The PPO enzyme family 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