Exploration of structure and chemistry in modular polyketide synthases

Nowadays the development of new-generation drugs has been rendered with high urgency in pharmaceutical industry, since existing medicines are not adequate to cope with new appeared diseases. Among these medicines, anticancer and antibiotic are the most important two categories which require new drugs to meet the need of frustrated patients. More efficacy and less toxicity are the target of developing new-generation drugs in the future. However, the intrinsic complexity of these chemicals compromises the effort for organic chemists to synthesize them in an eco-friendly and efficient way. In traditional organic chemistry, complex compound always means long synthetic routine, bad total yield and usually expensive and toxic chiral catalyst. Au contraire, biology now gives organic chemists an alternative way to mull the synthetic routine in a different view. In the natural world, plenty of useful complex potentially medicinal molecules are synthesized in an efficient and eco-friendly way with delicate stereochemistry. Room temperature, atmospheric pressure, aqueous environment, neutral pH and recyclable catalyst all lure chemists into the world of enzymatic biocatalysis. We study polyketide synthases to learn the mechanism how they function to produce polyketides. In our laboratory, we not only utilize enzyme to synthesize complex chiral compound but also explore the intrinsic structure of these enzymes. We first introduce the new developed general chemo-enzymatic polyketide synthesis using ketoreductases. We successfully synthesized a library of four complex polyketides with two chiral centers involving two ketoreductases. In order to prove our result, we found a strategy to chemically synthesize the same molecule as well involving high pressure catalytic hydrogenation. We also report a seven-enzyme cascade reaction to synthesize another complex polyketide derivative in one-pot with a technique to monitor the reaction in situ. We studied the mechanism of pyran synthase in a trans-acyltransferase assembly line. By mutagenesis we determined the catalytic domain of the pyran synthase using chemically prepared analogues. We also studied the mechanism of monomethylation and dimethylation in a methyltransferase module. Finally, we conducted docking motif swapping proving the portability of docking domain in trans-Acyltransferase PKS modules. With the development of our study, we not only hope to explore more structural and functional details about the PKSs but also strive to use our enzyme to create more complex pharmaceutically useful polyketide building blocks in an easier and greener way.Chemistr