Iterative-Acting Thioesterase from Polyketide Biosynthesis Accepts Diverse Nucleophilic Alcohols to Yield Oxazole-Containing Esters.

The biosynthesis of antitumor oxazole-containing conglobatin is directed by a multienzyme assembly line of nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS), in which an uncanonical iterative-acting C-terminal thioesterase domain, Cong-TE, ligated two fully elongated chains/conglobatin monomers on the terminal acylcarrier protein and subsequently cyclized the resulting dimer to a C 2 -symmetric macrodiolide. Screening of the conglobatin producer for secondary metabolites led to the discovery of two new compounds conglactones A ( 1 ) and B ( 2 ), possessing inhibitory activities to phytopathogenic microorganisms and cancer cells, respectively. The compounds 1 and 2 feature the ester bond-linked hybrid structures consisting of an aromatic polyketide benwamycin I ( 3 ) and one (for 1 )/two (for 2 ) molecules of the conglobatin monomer ( 5 ). Genetic mutational analysis revealed that the production of 1 and 2 was correlated with the biosynthetic pathways of 3 and 5 . Biochemical analysis indicated that 1 and 2 were produced by Cong-TE from 3 and an N -acetylcysteamine thioester form of 5 ( 7 ). Furthermore, the substrate compatibility of Cong-TE was demonstrated by enzymatically generating a bunch of ester products from 7 and 43 exotic alcohols. This property of Cong-TE was further validated by producing 36 hybrid esters in the fermentation of conglobatin producer fed with nonindigenous alcohols. This work shows a prospect of developing Cong-TE for green synthesis of valuable oxazole-containing esters, thus complementing the environmentally unfriendly chemosynthesis strategies.