While the cis-acyltransferase modular polyketide synthase assembly lines have largely been structurally dissected, enzymes from within the recently discovered trans-acyltransferase polyketide synthase assembly lines are just starting to be observed crystallographically. Here we examine the ketoreductase (KR) from the first polyketide synthase module of the bacillaene nonribosomal peptide synthetase/polyketide synthase at 2.35-Å resolution. This KR naturally reduces both α- and β-keto groups and is the only KR known to do so during the biosynthesis of a polyketide. The isolated KR not only reduced an N-acetylcysteamine-bound β-keto substrate to a D-β-hydroxy product, but also an N-acetylcysteamine-bound α-keto substrate to an L-α-hydroxy product. That the substrates must enter the active site from opposite directions to generate these stereochemistries suggests that the acyl-phosphopantetheine moiety is capable of accessing very different conformations despite being anchored to a serine residue of a docked acyl carrier protein. The features enabling stereocontrolled α-ketoreduction may not be extensive since a KR that naturally reduces a β-keto group within a cis-acyltransferase polyketide synthase was identified that performs a completely stereoselective reduction of the same α-keto substrate to generate the D-α-hydroxy product. A sequence analysis of trans-acyltransferase KRs reveals that a single residue, rather than a three-residue motif found in cis-acyltransferase KRs, is predictive of the orientation of the resulting β-hydroxyl group. Proteins 2014; 82:2067–2077. © 2014 Wiley Periodicals, Inc.
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