A Tylosin Ketoreductase Reveals How Chirality Is Determined in Polyketides

SummaryBecause it controls the majority of polyketide stereocenters, the ketoreductase (KR) is a central target in engineering polyketide synthases (PKSs). To elucidate the mechanisms of stereocontrol, the structure of KR from the first module of the tylosin PKS was determined. A comparison with a recently solved erythromycin KR that operates on the same substrate explains why their products have opposite α-substituent chiralities. The structure reveals how polyketides are guided into the active site by key residues in different KR types. There are four types of reductase-competent KRs, each capable of fixing a unique combination of α-substituent and β-hydroxyl group chiralities, as well as two types of reductase-incompetent KRs that control α-substituent chirality alone. A protocol to assign how a module will enforce substituent chirality based on its sequence is presented.