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.
Catalyzed reaction
Tyrosinase carries out the oxidation of phenols such as
tyrosine and
dopamine using dioxygen (O
2). 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
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, and activation characteristics. However, all tyrosinases have in common a
copper proteins within their
. Here, two copper atoms are each coordinated with three
histidine residues.
[File:Protein,
CAJ82935,
BAA02077,
BAV78831 and AAC17168), Snakes (Their genebank accession numbers
BBC55580,
XP032076040 and
BBC55647) and Human (Genebank accession number
AAA61242) using Clustal Omega. (Note: (*) shows a conserved region,(.) shows more conserved and (:) shows less conserved.)
]]
Plant
In vivo, plant PPOs are expressed as about 64–68 kDa
consisting of three domains: a chloroplastic transit
peptide (containing a ~4-9 kDa
thylakoid signal peptide), a catalytically active domain (~ 37–42 kDa) containing the
dinuclear copper center, and a
C-terminus 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 people and Mbuti people.
Bacterial
In peatlands,
tyrosinases are proposed to act as key regulators of carbon storage by removing
compounds, which inhibit the
Biodegradation of
Organic compound 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 (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 melanogenesis, 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 Liver spot.
Several polyphenols, including or stilbenoid, substrate analogues, free radical scavengers, and copper chelators, have been known to inhibit tyrosinase.
Inhibitors
Known Tyrosinase inhibitors are the following:
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
Water buffalo (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
Albinism 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).
| +
!Species
!Common name
!Amino Acid mutation
!GenBank
!Uniprot ID |
| Water buffalo | 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 . Miura et al. (2018) investigates the amino acid mutations in the tyrosinase gene in three albino : 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 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 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
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 perform various biological function including pigment formation,
innate immunity and oxygen transport. The combine genetic phylogenetic and structural analysis concluded that the original type-3
copper protein possessed a single
peptide and grouped into
Alpha subclass. The ancestral protein gene underwent to two duplication i.e., first one prior to divergence of unknown
Eukaryote lineage and second one before diversification. The prior duplication gave rise to cytosolic form(
Beta) and latter duplication gave
membrane bound form (
Gamma). The structural comparison concluded that active site of α and γ forms are covered by aliphatic amino acids and β form covered with aromatic residue. Thus, the evolution of these gene family is the lineage of multicellular
due to loss of one or more of these three subclasses and lineage-specific expansion of one or both of the remaining
Subclassis.
The
Genomics conserved nucleotide alignments of the tyrosinase among the
vertebrate family like frogs, snakes and human suggests that it has evolved from one ancestral tyrosinase gene. The
Gene duplication and
mutation of this gene is probably responsible for the emergence of a tyrosinase-related
gene.
Applications
In the food industry
In the food industry, tyrosinase inhibition is desired as tyrosinase catalyzes the oxidation of phenolic compounds found in fruits and vegetables into
, which gives an undesirable taste and color and also decreases the availability of certain essential amino acids as well as the digestibility of the products. As such, highly effective tyrosinase inhibitors are also needed in agriculture and the food industry.
Well known tyrosinase inhibitors include
kojic acid,
,
,
,
vanillin, and
vanillic alcohol.
In the cosmetic industry
Lighter skin complexion has been associated with youth and beauty across various Asian cultures. Recent research by cosmetic companies has been focused on the development of novel whitening agents that selectively suppress tyrosinase activity to reduce hyperpigmentation while avoiding cytotoxicity of healthy
.
[Qian, W., Liu, W., Zhu, D., Cao, Y., Tang, A., Gong, G., Su, H."Natural skin‑whitening compounds for the treatment of melanogenesis (Review)". Experimental and Therapeutic Medicine 20.1 (2020): 173-185.] Traditional pharmacological agents such as
,
hydroquinone, and amino numeric chloride lighten skin through the inhibition of melanocyte maturation.
[Lajis AFB and Ariff AB: Discovery of new depigmenting compounds and their efficacy to treat hyperpigmentation: Evidence from in vitro study. J Cosmet Dermatol. 18:703–727. 2019.] However, these agents are associated with adverse effects. Cosmetic companies have been focused on developing novel whitening agents that selectively suppress the activity of tyrosinase to reduce hyperpigmentation while avoiding melanocyte cytotoxicity as tyrosinase is the rate-limiting step of the melanogenesis pathway.
In insects
Tyrosinase has a wide range of functions in insects, including wound healing,
sclerotization, melanin synthesis and parasite encapsulation. As a result, it is an important enzyme as it is the defensive mechanism of insects. Some insecticides are aimed to inhibit tyrosinase.
[
]
In mussel-glue inspired polymers
Tyrosinase activated polymerization of , containing cysteine and tyrosine residues, lead to mussel-glue inspired . The tyrosine residues are oxidized to L-Dopaquinone, to which of cysteine could link by an intermolecular Michael addition. The resulting adsorb strongly to various surfaces with high adhesion energies.
External links
