Lyase that catalyzes the formation of isoprene from dimethylallyl diphosphate, but not from isopentenyl diphosphate or geranyl diphosphate. dimethylallyl diphosphate = diphosphate + isoprene Magnesium. Can also use other divalent metal cations, except copper and calcium. Optimum pH is 8.0. Active from pH 6.5 to 9.5. Optimum temperature is 40 degrees Celsius. Terpene metabolism. Predominantly expressed in leaves. Up-regulated by heat stress and continuous light. Down-regulated by continuous dark and cold stress. Not induced by pathogens or treatment with methyl jasmonate or methyl salicylate. 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.