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 / QuickGO

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.[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). 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.

Arctic apples are a suite of trademarked apples that contain a non-browning trait derived by gene silencing to suppress the expression of PPO, thus preventing the fruit from browning.[8]


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.[9] Tropolone is a grape polyphenol oxidase inhibitor.[10] Another inhibitor of this enzyme is potassium pyrosulphite (K2S2O5).[11] Banana root PPO is strongly inhibited by dithiothreitol and sodium metabisulfite.[12]

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.[13] 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.[14] Alternative spectrophotometric method that involves the coupling of o-quinones with nucleophilic reagents such as 3-methyl-2-benzothiazolinonehydrazone hydrochloride (MBTH) was also used.[15] Other techniques, such as activity staining assays with the use of polyacrylamide gel electrophoresis,[16] tritium-based radioactive assays,[17] oxygen consumption assay,[18] and nuclear magnetic resonance (NMR)-based assay were also reported and used.[19]

Related enzymes[edit]

Prophenoloxidase is a modified form of the complement response found in some invertebrates, including insects, crabs and worms.[20]

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.[21]

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): 2318–2331. doi:10.1016/j.phytochem.2006.08.006. PMID 16973188. 
  3. ^ Jolley Jr, RL; Mason, HS (1965). "The Multiple Forms of Mushroom Tyrosinase. Interconversion". The Journal of Biological Chemistry. 240: PC1489–91. PMID 14284774. 
  4. ^ Jolley Jr, RL; Robb, DA; Mason, HS (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 (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. ^ "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. 
  9. ^ 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. 
  10. ^ 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
  11. ^ 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. 
  12. ^ Wuyts, N; De Waele, D; Swennen, R (2006). "Extraction and partial characterization of polyphenol oxidase from banana (Musa acuminata Grande naine) roots". Plant Physiol Biochem. 44 (5-6): 308–314. doi:10.1016/j.plaphy.2006.06.005. PMID 16814556. 
  13. ^ García-Molina F, Muñoz JL, Varón R, Rodríguez-López JN, García-Cánovas F, Tudela J (November 2007). "A review on spectrophotometric methods for measuring the monophenolase and diphenolase activities of tyrosinase". J. Agric. Food Chem. 55 (24): 9739–49. doi:10.1021/jf0712301. PMID 17958393. 
  14. ^ Haghbeen K, Wue Tan E (January 2003). "Direct spectrophotometric assay of monooxygenase and oxidase activities of mushroom tyrosinase in the presence of synthetic and natural substrates". Anal. Biochem. 312 (1): 23–32. doi:10.1016/S0003-2697(02)00408-6. PMID 12479831. 
  15. ^ Espín JC, Morales M, Varón R, Tudela J, García-Cánovas F (October 1995). "A continuous spectrophotometric method for determining the monophenolase and diphenolase activities of apple polyphenol oxidase". Anal. Biochem. 231 (1): 237–46. doi:10.1006/abio.1995.1526. PMID 8678307. 
  16. ^ Rescigno A, Sollai F, Rinaldi AC, Soddu G, Sanjust E (March 1997). "Polyphenol oxidase activity staining in polyacrylamide electrophoresis gels". J. Biochem. Biophys. Methods. 34 (2): 155–9. doi:10.1016/S0165-022X(96)01201-8. PMID 9178091. 
  17. ^ Pomerantz SH (June 1964). "Tyrosine hydroxylation catalyzed by mammalian tyrosinase: an improved method of assay". Biochem. Biophys. Res. Commun. 16 (2): 188–94. doi:10.1016/0006-291X(64)90359-6. PMID 5871805. 
  18. ^ Naish-Byfield S, Riley PA (November 1992). "Oxidation of monohydric phenol substrates by tyrosinase. An oximetric study". Biochem. J. 288 (Pt 1): 63–7. doi:10.1042/bj2880063. PMID 1445282. 
  19. ^ 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. 
  20. ^ Immunity and the Invertebrates Beck, Gregory and Habicht, Gail S, Scientific American, November 1996, pages 60-66
  21. ^ Decker H, Tuczek F (August 2000). "Tyrosinase/catecholoxidase activity of hemocyanins: structural basis and molecular mechanism". Trends Biochem. Sci. 25 (8): 392–7. doi:10.1016/S0968-0004(00)01602-9. PMID 10916160.