Lyase that catalyzes the formation of isoprene from dimethylallyl diphosphate. dimethylallyl diphosphate = diphosphate + isoprene 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. Tpsb subfamily.