Tyrosinase

Tyrosinase (monophenol monooxygenase) (EC 1.14.18.1; CAS number: 9002-10-2) is an enzyme that catalyses the oxidation of phenols (such as tyrosine) and is widespread in plants and animals. 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.

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Pathology

When a person has a mutated tyrosinase gene they have albinism, a hereditary disease that one in every 17,000 person has in the United States.

An extremely high level of tyrosinase will induce melanoma^{[citation needed]}.

Chemical reactions

Tyrosinase carries out the oxidation of phenols such as tyrosine and catechol using dioxygen (O₂). In the presence of catechol, benzoquinone is formed (see reaction below). Hydrogens removed from catechol combine with oxygen to form water.

Tyrosinase structure

Tyrosinases have been isolated and studied from a wide variety of plant, animal and fungi species. Tyrosinases from different species are diverse in terms of their structural properties, tissue distribution and cellular location.^[1] It has been suggested that there is no common tyrosinase protein structure occurring across all species.^[2] The enzymes found in plant, animal and fungi 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 site. Here two copper atoms are each coordinated with three histidine residues.

Transmembrane protein and sorting

Human tyrosinase is a single membrane spanning transmembrane protein^[3]. In humans, tyrosinase is sorted into melanosomes^[4] and the catalytically active domain of the protein resides within melanosomes. Only a small enzymatically non-essential part of the protein extends into the cytoplasm.

Active site

The models below are formatted from the protein data bank file 1WX3. This pdb file contains the coordinates for the crystal structure of a Streptomyces derived tyrosinase in complex with a so called "caddie protein".^[5] 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 it can be clearly seen that each copper atom within the active site is indeed complexed with three histidine residues, forming a type 3 copper center. It can also be seen from models C and D that the active site for this protein sits within a pocket formed on the molecular surface of the molecule.

The two copper atoms within the active site of tyrosinase enzymes interact with dioxygen to form a highly reactive chemical intermediate that then oxidizes the substrate. The activity of tyrosinase is similar to catechol oxidase, a related class of copper oxidase. Tyrosinases and catechol oxidases are collectively termed polyphenol oxidases

References