Hydroalkoxylation enzyme phnH

UniProtKB accession:  A0A1S6PUA4

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UniProtKB description:  Hydroalkoxylation enzyme; part of the gene cluster that mediates the biosynthesis of phenalenones such as herqueinone, compounds that have been reported to treat tumors, bacterial infections and/or mycoses, and rheumatic diseases (PubMed:26978228). The non-reducing polyketide synthase phnA synthesizes the heptaketide backbone and cyclizes it into the angular, hemiketal-containing naphtho-gamma-pyrone prephenalenone. The product template (PT) domain of phnA catalyzes only the C4-C9 aldol condensation, which is unprecedented among known PT domains (PubMed:28240554, PubMed:26978228). The transformation of prephenalenone to phenalenones requires an FAD-dependent monooxygenase phnB, which catalyzes the C2 aromatic hydroxylation of prephenalenone and ring opening of the gamma-pyrone ring simultaneously (PubMed:28240554, PubMed:26978228). Subsequent intramolecular deprotonation of C3 phenolic oxygen accelerates phenalenone ring closure to yield the tricyclic phenalenone core with a C2 hydroxylation (PubMed:28240554, PubMed:26978228). The prenyltransferase phnF further catalyzes reverse C-prenylation of phenalenone by direct electrophilic substitution at C6, or possibly via first a forward O-prenylation of a neighboring phenol in phenalenone, followed by a Claisen rearrangement (PubMed:28240554). The hydroalkoxylation enzyme phnH catalyzes the 5-exo-trigcyclization via acid catalysis after the spontaneous deprotonation of 7-OH, which leads to the formation of the dihydrobenzofuran atrovenetin (PubMed:28240554). Atrovenetin is further converted to deoxyherqueinone by the O-methyltransferase phnC which can methylate C2-OH to stabilize the northern portion of the phenalenone core (PubMed:28240554). Finally, the oxidoreductase phnG converts deoxyherqueinone to herqueinone via C6 hydroxylation (PubMed:28240554).