Catalyzes the synthesis of curcumin by condensing feruloyl-CoA with a diketide-CoA in the curcuminoid biosynthesis. Also acts as a demethoxycurcumin synthase by accepting 4-coumaroyl-CoA as a starter substrate instead of feruloyl-CoA. (E)-feruloyl-CoA + (E)-feruloylacetyl-CoA + H2O = CO2 + 2 CoA + curcumin (E)-4-coumaroyl-CoA + (E)-feruloylacetyl-CoA + H2O = CO2 + 2 CoA + demethoxycurcumin (4-coumaroyl)acetyl-CoA + 4-coumaroyl-CoA + H2O = bisdemethoxycurcumin + CO2 + 2 CoA kcat is 0.19 min(-1) with feruloyl-CoA. kcat is 0.36 min(-1) with p-coumaroyl-CoA. Optimum pH is 8.0. Optimum temperature is 45-55 degrees Celsius. Secondary metabolite biosynthesis; flavonoid biosynthesis. Homodimer. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.