Macrodiolide formation by the thioesterase of a modular polyketide synthase

Article abstract of DOI:10.1002/anie.201500401

Elaiophylin is an unusual C₂-symmetric antibiotic macrodiolide produced on a bacterial modular polyketide synthase assembly line. To probe the mechanism and selectivity of diolide formation, we sought to reconstitute ring formation in vitro by using a non-natural substrate. Incubation of recombinant elaiophylin thioesterase/cyclase with a synthetic pentaketide analogue of the presumed monomeric polyketide precursor of elaiophylin, specifically its N-acetylcysteamine thioester, produced a novel 16-membered C₂-symmetric macrodiolide. A linear dimeric thioester is an intermediate in ring formation, which indicates iterative use of the thioesterase active site in ligation and subsequent cyclization. Furthermore, the elaiophylin thioesterase acts on a mixture of pentaketide and tetraketide thioesters to give both the symmetric decaketide diolide and the novel asymmetric hybrid nonaketide diolide. Such thioesterases have potential as tools for the in vitro construction of novel diolides. Think again: The thioesterase/cyclase enzyme of the elaiophylin polyketide synthase catalyzes symmetric diolide formation in vitro from a synthetic pentaketide substrate by an iterative mechanism. Unexpectedly, a tetraketide that is not itself a substrate can be co-opted in the presence of the pentaketide to produce an asymmetric macrodiolide.