Sesquiterpene synthase that catalyzes the formation of alpha-humulene. Can use farnesyl diphosphate (FPP) as substrate, but not geranyl diphosphate (GPP) or geranylgeranyl diphosphate (GGPP). (2E,6E)-farnesyl diphosphate = alpha-humulene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Sesquiterpene biosynthesis. Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed in trichomes, cones and young leaves. 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.