Identification and characterization of N 9-methyltransferase involved in converting caffeine into non-stimulatory theacrine in tea

Caffeine is a major component of xanthine alkaloids and commonly consumed in many popular beverages. Due to its occasional side effects, reduction of caffeine in a natural way is of great importance and economic significance. Recent studies reveal that caffeine can be converted into non-stimulatory theacrine in the rare tea plant Camellia assamica var. kucha (Kucha), which involves oxidation at the C 8 and methylation at the N 9 positions of caffeine. However, the underlying molecular mechanism remains unclear. Here, we identify the theacrine synthase CkTcS from Kucha, which possesses novel N9 -methyltransferase activity using 1,3,7-trimethyluric acid but not caffeine as a substrate, confirming that C 8 oxidation takes place prior to N 9-methylation. The crystal structure of the CkTcS complex reveals the key residues that are required for the N 9-methylation, providing insights into how caffeine N -methyltransferases in tea plants have evolved to catalyze regioselective N -methylation through fine tuning of their active sites. These results may guide the future development of decaffeinated drinks.