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Abstract

S‐adenosyl‐L‐methionine is a high energy compound and is the major source of methyl groups for a myriad of biologic transmethylation reactions. These highly specific single‐carbon transfers onto diverse nucleophilic centers in biomolecules are catalyzed by methyltransferase enzymes and play important regulatory and structural roles in the cell. Here we discuss the chemical mechanism of the methylation reactions, including structural features of the methylsulfonium center in the cofactor molecule, enzyme‐assisted activation of diverse nucleophilic targets by deprotonation or covalent catalysis, and spatial constraints of the reaction.