Tyrosinase

Tyrosinase (also called monophenol monooxygenases, EC 1.14.18.1) belongs to an oxidase family that has the catalytic center consisting of dinuclear type-3 copper. It is widely present in plant and animal tissues to catalyze the production of melanin and other pigments from tyrosine by oxidation, causing browning that occurs upon bruising or long-term storage. The catalytic reaction by tyrosinase is the rate-limiting for controlling the production of melanin from tyrosine, which mainly involves a cascade of two distinct reactions. The orthohydroxylation of monophenol and the subsequent oxidation of the diphenolic result in quinone, which is a reactive precursor and undergoes several reactions to eventually form melanin pigments. Tyrosinase is found inside melanosomes that are synthesized in the skin melanocytes. In humans, the tyrosinase enzyme is encoded by the TYR gene, which is regulated by the microphthalmia-associated transcription factor (MITF). Tyrosinase is involved in neurodegenerative disorders, such as Parkinson's disease, and also leads to melanin-browning reactions that important to the cosmetics and food industries.

Structure

Tyrosinases isolated from a wide variety of plant, animal, and fungal species, are diverse in terms of their primary structure, size, glycosylation pattern, activation characteristics, cellular location and tissue distribution. There is no common tyrosinase protein structure occurring across all species. However, all tyrosinases have in common a binuclear, type 3 copper center within their active sites, where the two copper atoms are each coordinated with three histidine residues.

Human tyrosinase is a single membrane-spanning transmembrane protein and is sorted into melanosomes. In contrast to fungal tyrosinase, human tyrosinase is a membrane-bound glycoprotein with 13% carbohydrate content. The catalytically active domain of human tyrosinase resides within melanosomes. Only a small, enzymatically inessential part of the protein extends into the cytoplasm of the melanocyte.

Catalyzed Reaction

Tyrosinase implements oxidation of phenols such as tyrosine and dopamine by using dioxygen (O₂). In the presence of catechol, benzoquinone is formed and hydrogen from catechol is combined with oxygen to produce water. The substrate specificity of mammalian tyrosinase is dramatically restricted, which uses only L-form of tyrosine or dihydroxyphenylalanine (DOPA) as substrates, and requires L-DOPA to be as cofactor. During the catalytic reaction, the type-3 copper center of tyrosinase is present in three redox forms. The deoxy form (Cu(I)–Cu(I)) is a reduced species that binds with oxygen to give the oxy form (Cu(II)–O22−–Cu(II)). In the oxy form, molecular oxygen is bound as peroxide in a μ-η²:η² side-on bridging mode, thus destabilizing the O–O bond and activating it. The met form (Cu(II)–Cu(II)) is considered as a resting enzymatic form, where Cu(II) ions are generally linked with a small ligand, such as a hydroxide ion or water molecule.

Figure 1. Reaction catalyzed by tyrosinase. (Lai X L; et al. 2018)

Significance

a. Clinical significance

Tyrosinase activity is critical for melanin synthesis. If uncontrolled, it results in an enhancement in melanin synthesis. In mammals, tyrosinase contributes to skin pigmentation abnormalities, such as flecks and defects. It has become a target to inhibit tyrosinase activity for the better treatment or prevention of conditions associated with the hyperpigmentation of the skin, such as melasma and age spots. Several polyphenols, including flavonoids, stilbenoid, free radical scavengers, substrate analogues, and copper chelators, have been identified to inhibit tyrosinase. In the medical and cosmetic industries, the development and screening of potent inhibitors of tyrosinase have become especially attractive to treat skin disorders. Moreover, a mutation in the tyrosinase gene can impair tyrosinase production and lead to type I oculocutaneous albinism, a hereditary disorder affecting one in every 20,000 people.

b. Significance in food industry

Tyrosinase is also of great significance in the fields of agriculture and food industry, where tyrosinase inhibition is desired since tyrosinase accelerates the conversion of phenolic compounds found in fruits and vegetables into quinones. This not only gives an undesirable taste and color, but also reduces the digestibility of the products and the availability of certain essential amino acids. Herein, tyrosinase inhibitors with high efficiency are urgenly needed in agriculture and the food industry. Some compounds like kojic acid, tropolone, coumarins, vanillic acid, vanillin, and vanillic alcohol are well known tyrosinase inhibitors.

c. Significance in insects

In insects, tyrosinase is acknowledged to be a beneficial enzyme with functions of sclerotization, wound healing, melanin synthesis and parasite encapsulation. Consequently, it is an important enzyme due to the defensive mechanism of insects. Some insecticides are developed by aiming at inhibiting tyrosinase.

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