monophenolase

Monophenolase is an enzymatic activity commonly attributed to polyphenol oxidases such as tyrosinase. It refers to the hydroxylation of monophenols to o-diphenols (the cresolase activity), a key early step in the biosynthesis of melanins and related pigments. In many organisms, this activity coexists with diphenolase activity, which oxidizes o-diphenols to o-quinones.

The enzyme typically contains a type 3 copper active site with two copper centers (CuA and CuB) coordinated by histidine residues. Monophenolase catalysis involves the initial hydroxylation of monophenol substrates, inserting an additional hydroxyl group to form an o-diphenol, followed by electron transfer steps that enable subsequent oxidation of the diphenol by molecular oxygen. Although the two activities are often described separately (monophenolase and diphenolase), they are usually part of the same catalytic protein, and their relative rates can vary with substrate and conditions.

Common substrates include phenolic compounds such as tyrosine (converted to L-DOPA) and other monophenols; the resulting o-diphenols can be oxidized further to o-quinones, which polymerize to form brown pigments. In plants, monophenolase activity contributes to wound-induced browning and pigment formation; in animals, it is a component of melanin biosynthesis.

Applications and significance include the control of enzymatic browning in fruits and vegetables, prevention of unwanted pigment formation in food, and use in biochemical assays of tyrosinase activity. Inhibitors or modulators of monophenolase are studied to manage browning and related processes, while deficiencies in tyrosinase can lead to oculocutaneous albinism type 1 in humans.