Polyphenol oxidase (PPO, EC 1.14.18.1) belongs to a subclass of enzymes known as tyrosinases which catalyze the o-hydroxylation of monophenols (phenol molecules in which the benzene ring contains a single hydroxyl substituent) to o-diphenols (phenol molecules containing two hydroxyl substituents) and subsequently to o-quinones. In the reaction, copper ion reacts with molecular oxygen to form an intermediate which acts on monophenol and diphenol.