Wikipedia
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.
PPO causes the rapid polymerization of o-quinones to produce black, brown or red pigments ( polyphenols) that cause fruit browning. The amino acid tyrosine contains a single phenolic ring that may be oxidised by the action of PPOs to form o-quinone. Hence, PPOs may also be referred to as tyrosinases.
Common foods producing the enzyme include mushrooms ( Agaricus bisporus), apples ( Malus domestica) and lettuce ( Lactuca sativa).