Figure 7.

Putative reaction mechanism of TgTCEA enzyme to convert 6-tuliposide A to tulipalin A. TgTCEA catalyzes the intramolecular transesterification reaction, where intramolecular nucleophilic attack by the terminal hydroxy group of substrate occurs without incorporating water after the elimination of Glc from the substrate. The hydrolysis reaction catalyzed by typical carboxylesterases proceeds via intermolecular nucleophilic attack by activated water, leading to 4-hydroxy-2-methylenebutanoic acid formation. Note that TgTCEA catalyzes a transesterification reaction but not a hydrolysis reaction. For the catalytic triad (Ser, Asp, and His) and oxyanion hole (Gly; Table IV; Supplemental Figure S1), only functional groups involved in catalysis are shown.