S163: Mechanisms of Modular Polyketide Synthase Beta-Carbon Processing Enzymes

Modular polyketide synthases synthesize complex polyketides such as the antibacterial erythromycin, the antifungal amphotericin, and the anticancer agent epothilone. These megasynthases are veritable assembly lines in which each enzymatic domain performs a separate catalytic step in the synthesis of a polyketide product. Enzymatic domains that process the α- and β- positions of growing polyketide intermediates include the ketoreductase, dehydratase, and enoylreductase. The functional groups and stereocenters produced by these enzymes are often critical for the biological activity of the polyketide. We have made progress in elucidating the mechanisms of these enzymes through structural and functional studies. The high-resolution structures of A1-, A2-, B1-, B2-, and C2-type ketoreductases enable descriptions of the active site features that epimerize α-substituents, select the appropriate epimer for reduction, and set the orientation of the resulting β-hydroxy group. High-resolution structures of dehydratases also enable insight into the mechanisms that catalyze the formation of trans- and cis- double bonds. Determining the precise mechanisms for these enzymes will enable advances in biocatalysis as well as the engineering of modular polyketide synthases that produce new polyketides.