(S)-tetrahydroprotoberberine N-methyltransferase () is an enzyme that catalyzes related methylation reactions in the biosynthesis of benzylisoquinoline alkaloids.
For example, (S)-stylopine is converted to (S)-cis-N-methylstylopine. The methyl group comes from the cofactor, S-adenosyl methionine (SAM), which becomes S-adenosyl-L-homocysteine (SAH). The enzyme was characterised from Eschscholtzia californica. It methylates (S)-canadine and (S)-tetrahydropalmatine in a similar way.
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:(S)-7,8,13,14-tetrahydroprotoberberine cis-N-methyltransferase. This enzyme is also called tetrahydroprotoberberine cis-N-methyltransferase. It participates in alkaloid biosynthesis.
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