Sesquiterpene synthase involved in the biosynthesis of volatile compounds (PubMed:21818683). Mediates the conversion of (2E,6E)-farnesyl diphosphate (FPP) into (1E,4E,8E)-alpha-humulene and (-)-(E)-beta-caryophyllene, and of (2Z,6Z)-farnesyl diphosphate ((ZZ)-FPP) into beta-bisabolene, gamma-curcumene and (Z)-gamma-bisabolene (PubMed:21818683). Can act with a low efficiency as a monoterpene synthase with geranyl diphosphate (GPP) as substrate, thus producing beta-myrcene, (E)-beta-ocimene, limonene and terpinolene (PubMed:21818683). (2E,6E)-farnesyl diphosphate = alpha-humulene + diphosphate (2E,6E)-farnesyl diphosphate = (-)-(E)-beta-caryophyllene + diphosphate (2Z,6Z)-farnesyl diphosphate = beta-bisabolene + diphosphate (2E)-geranyl diphosphate = diphosphate + terpinolene (2E)-geranyl diphosphate = diphosphate + limonene (2E)-geranyl diphosphate = beta-myrcene + diphosphate (2E)-geranyl diphosphate = (E)-beta-ocimene + diphosphate (2Z,6Z)-farnesyl diphosphate = (Z)-gamma-bisabolene + diphosphate (2Z,6Z)-farnesyl diphosphate = diphosphate + gamma-curcumene Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsa subfamily.