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Camptothecin (CAM) is a well-known, complex, plant-derived antitumor monoterpenoid indole alkaloid (MIA). Featuring a unique pentacyclic pyrroloquinoline scaffold, CAM is biosynthetically distinct from the other known MIAs, such as antitumor vincristine and vinblastine. Herein, CaCYP72A565 and CaCYP72A610 enzymes involved in the biosynthesis of the monoterpenoid moiety of CAM were cloned from CAM-producing Camptotheca acuminata. Heterologous overexpression and functional characterization assays showed that CaCYP72As catalyzes two consecutive reactions, the stereoselective hydroxylation at C-7 of 7-deoxyloganic acid and the subsequent carbon-carbon (C-C) bond cleavage between C-7 and C-8 of iridoid glucoside, to generate the intramolecular cyclopentane ring-opening secoiridoid glucoside. Comparative metabolite profiling analyses suggested that C. acuminata synthesizes loganic acid, secologanic acid, and strictosidinic acid as its MIA carboxylic acid intermediates. CaCYP72As are novel bifunctional enzymes that catalyze stereoselective hydroxylation and subsequent C-C bond cleavage reactions to give a ring-opening product with two functional groups, an aldehyde and a double bond.
Yang et al. (Wed,) studied this question.