Abstract The peptide bond is the defining feature of proteins, it co ntrols their structure by enforcing planarity and governs their chemistry by abolishing nucleophili city. This behaviour results from the delocalisation of the lone pair of electrons on the nitrogen atom into the adjacent carbonyl bond. Methylating an amide bond by conventional chemistry usually requires a metal to generate the amide anion. The OphA enzyme methyla tes amide bonds at pH 7 in water using S-adenosyl methionine (SAM) as co-factor. The st ructure of OphA reveals a complex catenane arrangement in which the substrate i s clamped adjacent to SAM. Site directed mutants have identified the key catalytic residue s. A mechanism in which conjugation of the peptide bond is disrupted by van der Waals in teractions between SAM and the amide nitrogen is proposed and tested. The methylation of amides is of particular interest as it greatly enhances the cell permeability of peptides.