Involved in viguiepinol biosynthesis. Catalyzes the conversion of geranylgeranyl diphosphate (GGDP) into copalyl diphosphate (ent-CDP). (2E,6E,10E)-geranylgeranyl diphosphate = ent-copalyl diphosphate Activity is highest with Mg(2+). Can also use Zn(2+), Co(2+), Mn(2+) and Ni(2+). kcat is 0.033 sec(-1). Optimum pH is 5.5. Optimum temperature is 35 degrees Celsius. Antibiotic biosynthesis. Homodimer. Belongs to the terpene synthase family.