Tyrosinase
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. In the presence of catechol, benzoquinone is formed. 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.
File:Protein nucleotides.png|thumb|400x400px|This is an alignment structure showing only the conserved region of protein nucleotide sequences of Frogs, Snakes and Human using Clustal Omega. shows a conserved region, shows more conserved and
Plant
In vivo, plant PPOs are expressed as about 64–68 kDa proteins consisting of three domains: a chloroplastic transit peptide, a catalytically active domain containing the dinuclear copper center, and a C-terminal domain 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 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.Fungal
In the fungus Neurospora crassa, four different forms of tyrosinase were distinguished among different strains. In each strain only one structure-determining genetic region was found for the enzyme.Gene regulation
The gene for tyrosinase is regulated by the microphthalmia-associated transcription factor.File:Mason raper pathway.png|left|thumb|The Raper–Mason pathway, depicting the steps in melanin synthesis'.' DHI- 5,6 dihydroxyindole, DHICA - 5,6- dihydroxyphenylalanine, GGT- Gamma-glutamyl transpeptidase, GST- Glutathione-S-transferase; L-Dopa - Levo- Dopa, TRP-2- Tyrosinase- related protein 2
File:Tyrosinase phylogeny.png|thumb|400x400px|A representative phylogenetic tree cladogram of tyrosinase proteins. Tyrosinase sequences from ten vertebrates species were analyzed. The multiple alignments are generated by the CLUSTAL W programand the phylogenetic trees were constructed by the Neighbour-joining method without distance correction. So Ambystoma and Xenopus'' do not cluster with other amphibians. Branches and nodes are drawn according to identical patterns.
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
- 4-Butylresorcinol
- Hydroquinone
- L-ascorbic acid - Vitamin C
- Tranexamic acid
Genetics
| Species | Common name | Amino Acid mutation | GenBank | Uniprot ID |
| Bubalus bubalis | Water Buffalo | W477 -> Stop codon | JN_887462 | J7FBF2 |
| Pelophylax nigromaculatus | Pond Frog | Deletion of a K228 | Q04604 | |
| Glandirana rugosa | Wrinkled Frog | G376 -> D376 | A0A1I9FZH0 | |
| Fejervarya kawamurai | Rice Frog | G57 -> R57 | A0A1E1G7U0 |
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. investigates the amino acid mutations in the tyrosinase gene in three albino frogs: Pelophylax nigromaculatus, Glandirana rugosa and Fejervarya kawamurai. 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.
File:Substituted amino acids of tyrosinase in albino frogs.png|thumb|Substituted amino acids of tyrosinase in albino frogs and corresponding amino acids in other vertebrate species. kW refers to the rice frog kawamurai Wild type, kA: kawamurai Albino type, rW: rugosa Wild, rA: rugosa Albino, nW nigromaculatus Wild, nAH: nigromaculatus Albino collected from Hiroshima. Numbers outside of the parenthesis refer to the amino acid position of the mutated species, and the number in the parenthesis refers to the associated amino acid position in the human sequence.
File:Tyrosinase Human Exon-Intron Representation.png|left|thumb|400x400px|This is a schematic representation of the intron-exon organization of tyrosinase gene in humans. Open and closed boxes represent protein-coding and untranslated regions of exons, respectively, with exons labeled by numbers. Intron sizes are indicated by small numbers.