Characterization of Methyltransferase Carbon-Oxygen Hydrogen Bonding and Sulfur-Oxygen Chalcogen Bonding with the Sulfonium of S-adenosyl-L-methionine

Abstract

Methyltransferases use S-adenosyl-L-methionine (AdoMet) to transfer the one-carbon group to nitrogen, carbon, oxygen, and sulfur nucleophiles, among other atoms. AdoMet is the most common biological methyl donor, and the methyl group is used in numerous biological processes such as signaling, metabolism, and gene regulation. The mechanism of transfer has been determined to be a bimolecular nucleophilic substitution (SN2) reaction, but the origin of the catalytic rate enhancement induced by methyltransferases remains a longstanding point of debate. The objective of this dissertation is to explore carbon-oxygen hydrogen bonding and sulfur-oxygen chalcogen bonding in several different families of methyltransferases to understand the importance of these interactions between active site oxygen atoms and the AdoMet sulfonium cation in substrate binding and catalysis. Carbon-oxygen hydrogen bonding was probed in two methyltransferase families, the Rossmann-like fold methyltransferase TylM1, and the reactivation domain of methionine synthase. Mutation to remove carbon-oxygen hydrogen bonding of TylM1 to the AdoMet methylene groups flanking the sulfur atom showed slight defects in both binding and catalysis. Furthermore, a second TylM1 active site residue involved in carbon-oxygen hydrogen bonding to the methyl group of AdoMet was shown to be crucial to enzyme catalysis through its role in binding and aligning both substrates. Carbon-oxygen hydrogen bonding to active site waters in methionine synthase was manipulated by mutation of active site glutamates which coordinated these waters to glutamine resulting in a loss of substrate binding and a lesser ability to discriminate substrate from product. In addition to carbon-oxygen hydrogen bonding, sulfur-oxygen chalcogen bonding between the AdoMet sulfonium cation and the active site of the lysine methyltransferase SET7/9, was found to facilitate AdoMet binding, discrimination between substrate and product, and catalysis. These results are among the first to establish chalcogen bonding as an important interaction in enzymes. Collectively, these studies further reinforce the roles of carbon-oxygen hydrogen bonding in substrate binding and catalysis across different methyltransferase families, and identifies the significance of sulfur-oxygen chalcogen bonding as a newly discovered interaction within these enzymes.