Covalent Warhead Assembly in Fostriecin Biosynthesis: Malonylation-Lactonisation by a Bifunctional Thioesterase and Enzymatic Demalonylation

This study investigates the enzymatic steps responsible for the formation of α,β-unsaturated δ-lactones (AUDLs), a key pharmacophore in fostriecin, an anticancer agent. The researchers reconstituted the biosynthetic pathway, demonstrating that the terminal polyketide synthase (PKS) module FosMod8 produces a 3-O-malonylthioester intermediate, which is then malonylated and lactonized by the bifunctional thioesterase FosTE. Structural modeling revealed two arginine residues that mediate malonyl-CoA binding and transesterification, similar to PKS acyltransferases. Additionally, the kinase FosH phosphorylates the compound, priming it for site-specific demalonylation by FosM. This highly regulated pathway ensures the optimized formation of bioactive phosphorylated AUDL metabolites, providing a blueprint for engineering AUDL polyketides through chemoenzymatic synthesis or