Polyphenol oxidase (tyrosinase) is a bifunctional, copper-containing oxidase having catecholase and cresolase activity. It is responsible for browning reactions through the phylogenetic scale. The enzyme has a molecular weight of 128 kDa. It is a tetramer containing four atoms of copper per molecule and two binding sites for aromatic compounds including phenolic substrates. There is a distinct binding site for oxygen. The optimum pH range is 6-7.