Structural and functional studies of a trans-acyltransferase polyketide assembly line enzyme that catalyzes stereoselective ?- and ?-ketoreduction.

ABSTRACT: 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 ?- and ?-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 ?-keto substrate to a D-?-hydroxy product, but also an N-acetylcysteamine-bound ?-keto substrate to an L-?-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 ?-ketoreduction may not be extensive since a KR that naturally reduces a ?-keto group within a cis-acyltransferase polyketide synthase was identified that performs a completely stereoselective reduction of the same ?-keto substrate to generate the D-?-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 ?-hydroxyl group.

SUBMITTER: Piasecki SK 

PROVIDER: S-EPMC4142079 | biostudies-literature | 2014 Sep

REPOSITORIES: biostudies-literature