Polyphenol oxidase is an enzyme found in the chloroplast of plants. It catalyzes the oxidation of phenolic compounds (like tyrosine) to make diphenols and quinones, which polymerize to form compounds that result in brown color development.[1] These reactions are also involved in the formation of melanin, the brown pigment in skin. Since tyrosine is a substrate, the enzyme is sometimes referred to as Tyrosinase. This page will focus on the plant polyphenol oxidases. Under normal physiological conditions, PPO is located in the chloroplast, the phenolic substrates are located in the cytosol, and oxygen concentrations are relatively low, so the rate of reaction is small. However, when the fruit is cut or bruised, the compartmentalization is lost, leading to reaction progress. These browning reactions have significant agricultural economic impact, as consumers will avoid products that have browned, either due to bruising or cutting.
Structural highlightsThe secondary structure of PPO is primarily alpha helical, a few small regions of beta strands. The quaternary structure is classified as a morpheein, as it switches between monomeric, dimeric, tetrameric, octameric, and dodecameric forms. The displayed form is a dimer . Polyphenol oxidase uses copper to catalyze the addition of oxygen to phenyl rings. The copper is held by histidine residues. The overall reaction involves introduction of one atom of oxygen from O2 onto the monophenol and reduction of the other oxygen to water[2]. Indeed, in thesingle oxygen can be seen bound to the two copper atoms in the protein structure.
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This page was last modified 14:06, 11 May 2016.