Genome Mining, Biosynthesis and Catalysis of a Novel Fungal Polyketide Synthase-Carnitine Acyltransferase Fusion Enzyme

This dissertation describes the genome mining, biosynthesis, and catalysis of a novel fungal polyketide synthase-carnitine acyltransferase fusion enzyme. Fungal highly-reducing polyketide synthases (HRPKS) catalyze the biosynthesis of polyketide natural products such as the cholesterol-lowering drug, lovastatin. Investigation into unexplored families of HRPKS enzymes such as the HRPKS-cAT fusion enzymes provides opportunities to discover new drug leads. This research will also improve our understanding of HRPKS enzymatic catalysis for future engineering purposes.Chapter 1 introduces the function, iterative catalysis, and biosynthetic framework of the fungal HRPKS enzymes. Partnering oxygenation enzymes which functionalize the polyketide scaffold are also assessed to emphasize the meticulous synchronization involved in HRPKS biosynthesis. Several examples of HRPKS biosynthesis are provided to illustrate the coordination required for the formation of the final natural product.Chapter 2 examines the genome mining of HRPKS-cAT enzymes and highlights their differences from canonical HRPKS catalysis. The HRPKS-cAT enzyme, Tv6-931, is reconstituted and the biosynthesis of polyketide-polyol conjugates bearing a rare gem-dimethyl moiety is discussed. Based on enzymatic assays using the Tv6-931 whole enzyme and dissected enzyme variants, an unusual polyketide “recapture” (oxyester-thioester transacylation) is proposed to explain the formation of the gem-dimethyl moiety.Chapter 3 investigates the biosynthetic, polyketide “recapture” mechanism introduced in Chapter 2. Enzyme kinetic studies show that the cofactor-independent, polyketide recapture is necessary for the gem-dimethyl adduct to be the major product. Based on the oxyester-thioester transacylation mechanism (recapture), a chemoenzymatic strategy was developed to conjugate other nucleophiles to create a variety of polyketide oxyester, thioester and amide adducts. A one-pot, multi-enzyme approach was used to demonstrate polyketide functionalization using HRPKS-cAT enzymes for proof-of-principle