Polyphenol oxidase

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Polyphenol oxidase
EC number1.14.18.1
CAS number9002-10-2
IntEnzIntEnz view
ExPASyNiceZyme view
MetaCycmetabolic pathway
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO

Polyphenol oxidase (PPO; also 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 at the 1, 2 positions, with no carbon between). 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[edit]

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 functions.[5]

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.[6]

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

Distribution and applications[edit]

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[7] 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.

Plants make use of polyphenol oxidase as one in a suite of chemical defences against parasites.[8]


Tyrosine is a key compound of pigmentation/browning reaction through the action of polyphenol oxidase. Discrimination of this pigmentation reaction is responsible for various types of diseases and disorders.[9] 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.[10] Tropolone is a grape polyphenol oxidase inhibitor.[11] Another inhibitor of this enzyme is potassium pyrosulphite (K2S2O5).[12] Banana root PPO is strongly inhibited by dithiothreitol and sodium metabisulfite.[13] Endogenous proteinous fractions act endogenous inhibitor/regulator of onion leaves PPO (monophenol monooxygenase and o-diphenol oxidase)[9] Potassium dithionite (or potassium hydrosulfite) is also an inhibitor of PPO.


Several assays were developed to monitor the activity of polyphenol oxidases and to evaluate the inhibition potency of polyphenol oxidase inhibitors. In particular, ultraviolet/visible (UV/Vis) spectrophotometry-based assays are widely applied.[14] The most common UV/Vis spectrophotometry assay involves the monitoring of the formation of o-quinones, which are the products of polyphenol oxidase-catalysed reactions, or the consumption of the substrate.[15] Alternative spectrophotometric method that involves the coupling of o-quinones with nucleophilic reagents such as 3-methyl-2-benzothiazolinonehydrazone hydrochloride (MBTH) was also used.[16] Other techniques, such as activity staining assays with the use of polyacrylamide gel electrophoresis,[17] tritium-based radioactive assays,[18] oxygen consumption assay,[19] and nuclear magnetic resonance (NMR)-based assay were also reported and used.[20]

Enzymatic browning[edit]

Polyphenol oxidase (PPO) is an enzyme found throughout the plant and animal kingdoms,[21] including most fruits and vegetables.[22] PPO has importance to the food industry because it catalyzes enzymatic browning when tissue is damaged from bruising, compression or indentations, making the produce less marketable and causing economic loss.[21][22][23] Enzymatic browning due to PPO can also lead to loss of nutritional content in fruits and vegetables, further lowering their value.[21][22]

Because the substrates of these PPO reactions are located in the vacuoles of plant cells damaged mainly by improper harvesting, PPO initiates the chain of browning reactions.[23][24] Exposure to oxygen when sliced or pureed also leads to enzymatic browning by PPO in fruits and vegetables.[22] Examples in which the browning reaction catalyzed by PPO may be desirable include prunes, sultana grapes, black tea, and green coffee beans.[22]

In mangoes[edit]

In mangoes, PPO catalyzed enzymatic browning is mainly caused by sap burn which leads to skin browning.[citation needed] Catechol oxidase-type PPO is located in the chloroplasts of mango skin cells and its phenolic substrates in the vacuoles. Sap burn is therefore the initiating event of PPO in mango skin, as it breaks down cell compartments.[24] PPO is located in mango skin, sap and pulp, with highest activity levels in skin.[22]

In apples[edit]

Present in the chloroplasts and mitochondria of all parts of an apple,[22] PPO is the major enzyme responsible for enzymatic browning of apples.[25] Due to an increase in consumer demand for pre-prepared fruits and vegetables, a solution for enzymatic browning has been a targeted area of research and new product development.[26] As an example, pre-sliced apples are an appealing consumer product, but slicing apples induces PPO activity, leading to browning of the cut surfaces and lowering their esthetic quality.[26] Browning also occurs in apple juices and purees when poorly handled or processed.[27]

Arctic® apples, an example of genetically modified fruit engineered to reduce PPO activity, are a suite of trademarked apples that contain a non-browning trait derived by gene silencing to suppress the expression of PPO, thus inhibiting fruit browning.[28]

In potatoes[edit]

