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Polyphenol oxidase

This article only describes one highly specialized aspect of its associated subject.
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Polyphenol oxidase (PPO) enzymes catalyse 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. Consequently PPOs may also be referred to as tyrosinases.^[1]

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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.