|PDB structures||RCSB PDB PDBe PDBsum|
Catechol oxidase is an enzyme that catalyses the oxidation of phenols such as catechol. Catechol oxidase is a copper-containing enzyme whose activity is similar to that of tyrosinase, a related class of copper oxidases. They are ubiquitous plant enzymes that catalyze oxidation of a broad range of ortho-diphenols to the corresponding o-quinones coupled with the reduction of oxygen to water. It is different from tyrosinase, Ec 18.104.22.168, which can catalyze both the monooxygenation of monophenols and the oxidation of catechols. 
Catechol oxidase carries out the oxidation of phenols such as catechol, using dioxygen (O2). In the presence of catechol, benzoquinone is formed (see reaction below). Hydrogens removed from catechol combine with oxygen to form water. This reaction, producing the yellow compound benzoquinone, is a form of enzymatic browning exhibited in many foods upon exposure to oxygen (e.g., in bananas). The benzoquinone is then oxidised by the air to form dark-brown melanin.
Catechol is present in small quantities in the vacuoles of cells of many plant tissues. Catechol oxidase is present in the cell cytoplasm. If the plant tissues are damaged, the catechol is released and the enzyme converts the catechol to ortho-quinone, which is a natural antiseptic. Catechol oxidase, therefore, has a role in plant defence mechanisms, helping to protect damaged plants against both bacterial and fungal disease.
It has been suggested that the quantity of catechol oxidase produced by a plant may be related to its susceptibility to fungal infection. Benzoquinone inhibits the growth of microorganisms and prevents damaged fruit from rotting. 
Catechol oxidase activity is also of economic importance. It has been estimated that half the world’s fruit and vegetable crop is lost due to post-harvest browning reactions due to the enzyme.
Catechol oxidase has been studied from sweet potatoes and assigned into a biological assembly. The catalytic copper center is accommodated in a central four-helix-bundle located in a hydrophobic pocket close to the surface. Both metal binding sites are composed of three histidine ligands. His 109, ligated to the CuA site, is covalently linked to Cys 92 by an unusual thioether bond.  An additional ligand component is Cu2O and contains a Cu-O-Cu linkage. 
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- "EC 22.214.171.124 Catechol oxidase.". The European Bioinformatics Institute.
- Wahlert, John. H. "Enzymes - Proteins That Act as Catalysts". Baruch College.
- Kiabunde, T; Eicken, C., Sacchettini, J.C., Krebs, B. (1998). "CATECHOL OXIDASE FROM IPOMOEA BATATAS (SWEET POTATOES) IN THE NATIVE CU(II)-CU(II) STATE". Nature Structural & Molecular Biology 5. doi:10.1038/4193.
- Virador, V.M.; Reyes Grajeda, J.P., Blanco-Labra, A., Mendiola-Olaya, E., Smith, G.M., Moreno, A., Whitaker, J.R. (2010). "Cloning, sequencing, purification, and crystal structure of Grenache (Vitis vinifera) polyphenol oxidase.". Journal of Agricultural and Food Chemistry 58: 1189–1201. doi:10.1021/jf902939q.