|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
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 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 polymerization 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.
Polyphenol oxidase 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 monomer, trimer, tetramer, octamer, dodecamer. Evidences for this are multiple (protein moonlighting functions, Substrate binding/turnover impacts multimerization, Different assemblies have different activities, Kinetic hysteresis).
In plants, PPO is a plastidic enzyme with unclear synthesis and function. In functional chloroplasts, it may be involved in some aspect of oxygen chemistry like mediation of pseudocyclic photophosphorylation.
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.
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 nonbrowning trait. Specifically, gene silencing is used to turn down the expression of polyphenol oxidase (PPO), thus preventing the fruit from browning. They are therefore genetically modified food.
There are two types of inhibitor of polyphenol oxidase. 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 polyphenol oxidase activity from seedlings of higher plants. Tropolone is a grape polyphenol oxidase inhibitor. Another inhibitor of this enzyme is potassium pyrosulphite (K2S2O5). Banana root PPO is strongly inhibited by dithiothreitol and sodium metabisulfite.
Potassium dithionite (or potassium hydrosulfite) is also an inhibitor of the polyphenol oxidase.
Hemocyanin is homologous to the phenol oxidases (e.g. tyrosinase) since both enzymes sharing type 3 Cu active site coordination. Hemocyanin also exhibits phenol oxidase activity, but with slowed kinetics from greater steric bulk at the active site. Partial denaturation actually improves hemocyanin’s phenol oxidase activity by providing greater access to the active site.
Aureusidin synthase is homologous to plant polyphenol oxidase, but contains certain significant modifications.
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