Phenolase
Polyphenol oxidase (PPO; also polyphenol oxidase i, chloroplastic), an enzyme involved in fruit browning, is a tetramer that contains four atoms of copper per molecule. PPO may accept monophenols and/or ''o''-diphenols as substrates. The enzyme works by catalyzing 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 at the 1, 2 positions, with no carbon between). It can also further catalyse the oxidation of ''o''-diphenols to produce ''o''-quinones. PPO catalyses 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 foods producing the enzyme include mushrooms (''Agaricus bisporus''), appl ...
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