroxylation, transforming paraherquamide F to paraherquamide G, and paraherquamide E to the final product paraherquamide A. The expansion from the 6-membered ring pyran (in paraherquamides F and G) to the 7-membered dioxepin ring (in paraherquamides A and E) represents a poorly understood but intriguing process that probably involves the 2-oxoglutarate-dependent dioxygenase phqC. Finally, the remaining members of the paraherquamide cluster, including phqI as well as phqM (or phqH), do not have a clearly prescribed role and appear to be redundant (Probable).