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Function(s) and Biological Relevance

Benzylisoquinoline alkaloids (BIAs), plant-specialized metabolites, is the organism that forms TNMT. Tetrahydroprotoberberine N-methyltransferase catalyzes the N-methylation of (S)-stylopine in the pathway leading to protopines and benzo [c] phenanthridines. Also acts on (S)-canadine in a different pathway that leads to the production of phthalideisoquinolines. The substrate specificity of TNMT enzymes appears to arise from the arrangement of subgroup-specific stereospecific recognition elements relative to catalytic elements that are more widely-conserved.

Broader Implications

Structural highlights and structure-function relationships

GfTNMT was co-crystallized with the cofactor S-adenosyl-L-methionine (dmin = 1.6 A), product S-adenosyl-L-homocysteine (dmin = 1.8 A), or in complex with S-adenosyl-L-homocysteine and (S)-cis-N-methylstylopine (dmin = 1.8 A), These structures reveal for the first time how a mostly hydrophobic L-shaped substrate recognition pocket selects for the (S)-cis configuration of the two central six-membered rings in protoberberine BIA compounds. The substrate specificity of TNMT enzymes appears to arise from the arrangement of subgroup-specific stereospecific recognition elements relative to catalytic elements that are more widely conserved among all BIA NMTs. The binding mode of protoberberine compounds to GfTNMT appears to be similar to coclaurine NMT, with the isoquinoline rings buried deepest in the binding pocket. This binding mode differs from that of pavine NMT, in which the benzyl ring is bound more deeply than the isoquinoline rings. The insights into substrate recognition and catalysis provided here form a sound basis for the rational engineering of NMT enzymes for chemoenzymatic synthesis and metabolic engineering.

Energy Transformation

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