Figure 1.

Reactions catalyzed by protein histidine N-methyltransferases. At pH ≈ 7, two neutral tautomers of histidine residues may exist in proteins: the N1-protonated π-tautomer and the N3-protonated τ-tautomer. Data show that different protein histidine methyltransferases catalyze the transfer of a methyl group from S-adenosyl-L-methionine (AdoMet) to specific nitrogen of the imidazole ring. HPM1, SETD3, METTL9, and METTL18 are the only enzymes characterized with this activity so far. AdoHcy—S-adenosyl-L-homocysteine.