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'''Polyphenol oxidase''' ('''PPO'''; also '''polyphenol oxidase i, chloroplastic'''), an [[enzyme]] involved in [[Browning (food process)|fruit browning]], is a [[tetramer]] that contains four atoms of copper per molecule.<ref>{{cite web|url=http://www.worthington-biochem.com/TY/default.html|title=Polyphenol Oxidase|publisher=Worthington Enzyme Manual|access-date=13 September 2011}}</ref>
PPO may accept [[monophenol | monophenols]]s and/or [[Pyrocatechol|''o''-diphenols]] as substrates.<ref name=pmid32441137>{{cite journal | authors = McLarin, Mark-Anthony; Leung, Ivanhoe K. H. | title = Substrate Specificity of Polyphenol Oxidase. | journal = [[Critical_Reviews_in_Biochemistry_and_Molecular_BiologyCritical |Reviews in Biochemistry and Molecular Biology|Crit. Rev. Biochem. Mol. Biol.]] | volume = 55| pages = 274–308| date = 2020 | issue = 3 | doi = 10.1080/10409238.2020.1768209 | pmid = 32441137 | s2cid = 218831573 }}</ref> The enzyme works by catalyzing the ''o''-[[hydroxylation]] of [[monophenol]] molecules in which the [[benzene ring]] contains a single [[hydroxyl]] substituent to [[Pyrocatechol|''o''-diphenols]] ([[phenol]] molecules containing two [[hydroxyl]] substituents at the 1, 2 positions, with no carbon between).<ref name=pmid7873577>{{cite journal | authors = A Sánchez-Ferrer, J N Rodríguez-López, F García-Cánovas, F García-Carmona | title = Tyrosinase: A Comprehensive Review of Its Mechanism. | journal = [[Biochimica_et_Biophysica_ActaBiochimica |et Biophysica Acta|Biochim. Biophys. Acta]] | volume =1247 | pages =1–11 | date =1995 | issue = 1 | doi = 10.1016/0167-4838(94)00204-t | pmid = 7873577 }}</ref> It can also further catalyse the [[oxidation]] of ''o''-diphenols to produce [[1,2-Benzoquinone|''o''-quinones]].<ref name=pmid10607672>{{cite journal | authors = C Eicken, B Krebs, J C Sacchettini | title = Catechol Oxidase - Structure and Activity. | journal = [[Current Opinion in Structural Biology]] | volume =9 | pages =677–683 | date =1999 | issue = 6 | doi = 10.1016/s0959-440x(99)00029-9 | pmid =10607672 | url = https://zenodo.org/record/896893 }}</ref> PPO catalyses the rapid [[polymerization]] of ''o''-quinones to produce black, brown or red pigments ([[polyphenol]]s) that cause [[Browning (food process)|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 [[tyrosinase]]s.<ref name=mayer>{{cite journal | vauthors = Mayer AM | title = Polyphenol oxidases in plants and fungi: going places? A review | journal = Phytochemistry | volume = 67 | issue = 21 | pages = 2318–31 | date = November 2006 | pmid = 16973188 | doi = 10.1016/j.phytochem.2006.08.006 }}</ref>
In plants, PPO is a [[plastid]]ic enzyme with unclear synthesis and function. In functional chloroplasts, it may be involved in oxygen chemistry like mediation of [[pseudocyclic photophosphorylation]].<ref>{{cite journal | vauthors = Vaughn KC, Duke SO | year = 1984 | title = Function of polyphenol oxidase in higher plants | journal = Physiologia Plantarum | volume = 60 | issue = 1| pages = 106–112 | doi = 10.1111/j.1399-3054.1984.tb04258.x }}</ref>
Enzyme [[nomenclature]] differentiates between monophenol oxidase enzymes ([[tyrosinase]]s) and ''o''-diphenol:oxygen oxidoreductase enzymes ([[catechol oxidase]]s). The [[Substrate (chemistry)|substrate]] preference of [[tyrosinase]]s and [[catechol oxidase]]s is controlled by the [[amino acid]]s around the two [[copper]] [[Ion|ionsion]]s in the [[active site]].<ref>{{cite journal | vauthors = Kampatsikas I, Rompel A | title = Similar but Still Different: Which Amino Acid Residues Are Responsible for Varying Activities in Type-III Copper Enzymes? | journal = ChemBioChem | pages = 1161–1175 | date = October 2020 | volume = 22 | issue = 7 | pmid = 33108057 | doi = 10.1002/cbic.202000647|issn=1439-4227 | pmc = 8049008 | doi-access = free }}</ref>
==Distribution and applications==
=== In black poplar ===
A [[Monomer|monomericmonomer]]ic [[catechol oxidase]] from ''[[Populus nigra]]'' converts [[caffeic acid]] to [[quinone]] and [[Melanin|melaninemelanin]]e at injured [[Cell (biology)|cells]].<ref>{{Cite journal|last1=Trémolières|first1=Michèle|last2=Bieth|first2=Joseph G.|date=1984|title=Isolation and characterization of the polyphenoloxidase from senescent leaves of black poplar|url=http://dx.doi.org/10.1016/s0031-9422(00)80367-2|journal=Phytochemistry|volume=23|issue=3|pages=501–505|doi=10.1016/s0031-9422(00)80367-2|issn=0031-9422}}</ref><ref>{{Cite journal|last1=Rompel|first1=Annette|last2=Fischer|first2=Helmut|last3=Meiwes|first3=Dirk|last4=Büldt-Karentzopoulos|first4=K.|last5=Dillinger|first5=Renée|last6=Tuczek|first6=Felix|last7=Witzel|first7=Herbert|last8=Krebs|first8=B.|date=1999|title=Purification and spectroscopic studies on catechol oxidases from Lycopus europaeus and Populus nigra: Evidence for a dinuclear copper center of type 3 and spectroscopic similarities to tyrosinase and hemocyanin|url=https://doi.org/10.1007/s007750050289|journal=JBIC Journal of Biological Inorganic Chemistry|language=en|volume=4|issue=1|pages=56–63|doi=10.1007/s007750050289|pmid=10499103|s2cid=29871864|issn=1432-1327}}</ref>
== Related enzymes ==
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