Polyphenol oxidase (PPO)

Polyphenol oxidase (PPO) is a copper-containing enzyme widely distributed in plants, fungi, and animals that catalyzes the oxidation of phenolic compounds, especially the o-hydroxylation of monophenols to o-diphenols and subsequent oxidation to quinones, leading to the formation of brown pigments (melanins) through polymerization[1][5][6]. In plants, PPO is primarily localized in plastids (chloroplasts) and participates in defense mechanisms against herbivores and pathogens by generating reactive quinones and forming physical barriers through melanin deposition[2][6]. PPO is responsible for enzymatic browning reactions observed in damaged fruits, vegetables, and other agricultural products, representing both a physiological defense process and a significant problem in the food industry due to economic loss and reduced produce quality[4][5]. The enzyme exists as multiple isoforms with diverse distribution and substrate specificity; structurally, it contains a conserved type-III copper center critical for catalytic activity, typically requiring activation under stress or proteolytic cleavage events[1][3][6]. Inhibitors of PPO are used industrially to control unwanted browning, but the enzyme lacks approved human therapeutic targeting, and its primary impact relates to food processing, plant biology, and industrial enzyme technology.

Other names

Catechol oxidaseTyrosinaseo-diphenol:oxygen oxidoreductaseEC 1.10.3.1 (catechol oxidase)EC 1.14.18.1 (tyrosinase)o-diphenol oxidase

Substrate oxidation: Catalyzes the oxidation of monophenols to o-diphenols (tyrosinase activity) and o-diphenols to o-quinones (catechol oxidase activity) using molecular oxygen, involving a type-III copper center[1][3][5] Resulting quinones polymerize to form melanins or brown pigments, especially upon tissue damage or exposure to air[5] Inhibitors act by chelating copper or blocking substrate access