Studies of unusual catalysis : a tale of four enzymes from diverse biosynthesis pathways

Sun, He, Ph. D.
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The diverse reactions that enzymes catalyze have fascinated enzymologists for decades. Continuing investigations in the biosynthesis of both primary and secondary metabolites have led to the discovery of enzymes that employ unusual ways to mediate bio-transformations. Exploration of such atypical biological catalysts is not only important for a comprehensive understanding of the natural products biosynthesis, but also providing new insights that are potentially valuable for developing novel compounds with enhanced biological activities. This dissertation describes the characterization of four enzymes that demonstrate unusual catalytic properties in different biosynthesis pathways. UDP-galactopyranose mutase (UGM) is required for cell wall biosynthesis in many microorganisms. It uses the common cofactor flavin adenine dinucleotide (FAD) in an unusual manner. Positional isotope exchange and kinetic linear free energy relationship studies provide a direct experimental evaluation of the nucleophilic participation by reduced FAD during UGM catalysis. MoeZ from Amycolatopsis orientalis is a unique sulfur carrier protein (SCP) activating enzyme that participates in the metabolism of sulfur. Unlike reported pathway specific homologues, MoeZ can activate multiple SCPs from different biosynthesis pathways found in A. orientalis. Herein, the enzyme is characterized biochemically, and a disulfide intermediate is suggested as part of its catalytic cycle for sulfur transfer from thiosulfate to SCPs. The last two enzymes, Fom3 and OxsB, belong to the cobalamin-dependent radical SAM class of enzymes. While these enzymes are believed to operate with unprecedented chemistry, they remain poorly understood and understudied. Fom3 is proposed to methylate an unactivated carbon center in the biosynthesis of fosfomycin, which is a clinically relevant antibiotic. OxsB is responsible for the biosynthesis of oxetanocin A, an oxetane ring containing nucleoside that exhibits antiviral activity. Efforts have been made to isolate active Fom3 and reconstitute the in vitro activity of OxsB. Reductive cleavage of SAM by the latter enzyme has been demonstrated for the first time and is described in this dissertation.