This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: biosynthesis of o-diquinones, fruit browning
|Superclasses:||Biosynthesis → Aromatic Compounds Biosynthesis|
Expected Taxonomic Range: Embryophyta
One of the reasons for fruit browning is the catalytic oxidation of phenolic substrates by polyphenoloxidase (PPO). Polyphenol oxidases catalyze two distinct reactions: the hydroxylation of monophenols to o-diphenols ( EC 188.8.131.52, tyrosinase), and the dehydrogenation of o-dihydroxyphenols to o-quinones ( EC 184.108.40.206, catechol oxidase). The products of the latter reaction are strong electrophiles that autoxidize, forming melanin, and are also capable of covalently modifying and crosslinking a variety of cellular nucleophiles. The formation of quinone adducts, usually brown or black, is the primary effect of PPO on fruit browning [Hunt93].
Some PPO endogenous substrates that have been identified in the pericarp tissues include flavan-3-ols and their dimers in lychee and longan, proanthocyanidins, (+)-catechin, and phlorizin in apple, dopamine in banana, and (-)-epicatechin and proanthocyanidin A2 in rambutan [Sun10].
This pathway shows the biosynthesis of a typical o-diquinone, 1,2-benzoquinone, by the action of catechol oxidases in potato tubers [Hunt93] and fruits of tomato plants [Newman93]. In tomato the PPOs belong to a gene family of seven members. They fall into three structural classes and all are located on chromosome 8. The PPOs encode copper-binding sites typical of bacterial and fungal PPOs [Newman93].
Understanding this pathway is crucial to finding ways of delaying or preventing browning, as pericarp browning of fruits has serious commercial implications for storage and transport
Newman93: Newman SM, Eannetta NT, Yu H, Prince JP, de Vicente MC, Tanksley SD, Steffens JC (1993). "Organisation of the tomato polyphenol oxidase gene family." Plant Mol Biol 21(6);1035-51. PMID: 8098228
Shahar92: Shahar T, Hennig N, Gutfinger T, Hareven D, Lifschitz E (1992). "The tomato 66.3-kD polyphenoloxidase gene: molecular identification and developmental expression." Plant Cell 4(2);135-47. PMID: 1633491
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