Polyphenol oxidase
Encyclopedia
Polyphenol oxidase 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
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...

 molecules in which the benzene ring contains a single hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 substituent) to o-diphenols
Pyrocatechol
Catechol, also known as pyrocatechol or 1,2-dihydroxybenzene, is an organic compound with the molecular formula C6H42. It is the ortho isomer of the three isomeric benzenediols. This colourless compound occurs naturally in trace amounts...

 (phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...

 molecules containing two hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 substituents). They can also further catalyse the oxidation of o-diphenols to produce o-quinones
Quinone
A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...

. It is the rapid polymerisation of o-quinones to produce black, brown or red pigments (polyphenol
Polyphenol
Polyphenols are a structural class of natural, synthetic, and semisynthetic organic chemicals characterized by the presence of large multiples of phenol structural units...

s) that is the cause of fruit browning. The amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

 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 tyrosinase
Tyrosinase
Tyrosinase also known as monophenol monooxygenase is an enzyme that catalyses the oxidation of phenols and is widespread in plants and animals...

s.

Enzyme nomenclature
Nomenclature
Nomenclature is a term that applies to either a list of names or terms, or to the system of principles, procedures and terms related to naming - which is the assigning of a word or phrase to a particular object or property...

 differentiates between monophenol oxidase enzymes (tyrosinase
Tyrosinase
Tyrosinase also known as monophenol monooxygenase is an enzyme that catalyses the oxidation of phenols and is widespread in plants and animals...

s) and o-diphenol:oxygen oxidoreductase enzymes (catechol oxidase
Catechol oxidase
Catechol oxidase is an enzyme that catalyses the oxidation of phenols such as catechol. Catechol oxidase is a copper-containing enzyme whose activity is similar to that of tyrosinase, a related class of copper oxidases....

s). Therefore please refer to the tyrosinase
Tyrosinase
Tyrosinase also known as monophenol monooxygenase is an enzyme that catalyses the oxidation of phenols and is widespread in plants and animals...

 and catechol oxidase
Catechol oxidase
Catechol oxidase is an enzyme that catalyses the oxidation of phenols such as catechol. Catechol oxidase is a copper-containing enzyme whose activity is similar to that of tyrosinase, a related class of copper oxidases....

 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
Desiccation tolerance
Desiccation tolerance refers to the ability of an organism to withstand or endure extreme dryness, or drought-like conditions. Plants and animals living in arid or periodically arid environments such as temporary streams or ponds may face the challenge of desiccation, therefore physiological or...

.

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
Tentoxin
Tentoxin is a natural cyclic tetrapeptide produced by phytopathogenic fungus Alternaria alternata. It selectively induces chlorosis in several germinating seedling plants. Therefore, tentoxin may be used as a potential natural herbicide....

 has also been used in recent research to eliminate the polyphenol oxidase activity from seedlings of higher plants. Tropolone
Tropolone
Tropolone is a derivative of tropone with a hydroxyl group in the 2-position.Two methods for the synthesis of tropolone are by bromination of 1,2-cycloheptanedione with N-bromosuccinimide followed by dehydrohalogenation at elevated temperatures and by acyloin condensation of the ethyl ester of...

 is a grape polyphenol oxidase inhibitor
Enzyme inhibitor
An enzyme inhibitor is a molecule that binds to enzymes and decreases their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides...

. Another inhibitor of this enzyme is potassium pyrosulphite (K2S2O5).
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