Fig. 2. The reaction mechanism of all sesquiterpene synthases start with the ionization of FPP. The resulting carbocation undergoes a range of cyclizations, some of which are
preceded by isomerization to a nerolidyl intermediate (A). Steps following the initial cyclization, which include secondary cyclization, deprotonation to a neutral
intermediate, hydride shifts, methyl shifts and Wagner–Meerwein rearrangements, are not shown except for the 5-epi-aristolochene synthase of Nicotiana tabacum. The
reaction mechanism of this enzyme proceeds via a neutral intermediate, germacrene A, that is re-protonated to form the bicyclic reaction products (B). The numbering of
carbon atoms of intermediates and products refers to that for FPP.