Polyphenol oxidase

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Polyphenol oxidase
EC number
CAS number 9002-10-2
IntEnz IntEnz view
ExPASy NiceZyme view
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / EGO

Polyphenol oxidase (PPO or monophenol monooxygenase) 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 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.[2]

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[3] 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 and cocoa[4] 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.[5] Tropolone is a grape polyphenol oxidase inhibitor.[6] Another inhibitor of this enzyme is potassium pyrosulphite (K2S2O5).[7]

Prophenoloxidase is a modified form of the complement response found in some invertebrates, including insects, crabs and worms.[8]

See also[edit]


  1. ^ http://www.worthington-biochem.com/TY/default.html, Polyphenol Oxidase - Worthington Enzyme Manual. Accessed 13 September 2011
  2. ^ Mayer, AM (November 2006). "Polyphenol oxidases in plants and fungi: Going places? A review". Phytochemistry 67 (21): 2318–2331. doi:10.1016/j.phytochem.2006.08.006. PMID 16973188. 
  3. ^ Sugumaran M, Lipke H (May 1983). "Quinone methide formation from 4-alkylcatechols: a novel reaction catalyzed by cuticular polyphenol oxidase". FEBS Letters 155 (1): 65–68. doi:10.1016/0014-5793(83)80210-5. 
  4. ^ QUESNEL V.C., JUGMOHUNSINGH K. (May 2006). "BROWNING REACTION IN DRYING CACAO". Journal of the Science of Food and Agriculture 21 (10): 537–541. doi:10.1002/jsfa.2740211011. 
  5. ^ Duke SO, Vaughn KC (April 1982). "Lack of involvement of polyphenol oxidase in ortho-hydroxylation of phenolic compounds in mung bean seedlings". Physiologia Plantarum 54 (4): 381–385. doi:10.1111/j.1399-3054.1982.tb00696.x. 
  6. ^ Time-dependent inhibition of grape polyphenol oxidase by tropolone. Edelmira Valero, Manuela Garcia-Moreno, Ramon Varon and Francisco Garcia-Carmona, J. Agric. Food Chem., 1991, 39 (6), pp 1043–1046, doi:10.1021/jf00006a007
  7. ^ Del Signore A, Romeoa F, Giaccio M (May 1997). "Content of phenolic substances in basidiomycetes". Mycological Research 101 (5): 552–556. doi:10.1017/S0953756296003206. 
  8. ^ Immunity and the Invertebrates Beck, Gregory and Habicht, Gail S, Scientific American, November 1996, pages 60-66