Functional of tyrosinase

The field of phenoloxidases is absolutely unclear with respect to the structure. Mammalian tyrosinases have transmembrane parts and are glycosylated, in arthropods they are freely soluble. The aggregation is unclear as well. In mammals and prokaryotes, tyrosinases seem to be monomers, in arthropods they occur as hexamers and look like hemocyanins. The fact that not much knowledge is available on the structme and function of tyrosinases is surprising especially since tyrosinases are of major concern in medical and cosmetic research. 

The function of tyrosinase in producing pigments in skin is obvious. The function of the corresponding plant enzymes is less clear. It has been proposed that they function in terminal respiration, but this has not been... 

Genetic disruption of dopamine synthesis in mice lacking TH shows that dopamine is not essential for development. However, dopamine deficient mice do not survive long after weaning unless treated with l-DOPA. These mice display severe aphagia and adipsia and loss of motor function. While these mice have a major reduction in dopamine levels some residual dopamine can be detected that is generated through the action of tyrosinases. 

Some non-silica sol-gel materials have also been developed to immobilize bioactive molecules for the construction of biosensors and to synthesize new catalysts for the functional devices. Liu et al. [33] proved that alumina sol-gel was a suitable matrix to improve the immobilization of tyrosinase for detection of trace phenols. Titania is another kind of non-silica material easily obtained from the sol-gel process [34, 35], Luckarift et al. [36] introduced a new method for enzyme immobilization in a bio-mimetic silica support. In this biosilicification process precipitation was catalyzed by the R5 peptide, the repeat unit of the silaffin, which was identified from the diatom Cylindrotheca fusiformis. During the enzyme immobilization in biosilicification the reaction mixture consisted of silicic acid (hydrolyzed tetramethyl orthosilicate) and R5 peptide and enzyme. In the process of precipitation the reaction enzyme was entrapped and nm-sized biosilica-immobilized spheres were formed. Carturan et al. [11] developed a biosil method for the encapsulation of plant and animal cells. 

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. 

Our biomimetic investigations have focused on the metalloproteins hemocyanin (He) (11-17) and tyrosinase (11,12,14,16,18,29), which contain two copper ions in their active center. The function of hemocyanin is to bind and transport dioxygen in the hemolymph of molluscs and arthropods. Studies employing EXAFS spectroscopy have shown that in the deoxy form, two (19-21) or three (13,21) imidazole units fiom protein histidine residues coordinate to each cuprous ion. Upon addition of O2 to give oxy-Hc, considerable changes take place in the coordination sphere giving rise to tetragonally coordinated Cu(II) ions... 

Fukuzumi and Itoh have jointly reported on a if-peroxo dicop-per(ll) complex that acts as a functional model for the phenolase activity of tyrosinase. lithium salts of para-substituted phenols were used as substrates, reaching yields between 60 and 90% with only the catechol product formed... 

Figure 2.22. Initial velocity of oxygen consumption as a function of the substrate (catechol) concentration in the presence of 0.074mg (7.11 x 1(T9M) tyrosinase (A), 2.0mg (2.8 x 1(T4M with a corresponding concentration of the mineral active sites, [M5]1.71xl(r6) of 8-Mn02 (B) and 10.0 mg (1.40 x 10 3M with a corresponding concentration of the mineral active sites, [M ]8.54 x 10-6) of S-Mn02 (C). Reprinted from Naidja, A., Liu, C., and Huang, P. M. (2002). Formation of protein-birnessite complex XRD, FTIR, and AFM analysis. J. Coll. Interface Sci. 251,46-56, with permission from Elsevier.

Theos AC, Tenza D, Martina JA, Hurbain I, Peden AA, et al. 2005. Functions of adaptor protein (AP)-3 and AP-1 in tyrosinase sorting from endosomes to melanosomes. Mol Biol Cell 16 5356-5372. 

Evolution of two phenoloxidases, an arthropod and molluscan type. A close relationship between phenoloxidase and hemocyantn was deduced based on their similar sequences, physico-chemical properties and similar functions. But sequence comparisons also revealed that there is not a common phenoloxidase type the enzymes found in animals, plants, and fungi are different with respect to their sequences, size, glycosylation, and activation. Two different types of tyrosinases can be distinguished based on their sequences, structure, and function. One type (m-phenoloxidase) is more related to molluscan hemocyanin with respect to the active site. The other type (a-phenoloxidase), which is very similar to arthropod hemocyanins, is found in arthropods together with hemocyanins (Figure 9). ... 

