Polyphenol oxidase

Polyphenol oxidase (PPO or monophenol monooxygenase) is a tetramer which contains four atoms of copper per molecule, and binding sites for two aromatic compounds and oxygen. The enzyme catalyses 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). They can also further catalyse the oxidation of o-diphenols to produce o-quinones. It is the rapid polymerisation of o-quinones to produce black, brown or red pigments (polyphenols) that is the cause of 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.

Enzyme nomenclature differentiates between monophenol oxidase enzymes (tyrosinases) and o-diphenol:oxygen oxidoreductase enzymes (catechol oxidases). Therefore please refer to the tyrosinase and catechol oxidase articles for more information on polyphenol oxidase enzymes.

A mixture of monophenol oxidase and catechol oxidase enzymes is present in nearly all plant tissues, and can also be found in bacteria, animals, and fungi. In insects, cuticular polyphenol oxidases are present and their products are responsible for desiccation tolerance.

In fact, browning by PPO is not always an undesirable reaction; the familiar brown color of tea, coffee and cocoa is developed by PPO enzymatic browning during product processing.

Tentoxin has also been used in recent research to eliminate the polyphenol oxidase activity from seedlings of higher plants. Tropolone is a grape polyphenol oxidase inhibitor. Another inhibitor of this enzyme is potassium pyrosulphite (K2S2O5).