-!- Found in a broad variety of bacteria, fungi, plants, insects, crustaceans, and mammals, which is involved in the synthesis of betalains and melanin. -!- The enzyme, which is activated upon binding molecular oxygen, can catalyze both a monophenolase reaction cycle or a diphenolase reaction cycle. -!- During the monophenolase cycle, one of the bound oxygen atoms is transferred to a monophenol (such as L-tyrosine), generating an O-diphenol intermediate, which is subsequently oxidized to an o-quinone and released, along with a water molecule. -!- The enzyme remains in an inactive deoxy state, and is restored to the active oxy state by the binding of a new oxygen molecule. -!- During the diphenolase cycle the enzyme binds an external diphenol molecule (such as L-dopa) and oxidizes it to an O-quinone that is released along with a water molecule, leaving the enzyme in the intermediate met state. -!- The enzyme then binds a second diphenol molecule and repeats the process, ending in a deoxy state. -!- The second reaction is identical to that catalyzed by the related enzyme catechol oxidase (EC 1.10.3.1). -!- However, the latter can not catalyze the hydroxylation or monooxygenation of monophenols. -!- Formerly EC 1.14.17.2.