Plots of reciprocals of tiag as a function of 1.-DOPA concentration at different concentrations of lu-tyrosinase (102)... 

Campanella, L. Favero, G. Sammartino, M. P. Tomassetti, M., Further development of catalase, tyrosinase and glucose oxidase based organic phase enzyme electrode response as a function of organic solvent properties, Talanta 1998, 46, 595-606... 

This similarity in spectral properties implies that haemocyanins should also have catalytic activity. From the available body of experimental data, it is clear that the distinction between the two major functions — oxygen transport and enzymatic activity — is determined by the presence or absence of a protein domain covering the active site. In the case of tyrosinase and catechol oxidase, inactive pro-enzyme forms are activated by removal of an amino acid which blocks the entrance channel to the active site (indicated by the black bar in Figure 14.7). Haemocyanins behave as silent inactive enzymes but can be activated in the same way if the blocking amino acid is removed. In arthropods, like crabs, this is located in the N-terminal domain of a subunit whereas in molluscs, like octopus, it is in the C-terminal domain of a functional unit. 

Tyrosine is converted to phenol by an enzyme, /3-tyrosinase, studied especially by Japanese workers (456, 654, 658, 879). The enzyme, which has been partially purified, is inhibited by carbonyl reagents and is dependent on pyridoxal phosphate. The reaction is mechanistically probably (593) very similar to the tryptophanase reaction and is discussed when considering the function of pyridoxal phosphate (p. 91). 

Tyrosinase inhibition may be a potential approach to prevent and control the enzymatic browning reactions and improve the quality and nutritional value of food products [20]. Tyrosinase also plays a major key role in the developmental and defensive functions of insects. Tyrosinase is involved in melanogenesis, wound healing, parasite encapsulation, and sclerotization in insects [21-23]. For these reasons, in recent years the development of tyrosinase inhibitors has become an active alternative approach to control insect pests [20]. Additionally, it is now well-recognized that tyrosinase inhibitors are important for their potential applications in medical and cosmetic products [24-26]. 

By this reaction sequence, highly functionalized bicyclo-[2.2.2]-octenes can be obtained from simple phenols and alkenes as starting materials. The overall yields reported are usually satisfactory (> 70%). The Diels-Alder products are racemic, probably because the Diels-Alder reaction proceeds in the bulk organic phase without involvement of tyrosinase. 

The active centers of tyrosinase and hemocyanins are very similar [254], although they fulfill very different functions. In both proteins, a binuclear center is coordinated by six histidines, leaving two coordination sites free to bind oxygen [237], The main difference in the active centers of the proteins is their position within the protein. In arthropod hemocyanin, the copper center is located in domain 2 of every subunit. Domain 3 folds over the copper center, rendering it inaccessible to substrates larger than oxygen [237]. In tyrosinase, a C-terminal peptide of 200 amino acid residues is removed by post-translational... 

Copper(I) complexes supported by certain capping ligands react with molecular oxygen in a 2 1 ratio to afford dinuclear copper dioxygen (CU2/O2) complexes. Such complexes can be regarded as structural and functional models of the reactive intermediates of tyrosinase and catechol oxidase. Numerous review articles on the subject have been published so far. ... 

We have also investigated the use of artificial cells containing tyrosinase to carry out some metabolic functions of the liver [31]. Results in animals show that tyrosinase artificial cells, retained in extracorporeal shunts perfused by blood, can effectively lower the systemic blood tyrosine levels of liver failure rats. Artificial cells, to carry out other metabolic functions of the liver, are also being studied. For instance, using artificial cells containing multienzyme systems with cofactor recycling [32], we have studied the in-vltro conversion of ammonia sequentially into different types of amino acids. This way, ammonia... 

Regarding the production of polymers (Kobayashi and Higashimura 2003) oxidases have been used to catalyze the chemoselective polymerization of phenolic monomers having a reactive functional group like methacryloyl and also the induced polymerization of syringic acid that cannot be polymerized by conventional metal catalysis. Other example is the use of tyrosinase to produce the coupling of phenoxy radicals from 2- and/or 6-unsubstituted phenols. For example, crystalline... 

FIGURE 11.5 Interrelation of the three functional states of tyrosinase. (From Lerch, K., Tyrosinase Molecular and active site structure, in Enzymatic Browning and Its Prevention, Lee, C.Y. and Whitaker, J.R., Eds., American Chemical Society, Washington, DC, 1995, 64. With permission.)... 

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