Amino acid tyrosinase

Gandia-Herrero, R, Escribano, J., and Garcfa-Carmona, R, Characterization of the activity of tyrosinase on betaxanthins derived from (i )-amino acids, J. Agric. Food Chem., 53, 9207, 2005. 

Melanin biosynthesis in animals is a complex process starting with the L-tyrosine amino acid. In the first step, L-tyrosine is converted first into DOPA and then into dopaquinone, a process catalyzed by tyrosinase. In the biosynthesis of eumelanins, dopaquinone undergoes a cyclization to form dopachrome and subsequently a tau-tomerization into 5,6-dihydroxyindole-2-carboxylic acid (DHICA). DHICA is further oxidized to indole-5,6-quinone2-carboxylic acid, the precnrsor of DHICA eumelanins. Tyrosinase-related proteins TRP-2 and TRP-1, respectively, are responsible for the last two steps, and they are under the control of the tyrosinase promoter. 

The pathway of melanin synthesis starts from the amino acid tyrosine (Fig. 1). The first two reactions are catalyzed by the copper-containing enzyme tyrosinase (EC 1.14.18.1). Tyrosine is hydroxylated to 3,4-dihy-... 

In a study of intermediate duration, dermal application of 0.5% p-cresol for 6 weeks produced permanent depigmentation of the skin and hair of mice (Shelley 1974). A caustic effect on the skin was noted in one strain of mouse, but not another. Neither o- nor m-cresol produced any color change in the mice. The author suggests that only p-cresol is active because it mimics the structure of tyrosine, the amino acid present in melanin, so that tyrosinase acts on it, liberating free radicals that damage melanocytes. NOAEL and LOAEL values were not derived from this study because the applied dose was not reported. 

In fluorine-18 chemistry some enzymatic transformations of compounds already labelled with fluorine-18 have been reported the synthesis of 6-[ F] fluoro-L-DOPA from 4-[ F]catechol by jS-tyrosinase [241], the separation of racemic mixtures of p F]fluoroaromatic amino acids by L-amino acylase [242] and the preparation of the coenzyme uridine diphospho-2-deoxy-2-p F]fluoro-a-o-glucose from [ F]FDG-1-phosphate by UDP-glucose pyrophosphorylase [243]. In living nature compounds exhibiting a carbon-fluorine bond are very rare. 

Use the techniques outlined in the experimental procedure to explore two enzymes you will study in later experiments. Study the two enzymes malate dehydrogenase (Experiment 10) and tyrosinase (Experiment 5). View structures and look at amino acid sequences as you did for human a-lactalbumin. 

Oxidative coupling polymerization provides great utility for the synthesis of high-performance polymers. Oxidative polymerization is also observed in vivo as important biosynthetic processes that, when catalyzed by metalloenzymes, proceed smoothly under an air atmosphere at room temperature. For example, lignin, which composes 30% of wood tissue, is produced by the oxidative polymerization of coniferyl alcohol catalyzed by laccase, an enzyme containing a copper complex as a reactive center. Tyrosine is an a-amino acid and is oxidatively polymerized by tyrosinase (Cu enzyme) to melanin, the black pigment in animals. These reactions proceed efficiently at room temperature in the presence of 02 by means of catalysis by metalloenzymes. Oxidative polymerization is observed in vivo as an important biosynthetic process that proceeds efficiently by oxidases. 

Tyrosine is one of the important a-amino acids and is oxidatively polymerized with tyrosinase to melanin, the black pigment in animals [25-30], Melanin plays a role in the prevention of damage to the organism that occurs through the absorption of ultraviolet light. Melanin is the only major paramagnetic organic com-... 

The NH2-terminal amino acid sequence of the activated wild-type tyrosinase precipitated with 5% (w/v) trichloroacetic acid was SDKIHLTDD (single-letter amino acid codes), which is known to be the NH2-terminal sequence of thioredoxin including the 10 residues from the Ser2 to Aspll. Although the initiating methinine residue was encoded in the construct, it seems to have disappeared after cleavage of the protein. 

Lerch, K. (1982). Primary structure of tyrosinase from Neurospora crassa. II. Complete amino acid sequence and chemical structure of a tripeptide containing an unusual thioether. J. Biol. Chem., 257, 6414-6419. 

Some of the spectral effects of specific covalent modifications of proteins, i.e., iodination, oxidation, tyrosinase action, etc., will be discussed briefly. Heme proteins, flavoproteins, and other conjugated proteins will not be discussed except as regards studies involving their amino acid components. 

Tyrosine phenol lyase (p-tyrosinase) has been shown to catalyze the efficient synthesis of the L-amino acids L-tyrosine and L-dopa from pyruvate, ammonia and phenol, or catechol, respectively (87-89). 

H. Vamada ai d H. Kumagai, Synthesis of L-iyrosine and related amino acids by p-tyrosinase, Adv. Appl. Microbiol-, 19 249 (1975). 

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. 

The conversion of tyrosine to 3,4-dihydroxyphenylalanine occurs both in vivo in man (590) and in vitro by the action of tissue tyrosinase (205, 688). Mammals can decarboxylate both tyrosine (402,407) and dihydroxyphenyl-alanine (406), tyrosine decarboxylase and dihydroxyphenylalanine (dopa) decarboxylases being quite distinct and separable (405), though both are dependent on pyridoxal phosphate (73, 758, and review 72). In mammals dihydroxyphenylalanine is the most readily decarboxylated of all amino acids, and it is therefore not unreasonable to assume that this is the substrate normally decarboxylated in adrenaline biosynthesis cf. 74, 75). Support for this concept derives from the fact that both the substrate and the product of the reaction (3,4-dihydroxyphenylethylamine diagram 11) can or do occur in the adrenal (298, 299, 802), and the amine is moreover, like adrenaline and noradrenaline, a normal urinary excretion product (245, 404). 

Pyridoxal phosphate is the coenzyme in a large number of amino acid reactions. At this point it is convenient to consider together 1,he mechanism of those pyridoxal-dependent reactions concerned with aromatic amino acids. The reactions concerned are (1) keto acid formation (e.g., from kynurenine, above), 2) decarboxylation (e.g., of 5-hydroxytrypto-phan to 5-hydroxytryptamine, p. 106), (3) scission of the side claain (e.g., 3-tyrosinase, p. 78 tryptophanase, p. 110 and kynureninase, above), and 4) synthesis (e.g., of tryptophan from indole and serine, p. 40). Many workers have considered the mechanism of one or more of these reactions (e.g., 24, 216, 361, 595), but a unified theory is primarily due to Snell and his colleagues (summarized in 593). Snell s experiments have been carried out largely in vitro, and it should be emphasized that in vivo it is the enzyme protein which probably directs the electromeric changes. 

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