Molecular Basis of Celmerʹs Rules: Stereochemistry of Catalysis by Isolated Ketoreductase Domains from Modular Polyketide Synthases Original Research Article

A system is reported for the recombinant expression of individual ketoreductase (KR) domains from modular polyketide synthases (PKSs) and scrutiny of their intrinsic specificity and stereospecificity toward surrogate diketide substrates. The eryKR1 and the tylKR1 domains, derived from the first extension module of the erythromycin PKS and the tylosin PKS, respectively, both catalyzed reduction of (2R, S)-2-methyl-3-oxopentanoic acid N-acetylcysteamine thioester, with complete stereoselectivity and stereospecificity, even though the substrate is not tethered to an acyl carrier protein or an intact PKS multienzyme. In contrast, and to varying degrees, the isolated enzymes eryKR2, eryKR5, and eryKR6 exercised poorer control over substrate selection and the stereochemical course of ketoreduction. These data, together with modeling of diketide binding to KR1 and KR2, demonstrate the fine energetic balance between alternative modes of presentation of ketoacylthioester substrates to KR active sites.