Polyphenol oxidase

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.^[1] 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.^[2]

Common foods producing the enzyme include mushrooms (Agaricus bisporus), apples (Malus domestica) and lettuce (Lactuca sativa).

Structure and function

PPO is listed as a morpheein, a protein that can form two or more different homo-oligomers (morpheein forms), but must come apart and change shape to convert between forms. It exists as a monomer, trimer, tetramer, octamer or dodecamer,^[3][4] creating multiple (protein moonlighting functions.^[5] Substrate binding/turnover impacts multimerization,^[6] Different assemblies have different activities,^[7] Kinetic hysteresis^[6]).

In plants, PPO is a plastidic enzyme with unclear synthesis and function. In functional chloroplasts, it may be involved in oxygen chemistry like mediation of pseudocyclic photophosphorylation.^[8]

Enzyme nomenclature differentiates between monophenol oxidase enzymes (tyrosinases) and o-diphenol:oxygen oxidoreductase enzymes (catechol oxidases).

Distribution and applications

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^[9] 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 (especially black tea) and cocoa^[10] is developed by PPO enzymatic browning during product processing.

Grape reaction product (2-S glutathionyl caftaric acid) is an oxidation compound produced by action of PPO on caftaric acid and found in wine. This compound production is responsible for the lower level of browning in certain white wines.

Arctic apples are a suite of trademarked apples that contain a non-browning trait derived by gene silencing to suppress the expression of PPO, thus preventing the fruit from browning.^[11]

Inhibitors

There are two types of inhibitor of PPO, those competitive to oxygen in the copper site of the enzyme and those competitive to phenolics. Tentoxin has also been used in recent research to eliminate the PPO activity from seedlings of higher plants.^[12] Tropolone is a grape polyphenol oxidase inhibitor.^[13] Another inhibitor of this enzyme is potassium pyrosulphite (K₂S₂O₅).^[14] Banana root PPO is strongly inhibited by dithiothreitol and sodium metabisulfite.^[15]

Potassium dithionite (or potassium hydrosulfite) is also an inhibitor of PPO.

Related enzymes

Prophenoloxidase is a modified form of the complement response found in some invertebrates, including insects, crabs and worms.^[16]

Hemocyanin is homologous to the phenol oxidases (e.g. tyrosinase) since both enzymes sharing type copper active site coordination. Hemocyanin also exhibits PPO activity, but with slowed kinetics from greater steric bulk at the active site. Partial denaturation actually improves hemocyanin’s PPO activity by providing greater access to the active site.^[17]

Aureusidin synthase is homologous to plant polyphenol oxidase, but contains certain significant modifications.

References