Catalyzes the synthesis of curcumin by condensing feruloyl-CoA with a diketide-CoA in the curcuminoid biosynthesis. (E)-feruloyl-CoA + (E)-feruloylacetyl-CoA + H2O = CO2 + 2 CoA + curcumin kcat is 1.1 min(-1) with feruloyl-CoA. kcat is 0.85 min(-1) with p-coumaroyl-CoA. Optimum pH is 9.0. Optimum temperature is 50 degrees Celsius. Secondary metabolite biosynthesis; flavonoid biosynthesis. Homodimer. Expressed in both the leaf and rhizome, with higher expression in the rhizome. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.