These conversions are catalysed by OSCs, known collectively as triterpene synthases [21]. Plants
biosynthesize diverse triterpenoids and their genomes encode multiple OSC enzymes to form these
skeletons. The level at which the structural diversity of triterpenes is generated depends on the
cyclization of 2,3-oxidosqualene by different OSCs such as cycloartenol synthase (CAS), lupeol
synthase (LS) and α/-amyrin synthase (AS) [25]. A phylogenetic tree analysis shows that OSCs have
the same enzyme function from respective branches in the tree even though they were derived from
different plants species. All triterpene synthases appear to have diverged from an ancestral CAS gene
[26], but an independent origin for -AS in eudicots and monocots has been proposed [19]. The
triterpenoid cyclases are distinct from LS and CAS, and form discrete subgroups within the OSC
superfamily