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Iterative type I polyketide synthases (iPKSs) are large multifunctional enzymes that assemble polyketide products by using a single module composed of several catalytic domains. Although iPKSs utilize the same repertoire o f catalytic domains as fatty acid synthases and modular PKSs, iPKSs are able to use a single set of catalytic domains to assemble the chemically and structurally diverse polyketide products in an iterative manner. How the iPKSs achieve the chemical and structural diversity by “programming” the catalytic domains remains one of the greatest mysteries in enzymology today. In this thesis, I describe the results from my studies on two iPKSs that aimed to understand the function and mechanism of the multifunctional iPKSs. The first iPKS under investigation is SgcE, the PKS putatively responsible for the biosynthesis of the functional enediyne moiety of the antitumor compound C-1027. Naturally occurring enediynes are mostly known for their unique molecular structure and remarkable biological activities largely owing to the central bicyclic enediyne functionality. Following the recent sequencing of the gene clusters for the synthesis of several enediyne natural products, my research work aimed to understand the role of SgcE in the biogenesis of the enediyne functionality o f C-1027. Together with the putative ancillary enzymes SgcE3 and SgcE10, SgcE were overexpressed in the heterologous expression systems that include E. coli and Saccharomyces cerevisiae. Several highly conjugated polyene products of SgcE generated by in vitro enzymatic reactions and in vivo protein co-expression were characterized to suggest that SgcE is ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library

A study of the iterative type I polyketide synthases in enediyne and mellein biosynthesis