Ergothioneine Biosynthetic Methyltransferase EgtD Reveals the Structural Basis of Aromatic Amino Acid Betaine Biosynthesis

Ergothioneine is an N-alpha-trimethyl-2-thiohistidine derivative that occurs in human, plant, fungal, and bacterial cells. Biosynthesis of this redox-active betaine starts with trimethylation of the a-amino group of histidine. The three consecutive methyl transfers are catalyzed by the S-adenosylmethionine-dependent methyltransferase EgtD. Three crystal structures of this enzyme in the absence and in the presence of N-alpha-dimethyl-histidine and S-adenosylhomocysteine implicate a preorganized array of hydrophilic interactions as the determinants for substrate specificity and apparent processivity. We identified two active site mutations that change the substrate specificity of EgtD 10(7)-fold and transform the histidine-methyltransferase into a proficient tryptophan-methyltransferase. Finally, a genomic search for EgtD homologues in fungal genomes revealed tyrosine and tryptophan trimethylation activity as a frequent trait in ascomycetous and basidomycetous fungi.