A Thioesterase from an Iterative Fungal Polyketide Synthase Shows Macrocyclization and Cross Coupling Activity and May Play a Role in Controlling Iterative Cycling through Product Offloading

Zearalenone, a fungal macrocyclic polyketide, is a member of the resorcylic acid lactone family. Herein, we characterize in vitro the thioesterase from PKS13 in zearalenone biosynthesis (Zea TE). The excised Zea TE catalyzes macrocyclization of a linear thioester-activated model of zearalenone. Zea TE also catalyzes the cross coupling of a benzoyl thioester with alcohols and amines. Kinetic characterization of the cross coupling is consistent with a ping-pong bi-bi mechanism, confirming an acyl-enzyme intermediate. Finally, the Substrate specificity of the Zea TE indicates the TE may help control iterative cycling on PKS13 by rapidly offloading the final resorcylate-containing product.