A simplified scheme of triterpenoid biosynthesis in Centella. Farnesyl
diphosphate synthase (FPS) isomerizes isopentyl diphosphate (IPP) and dimethylallyl
diphosphate (DMAPP) to farnesyl diphosphate (FPP), which squalene synthase (SQS)
converts to squalene. Squalene epoxidase (SQE) oxidises squalene to 2,3-oxidosqualene.
Oxidosqualene cyclase (OSC) enzymes cyclize 2,3-oxidosqualene through cationic
intermediates (e.g. dammarenyl cation) to one or more cyclic triterpene skeletons. Other
enzymes involved include α/-amyrin synthases (α/-AS) which can also form the
lupenyl cation but further ring expansion and rearrangements are required before the
deprotonation to α/-amyrin, the precursors of the sapogenins, to generate the products
listed in Table 1. Adapted from[ 19,21].