InterPro: IPR000940 Methyltransferase, NNMT/PNMT/TEMT

Methyl transfer from the ubiquitous S-adenosyl-L-methionine (AdoMet) to either nitrogen, oxygen or carbon atoms is frequently employed in diverse organisms ranging from bacteria to plants and mammals. The reaction is catalysed by methyltransferases (Mtases) and modifies DNA, RNA, proteins and small molecules, such as catechol for regulatory purposes. The various aspects of the role of DNA methylation in prokaryotic restriction-modification systems and in a number of cellular processes in eukaryotes including gene regulation and differentiation is well documented.

Three classes of DNA Mtases transfer the methyl group from AdoMet to the target base to form either N-6-methyladenine, or N-4-methylcytosine, or C-5- methylcytosine. In C-5-cytosine Mtases, ten conserved motifs are arranged in the same order [1]. Motif I (a glycine-rich or closely related consensus sequence; FAGxGG in M.HhaI [2]), shared by other AdoMet-Mtases [3], is part of the cofactor binding site and motif IV (PCQ) is part of the catalytic site. In contrast, sequence comparison among N-6-adenine and N-4-cytosine Mtases indicated two of the conserved segments [4], although more conserved segments may be present. One of them corresponds to motif I in C-5-cytosine Mtases, and the other is named (D/N/S)PP(Y/F). Crystal structures are known for a number of Mtases [5, 2, 1, 6]. The cofactor binding sites are almost identical and the essential catalytic amino acids coincide. The comparable protein folding and the existence of equivalent amino acids in similar secondary and tertiary positions indicate that many (if not all) AdoMet-Mtases have a common catalytic domain structure. This permits tertiary structure prediction of other DNA, RNA, protein, and small-molecule AdoMet-Mtases from their amino acid sequences [7].

Several cytoplasmic vertebrate methyltransferases are evolutionary related [8], including nicotinamide N-methyltransferase (EC:2.1.1.1) (NNMT); phenylethanolamine N-methyltransferase (EC:2.1.1.28) (PNMT); and thioether S-methyltransferase (EC:2.1.1.96) (TEMT). NNMT catalyzes the N-methylation of nicotinamide and other pyridines to form pyridinium ions. This activity is important for the biotransformation of many drugs and xenobiotic compounds. PNMT catalyzes the last step in catecholamine biosynthesis, the conversion of noradrenalin to adrenalin; and TEMT catalyzes the methylation of dimethyl sulphide into trimethylsulphonium. These three enzymes use S-adenosyl-L-methionine as the methyl donor. They are proteins of 30 to 32 kDa.