Tyrosinase mechanism

Sanchez-Ferrer A, Rodriguez-Lopez JN, Garcia-Canovas F and Garcia-Carmona F. 1995. Tyrosinase a comprehensive review of its mechanism. Biochim Biophys Acta 1247(1) 1—11. 

Adults require 1-2 mg of copper per day, and eliminate excess copper in bile and feces. Most plasma copper is present in ceruloplasmin. In Wilson s disease, the diminished availability of ceruloplasmin interferes with the function of enzymes that rely on ceruloplasmin as a copper donor (e.g. cytochrome oxidase, tyrosinase and superoxide dismutase). In addition, loss of copper-binding capacity in the serum leads to copper deposition in liver, brain and other organs, resulting in tissue damage. The mechanisms of toxicity are not fully understood, but may involve the formation of hydroxyl radicals via the Fenton reaction, which, in turn initiates a cascade of cellular cytotoxic events, including mitochondrial dysfunction, lipid peroxidation, disruption of calcium ion homeostasis, and cell death. 

This discussion of copper-containing enzymes has focused on structure and function information for Type I blue copper proteins azurin and plastocyanin, Type III hemocyanin, and Type II superoxide dismutase s structure and mechanism of activity. Information on spectral properties for some metalloproteins and their model compounds has been included in Tables 5.2, 5.3, and 5.7. One model system for Type I copper proteins39 and one for Type II centers40 have been discussed. Many others can be found in the literature. A more complete discussion, including mechanistic detail, about hemocyanin and tyrosinase model systems has been included. Models for the blue copper oxidases laccase and ascorbate oxidases have not been discussed. Students are referred to the references listed in the reference section for discussion of some other model systems. Many more are to be found in literature searches.50... 

In the present paper we describe the catalytic mechanisms of synthetic polymer-Cu complexes a catalytic interaction between the metal ions which attached to a polymer chain at high concentration and an environmental effect of polymer surrounding Cu ions. In the latter half, the catalytic behavior is compared with the specific one of tyrosinase enzyme in the melanin-formation reaction which is a multi-step reaction. To the following polymers Cu ions are combined. 

Hydroquinone interferes with the production of the pigment melanin by epidermal melanocytes through at least two mechanisms it competitively inhibits tyrosinase, one of the principal enzymes responsible for converting tyrosine to melanin, and it selectively damages melanocytes and melanosomes (the organelles within which melanin is stored). 

Mecfianism of Action The mechanism of action is not fully understood. Monobenzone maybe converted to hydroquinone, which inhibits the enzymatic oxidation of tyrosine to DOPA it may have a direct action on tyrosinase, or it may act as an antioxidant to prevent SH-group oxidation so that more SH groups are available to inhibit tyrosinase. Therapeutic Effect Depigmentation in extensive vitiligo. 

Stack and co-workers recently reported a related jx-rf / -peroxodi-copper(II) complex 28 with a bulky bidentate amine ligand capable of hydroxylating phenolates at - 80 °C. At - 120 °C, a bis(yu,-oxo)dicopper(III) phenolate complex 29 with a fully cleaved 0-0 bond was spectroscopically detected (Scheme 13) [190]. These observations imply an alternative mechanism for the catalytic hydroxylation of phenols, as carried out by the tyrosinase metalloenzyme, in which 0-0 bond scission precedes C - 0 bond formation. Hence, the hydroxylation of 2,4-di-tert-butylphenolate would proceed via an electrophilic aromatic substitution reaction. 

One of the questions surrounding the mechanism of tyrosinase concerns the initial site of attack. As a control, LFMD simulations of a model for the sTy active site, Meim6 (Fig. 28), give identical behavior for each Cu center consistent with its symmetry. In contrast, the LFMD simulations clearly distinguish the two copper sites in the sTy enzyme which must result from the protein environment (Fig. 29). 

The mechanism of action of these compounds appears to involve inhibition of the enzyme tyrosinase, thus interfering with the biosynthesis of melanin. In addition, monobenzone may be toxic to melanocytes, resulting in permanent loss of these cells. Some percutaneous absorption of these compounds takes place, because monobenzone may cause hypopigmentation at sites distant from the area of application. Both hydroquinone and monobenzone may cause local irritation. Allergic sensitization to these compounds can occur. Prescription combinations of hydroquinone, fluocinolone... 

With tyrosinase, on the contrary, a two-electron oxidation occurs, as no EPR signal was detected in the catechol oxidation at pH 5.3 Melanins are polymerization products of tyrosine, whereby tyrosinase catalyses the first steps the formation of dopa (3,4-dihydroxyphenylalanine) and of dopaquinone, leading to an indolequi-none polymer The peroxidase mechanism for the conversion of tyrosine into dopa in melanogenesis was not substantiated In natural and synthetic melanins free radicals of a semiquinone type were detected by EPR 4-10 x 10 spins g of a hydrated suspension (the material was modified on drying and the number of free spins increased). The fairly symmetrical EPR signal had a g-value of 2.004 and a line-width of 4-10 G The melanins seem to be natural radical scavengers. 

