Structural and functional studies of a trans ‐acyltransferase polyketide assembly line enzyme that catalyzes stereoselective α‐ and β‐ketoreduction

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.