Monoterpene synthase (mono-TPS) involved in the biosynthesis of monoterpenes natural products, constituent of coffee beverage aroma. Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Monomer. Confined to flowers. 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. Tpsg subfamily.