A tylosin ketoreductase reveals how chirality is determined in polyketides

Because 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 ocsubstituent 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 oc-substituent and beta-hydroxyl group chiralities, as well as two types of reductase-incompetent KRs that control alpha-substituent chirality alone. A protocol to assign how a module will enforce substituent chirality based on its sequence is presented.