6pvf
Crystal structure of PhqK in complex with malbrancheamide B
Structural highlights
FunctionPHQK_PENFE 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).[1] [2] [3] Publication Abstract from PubMedThe paraherquamides are potent anthelmintic natural products with complex heptacyclic scaffolds. One key feature of these molecules is the spiro-oxindole moiety that lends a strained three-dimensional architecture to these structures. The flavin monooxygenase PhqK was found to catalyze spirocycle formation through two parallel pathways in the biosynthesis of paraherquamides A and G. Two new paraherquamides (K and L) were isolated from a DeltaphqK strain of Penicillium simplicissimum, and subsequent enzymatic reactions with these compounds generated two additional metabolites, paraherquamides M and N. Crystal structures of PhqK in complex with various substrates provided a foundation for mechanistic analyses and computational studies. While it is evident that PhqK can react with various substrates, reaction kinetics and molecular dynamics simulations indicated that the dioxepin-containing paraherquamide L is the favored substrate. Through this effort, we have elucidated a key step in the biosynthesis of the paraherquamides and provided a rationale for the selective spirocyclization of these powerful anthelmintic agents. Molecular Basis for Spirocycle Formation in the Paraherquamide Biosynthetic Pathway.,Fraley AE, Caddell Haatveit K, Ye Y, Kelly SP, Newmister SA, Yu F, Williams RM, Smith JL, Houk KN, Sherman DH J Am Chem Soc. 2020 Jan 16. doi: 10.1021/jacs.9b09070. PMID:31904957[4] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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