Biochemical investigation of pikromycin biosynthesis employing native penta- and hexaketide chain elongation intermediates.

The unique ability of the pikromycin (Pik) polyketide synthase to generate 12- and 14-membered ring macrolactones presents an opportunity to explore the fundamental processes underlying polyketide synthesis, specifically the mechanistic details of chain extension, keto group processing, acyl chain release, and macrocyclization. We have synthesized the natural pentaketide and hexaketide chain elongation intermediates as N-acetyl cysteamine (NAC) thioesters and have used them as substrates for in vitro conversions with engineered PikAIII+TE and in combination with native PikAIII (module 5) and PikAIV (module 6) multifunctional proteins. This investigation demonstrates directly the remarkable ability of these monomodules to catalyze one or two chain extension reactions, keto group processing steps, acyl-ACP release, and cyclization to generate 10-deoxymethynolide and narbonolide. The results reveal the enormous preference of Pik monomodules for their natural polyketide substrates and provide an important comparative analysis with previous studies using unnatural diketide NAC thioester substrates.