Post-Cyclase Skeletal Rearrangements in Plant Triterpenoid Biosynthesis by a Pair of Branchpoint Isomerases

Triterpenoids possess potent biological activities, but their polycyclic skeletons are challenging to synthesise. In biochemistry, the skeletal diversity of plant triterpenoids is normally generated by oxidosqualene cyclases and remains unaltered during subsequent tailoring steps. In contrast, we report here enzyme-mediated skeletal rearrangements after the initial cyclisation, controlling the pathway bifurcation between different plant triterpenoid classes. Using a combination of bioinformatics, heterologous expression in plants and chemical analyses, we identified a cytochrome P450 monooxygenase and two isomerases for this process. The two isomerases share one epoxide substrate but generate two different rearrangement products, one containing a cyclopropane ring. Our findings reveal a new strategy how triterpenoid skeletal diversity is generated in Nature and are crucial for the biotechnological production of limonoid, quassinoid, isoprotolimonoid and glabretane triterpenoids.