Polyphenol oxidase

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), apples (Malus domestica), avocados (Persea americana), and lettuce (Lactuca sativa). 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 functions. 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. Enzyme nomenclature differentiates between monophenol oxidase enzymes (tyrosinases) and o-diphenol:oxygen oxidoreductase enzymes (catechol oxidases). The substrate preference of tyrosinases and catechol oxidases is controlled by the amino acids around the two copper ions in the active site. A mixture of monophenol oxidase and catechol oxidase enzymes is present in nearly all plant tissues, and can also be found in bacteria, animals, and fungi. In insects, cuticular polyphenol oxidases are present and their products are responsible for desiccation tolerance.