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(詳細はCatechol oxidaseを参照) 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.〔(【引用サイトリンク】title=Polyphenol Oxidase )〕 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. Common plant food examples producing the enzyme are mushrooms (''Agaricus bisporus''), apples (''Malus domestica'') and lettuce (''Lactuca sativa''). ==Structure and function== 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, creating multiple (protein moonlighting functions. Substrate binding/turnover impacts multimerization, Different assemblies have different activities, Kinetic hysteresis〔). 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.〔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, 〕 Enzyme nomenclature differentiates between monophenol oxidase enzymes (tyrosinases) and ''o''-diphenol:oxygen oxidoreductase enzymes (catechol oxidases). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「polyphenol oxidase」の詳細全文を読む スポンサード リンク
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