TYR gene

Tyrosinase is an oxidase that is the rate-limiting enzyme for controlling the production of melanin. The enzyme is mainly involved in two distinct reactions of melanin synthesis otherwise known as the Raper Mason pathway. 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. It is found inside melanosomes which are synthesized in the skin melanocytes. In humans, the tyrosinase enzyme is encoded by the ''TYR'' gene.

Catalyzed reaction

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

The substrate specificity becomes dramatically restricted in mammalian tyrosinase which uses only L-form of tyrosine or DOPA as substrates, and has restricted requirement for L-DOPA as cofactor.

Active site

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.

Structure

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 histidine residues.

Plant

''In vivo'', plant PPOs are expressed as about 64–68 k Da proteins consisting of three domains: a chloroplastic transit peptide (containing a ~4-9 k Da thylakoid signal peptide), a catalytically active domain (~ 37–42 kDa) containing the dinuclear copper center, and a C-terminal domain (~15–19 kDa) shielding the active site.

Mammalian

Mammalian 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.

As opposed to fungal tyrosinase, human tyrosinase is a membrane-bound glycoprotein and has 13% carbohydrate content.

The derived TYR allele (rs2733832) is associated with lighter skin pigmentation in human populations. It is most common in Europe, but is also found at lower, moderate frequencies in Central Asia, the Middle East, North Africa, and among the San and Mbuti Pygmies.

Bacterial

In peatlands, bacterial tyrosinases are proposed to act as key regulators of carbon storage by removing phenolic compounds, which inhibit the degradation of organic carbon.

Gene regulation

The gene for tyrosinase is regulated by the microphthalmia-associated transcription factor (MITF).

Clinical significance

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 20,000 people.

Tyrosinase activity is very important. If uncontrolled during the synthesis of melanin, it results in increased melanin synthesis. Decreasing tyrosinase activity has been targeted for the improvement or prevention of conditions related to the hyperpigmentation of the skin, such as melasma and age spots.

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.

Inhibitors

Known Tyrosinase inhibitors are the following:

* Azelaic acid
* Hydroquinone
* ''L''-ascorbic acid - Vitamin C
* Tranexamic acid

Genetics

While albinism is common, there have only been a few studies about the genetic mutations in the tyrosinase genes of animals. One of them was on ''Bubalus bubalis'' (water buffalo). The tyrosinase mRNA sequence of the wild-type ''B. bubalis'' is 1,958 base pairs (bp) with an open reading frame (ORF) of 1,593 bp long, which translates to 530 amino acids. Meanwhile, the tyrosinase gene of the albino ''B. bubalis'' (GenBank JN_887463) is truncated at position 477, caused by a point mutation in nucleotide 1431 which converts a Tryptophan (TGG) into a stop codon (TGA), resulting in a shorter and inactive tyrosinase gene. Other albinos have point mutations that appear to inactivate Tyrosinase without truncation (see table and figure for examples).

Knowing that there are a few studies about the genomic data of the tyrosinase gene, there are only a handful of studies on the mutations in albino amphibians. Miura et al. (2018) investigates the amino acid mutations in the tyrosinase gene in three albino frogs: ''Pelophylax nigromaculatus'' (pond frog), ''Glandirana rugosa'' (wrinkled frog) and ''Fejervarya kawamurai'' (rice frog). In total, five different populations were studied of which three were ''P. nigromaculatus'' and one each of ''G. rugosa'' and ''F. kawamurai''. In two of the three ''P. nigromaculatus'' populations, there was a frameshift mutation because of the insertion of a thymine within exons 1 and 3, and the third population lacked three nucleotides that encoded a Lysine in exon 1. The population of ''G. rugosa'' had a missense mutation where there was an amino acid substitution from a Glycine to Aspartic acid, and the mutation of ''F. kawamurai'' was also an amino acid substitution from Glycine to Arginine. The mutation for ''G. rugosa'' and ''F. kawamurai'' occurs in exons 1 and 3. The mutations of the third population of ''P. nigromaculatus'', and the mutations of ''G. rugosa'' and ''F. kawamurai'' occurred in areas that are highly conserved among vertebrates which could result in a dysfunctional tyrosinase gene.

Evolution

Tyrosinase is a highly conserved protein in animals and apparently arose already in bacteria.The tyrosinase related protein(Tyrp1) and dopachrome tautomerase (Dtc), which encode for protein implicated in melanin synthesis which are the common regulatory elements of exon/intron structure. The development of the three types of vertebrate pigment cells, although different, thus converge at a certain point to allow the expression of members of the ''tyrosinase'' family, in order to produce melanin pigments. Tyrosinase family related genes plays an important role in the evolution, genetics, and developmental biology of pigment cells, as well as to approach human disorders associated with defects in their synthesis, regulation or function in vertebrates three types of melanin producing pigment cells are well known since embryonic origin i.e., from the neural crest, neural tube and pineal body. All of them have the capacity to produce melanin pigments. Their biosynthesis is governed by evolutionary conserved enzymes of the tyrosinase family( tyr, tyr1 and tyr2) also called DOPAchrome tautomerase ( dct). Among them Tyr plays significance role in melanin production. However, sequenced genome from the different taxa for evolutionary analysis in the depth become more crucial in present study. Similarly, the type-3 copper protein family are preform various biological function including pigment formation, innate