Tyrosinases

Tyrosinases are copper-containing enzymes that catalyze the oxidation of phenols and are central to melanin biosynthesis in many organisms as well as to the browning of fruits and vegetables. They exhibit two related enzymatic activities: monophenolase (cresolase), which oxidizes monophenols such as tyrosine to o-diphenols, and diphenolase (catechol oxidase), which further oxidizes o-diphenols to o-quinones. The catalytic mechanism depends on a binuclear copper center in the active site, with two copper ions (CuA and CuB) coordinated by histidine residues. Oxygen is consumed and water is produced during the reaction.

Distribution and roles vary by organism. In plants and fungi, tyrosinases and related polyphenol oxidases contribute to defense, pigmentation, and wound responses. In animals, especially mammals, tyrosinase is the key enzyme in melanin production within melanocytes and melanosomes, influencing skin, hair, and eye color. The TYR gene encodes the enzyme in many vertebrates, often with tissue-specific regulation and post-translational modifications that affect activity and stability. Related proteins (TYRP1, TYRP2) participate in the melanin pathway but have distinct catalytic roles.

Regulation and inhibition are agriculturally and medically relevant. In fruits and vegetables, tyrosinase drives browning after injury. In humans, abnormal tyrosinase activity relates to pigmentary disorders. Inhibitors such as kojic acid and hydroquinone are used in cosmetics and food industries to slow browning. Assays typically monitor formation of quinones or consumption of substrates, reflecting monophenolase and diphenolase activities.