Figure 92 (a) Structural mechanism for the hydroxylation of monophenolic substrates by oxytyrosinase (b) reaction coordinate diagram for associative ligand substitution at the copper site of tyrosinase... 

The mechanism of iris pigmentation due to latanoprost is unknown. In an in vitro experiment using uveal melanocytes, the addition of latanoprost increased melanin content, melanin production, and tyrosinase activity (18). Alpha-methyl-para-tyrosine, an inhibitor of tyrosinase (the enzyme that transforms tyrosine to levodopa), completely prevented the latanoprost-induced stimulation of melanogenesis. 

Comparison of reaction mechanisms and products of transformation of catechol by biotic (tyrosinase) and abiotic (birnessite) catalysts... 

Figure 2.21. Mechanisms of the oxidative polymerization of catechol to melanins (humic polymers) in the presence of tyrosinase or birnessite. Reprinted with permission from Naidja, A., Huang, P. M., Dec, J., and Bollag, J.-M. (1999). Kinetics of catechol oxidation catalyzed by tyrosinase or 8-Mn02. In Effect of Mineral-Organic-Microorganism Interactions on Soil and Freshwater Environments, Berthelin, J., Huang, P. M., Bollag, J.-M., and Andreux, F., eds., Kluwer Academic/Plenum Publishers, New York, 181-188.

Mimicking and understanding tyrosinase activity (o-hydroxylation of phenols) have been of longtime interest because this was one of the earliest copper monooxygenases described and the significance of elucidating dioxygen activation mechanism(s) has widespread implications and potential applications. 

Many marine species also possess the tyrosinase-mediated pathway to synthesize the UV-absorbing pigment melanin. Melanin occurs in a wide range of taxa including bacteria, fungi, invertebrates, and chordates. While much is known about the role of melanin in the UV protection of mammalian skin, very little research has been conducted to examine the efficiency of melanin as a UV-protective mechanism in aquatic taxa.9 It is known that melanin levels in juvenile hammerhead sharks, Sphyrna lewini, are directly correlated to solar UV exposure in the freshwater crustacean Daphnia pulex, melanin concentrations are genetically determined within populations and are correlated to UV sensitivity.50-51 The few studies that have been undertaken suggest that melanin has an important role in UV protection in aquatic environments. 

It is considered that A. oryzae tyrosinase forms the inactive tetrameric conformation to regulate the activity. A. oryzae produces many kinds of secondary metabolites, organic acid, kojic acid, and citrate, etc. [163], When a cell is injured, protyrosinase is activated by extracellular acidic environment to produce melanin, which defends the interior of the cell against the acidic environment. It is considered that the acid activation is an instantaneous response to an acid stress. The acid activation is a quite unique mechanism, in which the... 

The interest in catechol oxidase, as well as in other copper proteins with the type 3 active site, is to a large extent due to their ability to process dioxygen from air at ambient conditions. While hemocyanin is an oxygen carrier in the hemolymph of some arthropods and mollusks, catechol oxidase and tyrosinase utilize it to perform the selective oxidation of organic substrates, for example, phenols and catechols. Therefore, establishment of structure-activity relationships for these enzymes and a complete elucidation of the mechanisms of enzymatic conversions through the development of synthetic models are expected to contribute greatly to the design of oxidation catalysts for potential industrial applications. 

Melanin granules are secreted by melanocytes in the hair papilla and distributed to keratin in the hair cortex and inner layers of the hair sheath during normal development. Melanogenesis is subject to hormonal control and has been the focus of intensive genetic studies. Two main forms of melanin exist in human skin—eumelanin and phaeomelanin, both of which are derived from tyrosine through the action of tyrosinase (a cupro-enzyme) and possibly other key enzymes (with nickel, chromium, iron, and manganese as cofactors). Tyrosine is converted to dihydroxyphenylalanine and, via a series of intermediate steps, to indole-5,6-quinone, which polymerizes to eumelanin. Phaeomelanins are produced by a similar mechanism but with the incorporation of sulfur (as cysteine) by a nonenzymatic step in the oxidation process. 

Selinheimo, E., Autio, K., Kruus, K., Buchert, J. 2007. Elucidating the mechanism of laccase and tyrosinase in wheat bread making. JAgric Food Chem 55 6357-6365. 

Tyrosinase. The oxidation of catechol with 02, catalyzed by tyrosinase, was concluded by Mason (I) in 1961 not to involve any radical species therefore, an ionic mechanism was proposed by Hamilton (2). A possible activation mechanism seems to involve the interaction between Cu(I)-protein and 02 to give an active Cu(II)—O—OH species in which Cu(II) and OH act as electron-deficient centers, withdrawing electrons from the substrate (Figure 1). 

Activation mechanism for tyrosinase-catalyzed oxidation of catechol with 02... 

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