Dimethylhistidine N-methyltransferase () is an enzyme that catalyzes a sequence of methylation reactions:
The overall effect is to convert L-histidine to its trimethylated product, hercynine. Both the singly and doubly methylated compounds are also substrates for the reaction. The methyl groups come from the cofactor, S-adenosyl methionine (SAM), which is converted to S-adenosyl-L-homocysteine (SAH).
This enzyme belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases. The systematic name of this enzyme class is S-adenosyl-L-methionine:Nalpha,Nalpha-dimethyl-L-histidine Nalpha-methyltransferase. Other names in common use include dimethylhistidine methyltransferase, histidine-alpha-N-methyltransferase, S-adenosyl-L-methionine:alpha-N,alpha-N-dimethyl-L-histidine, and alpha-N-methyltransferase.
In the bacterium Mycobacterium smegmatis, this enzyme is part of the pathway that converts histidine to ergothioneine.
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