Found in high concentrations in potato tuber peel and 1-2 mm of the outer cortex tissue, PPO is used in the potato as a defense against insect predation, leading to enzymatic browning from tissue damage.[citation needed] Damage in the skin tissue of potato tuber causes a disruption of cell compartmentation, resulting in browning. The brown or black pigments are produced from the reaction of PPO quinone products with amino acid groups in the tuber.[23] In potatoes, PPO genes are not only expressed in potato tubers, but also in leaves, petioles, flowers and roots.[23]

Related enzymes[edit]

Prophenoloxidase is a modified form of the complement response found in some invertebrates, including insects, crabs and worms.[29]

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.[30]

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

See also[edit]


  1. ^ "Polyphenol Oxidase". Worthington Enzyme Manual. Retrieved 13 September 2011.
  2. ^ Mayer AM (November 2006). "Polyphenol oxidases in plants and fungi: going places? A review". Phytochemistry. 67 (21): 2318–31. doi:10.1016/j.phytochem.2006.08.006. PMID 16973188.
  3. ^ Jolley RL, Mason HS (March 1965). "The Multiple Forms of Mushroom Tyrosinase. Interconversion". The Journal of Biological Chemistry. 240: PC1489–91. PMID 14284774.
  4. ^ Jolley RL, Robb DA, Mason HS (March 1969). "The multiple forms of mushroom tyrosinase. Association-dissociation phenomena". The Journal of Biological Chemistry. 244 (6): 1593–9. PMID 4975157.
  5. ^ Mallette MF, Dawson CR (August 1949). "On the nature of highly purified mushroom tyrosinase preparations". Archives of Biochemistry. 23 (1): 29–44. PMID 18135760.
  6. ^ Function of polyphenol oxidase in higher plants. Kevin C. Vaughn and Stephen O. Duke, Physiologia Plantarum, January 1984, Volume 60, Issue 1, pages 106–112, doi:10.1111/j.1399-3054.1984.tb04258.x
  7. ^ 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.
  8. ^ Thaler JS, Karban R, Ullman DE, Boege K, Bostock RM (April 2002). "Cross-talk between jasmonate and salicylate plant defense pathways: effects on several plant parasites". Oecologia. 131 (2): 227–235. doi:10.1007/s00442-002-0885-9. PMID 28547690.
  9. ^ a b Anita Goswami-Giri, Neha Sawant. "Separation of polyphenol oxidase and its inhibitor from onion leaves on Lignin Column".
  10. ^ 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.
  11. ^ 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
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  18. ^ Pomerantz SH (June 1964). "Tyrosine hydroxylation catalyzed by mammalian tyrosinase: an improved method of assay". Biochemical and Biophysical Research Communications. 16 (2): 188–94. doi:10.1016/0006-291X(64)90359-6. PMID 5871805.
  19. ^ Naish-Byfield S, Riley PA (November 1992). "Oxidation of monohydric phenol substrates by tyrosinase. An oximetric study". The Biochemical Journal. 288 ( Pt 1) (Pt 1): 63–7. doi:10.1042/bj2880063. PMC 1132080. PMID 1445282.
  20. ^ Li Y, Zafar A, Kilmartin PA, Reynisson J, Leung IK (November 2017). "Development and Application of an NMR-Based Assay for Polyphenol Oxidases". ChemistrySelect. 2 (32): 10435–41. doi:10.1002/slct.201702144.
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  24. ^ a b Robinson SP, Loveys BR, Chacko EK (1993). "Polyphenol Oxidase Enzymes in the Sap and Skin of Mango Fruit". Functional Plant Biology. 20 (1): 99–107. doi:10.1071/pp9930099. ISSN 1445-4416.
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  28. ^ "Novel Food Information - Arctic Apple Events GD743 and GS784". Novel Foods Section, Food Directorate, Health Products and Food Branch, Health Canada, Ottawa. 20 March 2015. Retrieved 5 November 2016.
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  30. ^ Decker H, Tuczek F (August 2000). "Tyrosinase/catecholoxidase activity of hemocyanins: structural basis and molecular mechanism". Trends in Biochemical Sciences. 25 (8): 392–7. doi:10.1016/S0968-0004(00)01602-9. PMID 10916160.