Polyphenol oxidase

Polyphenol oxidase
Identifiers
EC number	1.14.18.1
CAS number	9002-10-2
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / EGO
Search PMC articles PubMed articles

Polyphenol oxidase (PPO or monophenol monooxygenase) is a tetramer which 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, coffee and cocoa 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.^[4] Tropolone is a grape polyphenol oxidase inhibitor.^[5] Another inhibitor of this enzyme is potassium pyrosulphite (K₂S₂O₅).^[6]

See also

• catechol oxidase

References

1. ^ http://www.worthington-biochem.com/TY/default.html, Polyphenol Oxidase - Worthington Enzyme Manual. Accessed 13th 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. ^ 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. 
5. ^ 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
6. ^ 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.