Polyphenol oxidase (PPO or monophenol monooxygenase or Polyphenol oxidase I, chloroplastic) is a
tetramer that contains four atoms of copper per molecule, and binding sites for two aromatic compounds and oxygen. The enzyme catalyses the
o-
hydroxylation of
monophenol molecules in which the
benzene ring contains a single
hydroxyl substituent to
o-diphenols (
phenol molecules containing two
hydroxyl substituents). It can also further catalyse the
oxidation of
o-diphenols to produce
o-quinones.