Converts beta-carotene to canthaxanthin via echinenone. 2 AH2 + all-trans-beta-carotene + 2 O2 = 2 A + echinenone + 3 H2O 2 AH2 + echinenone + 2 O2 = 2 A + canthaxanthin + 3 H2O Carotenoid biosynthesis; astaxanthin biosynthesis.