title: Polyketide double bond biosynthesis. Mechanistic analysis of the dehydratase-containing module 2 of the picromycin/methymycin polyketide synthase.
abstract: Picromycin/methymycin synthase (PICS) is a modular polyketide synthase (PKS) that is responsible for the biosynthesis of both 10-deoxymethynolide (1) and narbonolide (2), the parent 12- and 14-membered aglycone precursors of the macrolide antibiotics methymycin and picromycin, respectively. PICS module 2 is a dehydratase (DH)-containing module that catalyzes the formation of the unsaturated triketide intermediate using malonyl-CoA as the chain extension substrate. Recombinant PICS module 2+TE, with the PICS thioesterase domain appended to the C-terminus to allow release of polyketide products, was expressed in Escherichia coli. Purified PICS module 2+TE converted malonyl-CoA and 4, the N-acetylcysteamine thioester of (2S,3R)-2-methyl-3-hydroxypentanoic acid, to a 1:2 mixture of the triketide acid (4S,5R)-4-methyl-5-hydroxy-2-heptenoic acid (5) and (3S,4S,5R)-3,5-dihydroxy-4-methyl-n-heptanoic acid-delta-lactone (10) with a combined kcat of 0.6 min(-1). The triketide lactone 10 is formed by thioesterase-catalyzed cyclization of the corresponding d-3-hydroxyacyl-SACP intermediate, a reaction which competes with dehydration catalyzed by the dehydratase domain. PICS module 2+TE showed a strong preference for the syn-diketide-SNAC 4, with a 20-fold greater kcat/K(m) than the anti-(2S,3S)-diketide-SNAC 14, and a 40-fold advantage over the syn-(2R,3S)-diketide-SNAC 13. PICS module 2(DH(0))+TE, with an inactivated DH domain, produced exclusively 10, while three PICS module 2(KR(0))+TE mutants, with inactivated KR domains, produced exclusively or predominantly the unreduced triketide ketolactone, (4S,5R)-3-oxo-4-methyl-5-hydroxy-n-heptanoic acid-delta-lactone (7). These studies establish for the first time the structure and stereochemistry of the intermediates of a polyketide chain elongation cycle catalyzed by a DH-containing module, while confirming the importance of key active site residues in both KR and DH domains.
authors.affiliation: Department of Chemistry, Box H, Brown University, Providence, Rhode Island 02912-9108, USA.
authors.forename: Jiaquan
authors.initials: J
authors.lastname: Wu
authors.forename: Toby J
authors.initials: TJ
authors.lastname: Zaleski
authors.forename: Chiara
authors.initials: C
authors.lastname: Valenzano
authors.forename: Chaitan
authors.initials: C
authors.lastname: Khosla
authors.forename: David E
authors.initials: DE
authors.lastname: Cane
mesh_terms: D000595:Amino Acid Sequence; D018942:Macrolides / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D015394:Molecular Structure; D016297:Mutagenesis, Site-Directed; D048630:Polyketide Synthases / Q000737:chemistry* / Q000378:metabolism*; D021122:Protein Subunits; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
chemical_list: D018942:Macrolides; D021122:Protein Subunits; C008517:methymycin; D048630:Polyketide Synthases; C003145:picromycin