Two methods for accessing synthetic polyketides

Polyketides are diverse natural products with high biological activity. Naturally occurring polyketides can be antibiotics, anti-cancer agents, or some of the most toxic compounds known. Because of their potency, adding novel polyketides to compound pools or having the ability to synthesize a desired polyketide could have a far-reaching impact on drug development. Unfortunately, new polyketides are traditionally hard to synthesize chemically due to many tightly-controlled stereocenters. Presented here are two methods to produce synthetic polyketides with stereocenters. In nature, polyketides are produced from complex molecular machines called polyketide synthases (PKSs). Assembly line PKSs are organized into domains which each have a catalytic activity and modules which consist of all the domains to catalyze one two-carbon addition. The chemoenzymatic method employed here uses the ketoreductase (KR), one of the domains within PKSs which reduces carbonyl groups stereoselectively and controls stereocenters in the polyketide – the biggest challenge in traditional synthesis. KRs have been used here to produce 2 stereotriads that would traditionally be difficult to synthesize. A diketide was first synthesized chemically and reduced with a tylosin KR from Streptomyces fradiae (TylKR2). This reduced diketide was then extended into a triketide through the Masamune C-acylation reaction. Finally, TylKR2 and a mycolactone KR from a bacterial artificial chromosome (MycKR6) were found to reduce the triketide and analyzed through liquid chromatography – mass spectrometry. An alternate enzyme engineering method involves constructing chimeric PKSs to produce diketides completely enzymatically given starting material and cofactors. The engineered PKSs were constructed from the first module of Venemycin with either the termination module of Erythromycin or Oleandomycin. By mixing modules from different PKSs, polyketides were synthesized that are not found in nature. The currently cloned constructs have shown reactant consumption in vitro by NMR. Both constructs contain a KR, giving chirality to this class of synthetic polyketide.