Elucidation of novel enzymatic system involved in the activation of the defensive secondary metabolites in the tulip

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This study was performed to identify the enzymes involved in the conversion of tuliposides (Pos), the major secondary metabolites in tulip, to antimicrobial tulipalins (Pa), the lactonized aglycons of Pos. I found that PosA and PosB, which are the major forms of Pos in the tulip cultivar, are converted to the corresponding PaA and PaB by PosA-converting enzyme and PosB-converting enzyme, respectively. Amino acid sequences of those enzymes were similar to the carboxylesterases, which typically catalyze hydrolysis reactions. However, the Pos-converting enzymes catalyzed no hydrolysis of Pos, but the formation of Pa through lactonization. Pos-converting enzymes were identified as the unique lactone-forming carboxylesterases, specifically catalyzing intramolecular transesterification.