catecholase

Catecholase, often referred to as catechol oxidase, is an enzyme that catalyzes the oxidation of o-diphenols such as catechol to o-quinones. The net reaction can be written as catechol + 1/2 O2 → o-benzoquinone + H2O. This activity is a subset of the larger polyphenol oxidase (PPO) family, which also includes tyrosinases and related enzymes.

Catalysis by catecholase relies on a copper-containing active site, typically a dicopper center, with histidine ligands coordinating the copper ions. In the catalytic cycle, substrate electrons are transferred to oxygen that is bound at the active site, reducing O2 to water while the substrate is oxidized to the corresponding quinone. The mechanism involves changes in the oxidation state of the copper ions and is sensitive to factors such as pH, temperature, and the presence of inhibitors or reducing agents.

Catecholase is widespread in plants and is also found in some fungi and bacteria. In plants, these enzymes are commonly located in plastids and become active when tissues are damaged, releasing substrates from vacuoles. The produced quinones rapidly polymerize to brown, melanin-like pigments, a reaction collectively responsible for enzymatic browning. This browning can deter herbivory and help limit pathogen spread, but it also affects the appearance and quality of harvested fruits and vegetables.

In research and industry, catecholase activity serves as a diagnostic measure of PPO activity and as a model for studying copper-containing oxidoreductases. Practical considerations include strategies to inhibit browning in foods (e.g., reducing agents, chelators, acidity) and the use of PPO assays in enzyme kinetics.