Title

Characterizing structure and function of thioesterase from polyketide synthase

Date of Award

2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Christopher N. Boddy

Keywords

Thioesterase, Polyketide synthase, Chain termination

Subject Categories

Chemistry

Abstract

Polyketides are a large class of structurally diverse natural products with important biological activities. A number of commercially available drugs are developed from polyketides, such as simvastin, erythromycin and tetracyclines. Polyketides are generated by polyketide synthases (PKSs) via condensing a starter unit with a series of extender units. Chain termination step is critical in polyketide biosynthesis. Although nature uses a variety of enzymes to catalyze the chain termination step in polyketide biosynthesis, which leads to the large chemical diversity of polyketides. Thioesterase (TEs)- mediated chain termination mechanism is the most common termination mechanism in polyketide biosynthesis. In this dissertation, using novel model substrates generated through organic synthesis, we systematically studied the structure and function of the thioesterase domain from 6-deoxyerythronolide B synthase (DEBS TE) via site-directed mutagenesis and steady state kinetics analysis. Our results provide more insights on substrates tolerance and stereo-selectivity of TE catalyzed hydrolysis and macrocyclization, which supports a novel model for substrate recognition in DEBS TE based on hydrophobic interactions. In addition, we also systematically investigated the structure and function of thioesterase domain from fungal iterative polyketide synthase, such as zearalenone and radicicol biosynthesis pathways (Zea TE and Rad TE respectively). The substrate tolerance of ring size and stereo-selectivity of O-nucleophile of TE catalyzed macrocyclization were characterized. We addressed that Zea TE and Rad TE can produce macrocycles with different ring size (10, 14, 15 and 16 member ring) and stereochemistry. We also discovered that Zea TE and Rad TE can catalyze novel cross-coupling reaction. Our results indicate that TEs are potential useful tools in organic synthesis. Lastly, we addressed that the kinetic behavior of fungal TEs suggests that they may play an important role in controlling the iterative cycling in fungal polyketide biosynthesis. All the results presented in the dissertation not only provide better understanding of TE mediated chain termination in polyketide biosynthesis, but also can serve as a guideline for engineering novel polyketides.

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