PKSE biosynthesizes an enediyne core precursor from decarboxylative condensation of eight malonyl-CoAs. The KR domain of PKSE is responsible for iterative β-ketoreduction in each round of polyketide chain elongation. KRs from selected PKSEs were investigated in vitro with β-ketoacyl-SNACs as substrate mimics. Each of the KRs reduced the β-ketoacyl-SNACs stereoselectively, all affording the corresponding β-D-hydroxyacyl-SNACs, and the catalytic efficiencies (k(cat)/K(M)) of the KRs increased significantly as the chain length of the β-ketoacyl-SNAC substrate increases.