Tyrosinase is an oxidase that is the rate-limitingenzyme for controlling the production of melanin. It is mainly involved in two distinct reactions of melanin synthesis; firstly, the hydroxylation of a monophenol and secondly, the conversion of an o-diphenol to the corresponding o-quinone. o-Quinone undergoes several reactions to eventually form melanin. Tyrosinase is a copper-containing enzyme present in plant and animal tissues that catalyzes the production of melanin and other pigments from tyrosine by oxidation, as in the blackening of a peeled or sliced potato exposed to air. It is found inside melanosomes which are synthesised in the skin melanocytes. In humans, the tyrosinase enzyme is encoded by the TYRgene.
A mutation in the tyrosinase gene resulting in impaired tyrosinase production leads to type I oculocutaneous albinism, a hereditary disorder that affects one in every 17,000 people.
Tyrosinase activity is very important. If uncontrolled during melanoma, it results in increased melanin synthesis.
Several polyphenols, including flavonoids or stilbenoid, substrate analogues, free radical scavengers, and copper chelators, have been known to inhibit tyrosinase. Henceforth, the medical and cosmetic industries are focusing research on tyrosinase inhibitors to treat skin disorders.
Tyrosinases have been isolated and studied from a wide variety of plant, animal, and fungal species. Tyrosinases from different species are diverse in terms of their structural properties, tissue distribution, and cellular location. No common tyrosinase protein structure occurring across all species has been found. The enzymes found in plant, animal, and fungal tissue frequently differ with respect to their primary structure, size, glycosylation pattern, and activation characteristics. However, all tyrosinases have in common a binuclear, type 3 copper centre within their active sites. Here, two copper atoms are each coordinated with three histidineresidues.
Human tyrosinase is a single membrane-spanning transmembrane protein. In humans, tyrosinase is sorted into melanosomes and the catalytically active domain of the protein resides within melanosomes. Only a small, enzymatically inessential part of the protein extends into the cytoplasm of the melanocyte.
Crystallographic structure of a Streptomyces-derived tyrosinase in complex with a so-called "caddie protein" In all models, only the tyrosinase molecule is shown, copper atoms are shown in green and the molecular surface is shown in red. In models D and E, histidine amino acids are shown as a blue line representation. From model E, each copper atom within the active site is indeed complexed with three histidine residues, forming a type 3 copper center. From models C and D, the active site for this protein can be seen to sit within a pillus formed on the molecular surface of the molecule.
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