Structural and Functional Characterization of Sulfonium Carbon-Oxygen Hydrogen Bonding in the Deoxyamino Sugar Methyltransferase TylM1.

The N-methyltransferase TylM1 from Streptomyces fradiaecatalyzes the final step in the biosynthesis of the deoxyamino sugar mycaminose, a substituent of the antibiotic tylosin. The high-resolution crystal structure of TylM1 bound to the methyl donor S-adenosylmethionine (AdoMet) illustrates a network of carbon–oxygen (CH···O) hydrogen bonds between the substrate’s sulfoniumcation and residues within the active site. These interactions include hydrogen bonds between the methyl and methylene groups of the AdoMet sulfoniumcation and the hydroxyl groups of Tyr14 and Ser120 in the enzyme. To examine the functions of these interactions, we generated Tyr14 to phenylalanine (Y14F) and Ser120 to alanine (S120A) mutations to selectively ablate the CH···O hydrogen bonding to AdoMet. The TylM1 S120A mutant exhibited a modest decrease in its catalytic efficiency relative to that of the wild type (WT) enzyme, whereas the Y14F mutation resulted in an approximately 30-fold decrease in catalytic efficiency. In contrast...