A Polyketide Synthase Component for Oxygen Insertion into Polyketide Backbones

Abstract

Enzymatic core components from trans‐acyltransferase polyketide synthases (trans‐AT PKSs) catalyze exceptionally diverse biosynthetic transformations to generate structurally complex bioactive compounds. Here we focus on a group of oxygenases identified in various trans‐AT PKS pathways, including those for pederin, oocydins, and toblerols. Using the oocydin pathway homologue (OocK) from Serratia plymuthica 4Rx13 and N‐acetylcysteamine (SNAC) thioesters as test surrogates for acyl carrier protein (ACP)‐tethered intermediates, we show that the enzyme inserts oxygen into β‐ketoacyl moieties to yield malonyl ester SNAC products. Based on these data and the identification of a non‐hydrolyzed oocydin congener with retained ester moiety, we propose a unified biosynthetic pathway of oocydins, haterumalides, and biselides. By providing access to internal ester, carboxylate pseudostarter, and terminal hydroxyl functions, oxygen insertion into polyketide backbones greatly expands the biosynthetic scope of PKSs.

Identification of trans-AT polyketide clusters in two marine bacteria reveals cryptic similarities between distinct symbiosis factors

“…2C) provides a rationale for this feature. The internal ester moiety of this congener is colinear with a split module comprising the SbnOQ interface and the monooxygenase SbnP, a module type recently shown to insert oxygen by Baeyer-Villiger oxidation resulting in ester formation (32). The lipid moiety of sesbanimide F matches well to the subsequent PKS modules, but the compound lacks the final incorporation of an amino acid suggested by the terminal NRPS module of SbnQ, hypothetically an arginine according to NRPS2 Predictor (42).…”

“…The sbn ORFs and their homologous protein are shown in Table S3. Auxiliary enzymes are trans-AT responsible for loading of the acyl units that serve as building blocks of polyketide assembly; trans-acyl hydrolase (AH) aiding the correct acyl-unit incorporation; a 4'-phosphopantetheinyl transferase (PPT) responsible for transferring the essential prosthetic group 4'-phosphopantetheine to the acyl carrier proteins (ACPs); a β branching cassette comprising a hydroxymethylglutaryl-CoA synthase (HMGS) homolog, an ACP, a ketosynthase (KS) and an enoyl-CoA hydratase (ECH), present in other characterized polyketide clusters (28-30); a stand-alone ACP and an amidotransferase (AMT) with a glutamine hydrolyzing domain involved in glutaramide formation (6,15); a flavin-dependent monooxygenase (OXY) homologous to a polyketide synthase component responsible for Baeyer-Villiger oxidation (31,32); tailoring enzymes such a metallophosphoesterase (EST), a cytochrome P450, a methyltransferase (MT) (only present in PHM037 strain); and three putative ABC transport proteins.…”

A multiproducer microbiome generates chemical diversity in the marine sponge Mycale hentscheli

“…An interesting feature shared with the pederin system is the large PKS gene mycH that does not correspond to any part of the polyketide structure. MycG encoded directly upstream of mycH belongs to a group of PKS-associated monooxygenases, for which biochemical data support a function as oxygen-inserting Baeyer-Villigerase acting on growing polyketide chains (50). In the case of pederin and mycalamide, hydrolytic cleavage of the resulting ester moiety would be in agreement with the oxygenated polyketide terminus and missing polyketide portion.…”

The hidden enzymology of bacterial natural product biosynthesis

“…Phylogenetic analysis can thus be utilized to identify distinct 'orphan' KS domain clades for which the chemical structure of the substrate intermediate thioester region is unknown, thereby revealing candidates for possible novel enzymology in preceding modules. This approach is exemplified by the recent identification of a KS domain proposed to accept an ester function as a common progenitor moiety, and led to the biochemical characterization of an unusual Baeyer-Villiger-type monooxygenase that catalyses a rare oxygen insertion during chain elongation during oocydin (14) biosynthesis by Serratia plymuthica 90 (FIG. 6d).…”

“…rare cysteine lyase domains during leinamycin biosynthesis 85 . d | Oxygen insertion into the growing oocydin polyketide backbone catalysed by the trans-acting Baeyer-Villiger monooxygenase, OocK 90 . OocK and its flavin cofactor are coloured purple.…”