Abstract

Securinega alkaloids represent a class of plant secondary metabolites with intriguing bridged tetracyclic structures and promising biological activities. Despite extensive synthetic efforts and a number of innovative total syntheses, no relevant biosynthetic genes have been reported to date. In this study, fluesuffine A (1), a C-2 and C-3 functionalized derivative of Securinega alkaloids with a fused 5/5/5/6/5/6/5 congested heptacyclic ring system, was isolated from Flueggea suffruticosa. In the biogenetic study of 1, we found a berberine bridge enzyme (BBE)-like enzyme (FsBBE), which could efficaciously catalyze the condensation of allosecurinine and l-ascorbic acid (l-AA) or dehydroascorbic acid (DHA), resulting in the chemical skeleton of 1. FsBBE also could catalyze the condensation of allosecurinine and isoascorbic acid to form another unnatural Securinega alkaloid fluesuffine B (2), an epimer of 1. Moreover, the catalytic mechanism study of FsBBE revealed the existence of an enamine intermediate (3), which was previously proposed as a key intermediate in the biosynthesis of C-2 and C-3 functionalized derivatives of Securinega alkaloids. The formation of 3 suggested that the epimers were synthesized via an intermolecular enamine-type addition reaction. Hence, our findings provide further insight into the biosynthesis of C-2 and C-3 functionalized derivatives of Securinega alkaloids and expand the diversity of ascorbylated natural product.