Tyrosinase oxidation

SCHEME 68. Mushroom tyrosinase oxidation of 2,6-disubstituted phenols... 

The natural colour of animal fibre is closely related to the character of environment in which the animal lives [19]. Wool lots completely free of dark fibres do not exist [20]. In animal (and human) hair two kinds of pigments occur, namely eumelanin (responsible for black, dark brown and grey colours and commonly referred to as melanin) and pheomelanin (present in yellow, reddish-brown and red hair). Both are thought to be formed by different mechanisms and chemically differed [21]. Eumelanin is formed by enzymatic (tyrosinase) oxidation of tyrosine and polymerisation of several oxidation product [22]. Pheomelanin occurs in form of discrete grannules. Melanin grannules can occur in the cortex or in the cuticle. 

Similar redox equilibria are felt [68] to be present in melanins, the final polymeric products of tyrosinase oxidation of catechols and catecholamines. Whereas natural... 

Phaeomelanins are reddish-brown, nitrogen and sulfur-containing macro-molecular pigments, which are found in phaeomelanocytes. They are derived from tyrosinase oxidation of tyrosine and subsequent reaction with cysteine. Trichochromes are produced in a similar manner, but they are of low molecular weight and usually yellow, red, or violet. The commonly encountered trichochromes have the structures 13 and 14. 

Elastin, collagen formation Tyrosinase oxidation, skin pigment (melanin) formation Epinephrine biosynthesis... 

LC-MS studies indicate that tyrosinase oxidation of 4-fluorocatechol gives several products including 2,3-dihydroxy-6-fluorodioxin, formed via fluoride displacement from 4-fluoro-l,2-benzoquinone and cyclisa-tion of the resulting 4-(2-hydroxyaryloxy)-l,2-benzoquinone (08MI1). 

Hemocyanine and tyrosinase are of this type, Hemocyanine is the oxygen carrier in molluscs and arthropods. Two Cu ions are involved in a subunit and bind one O2. A deoxy structure of a lobster hemocyanine from Panulitrus interruptus has been reported [43] for which both the Cu are monovalent. Three His coordinate to each Cu, and two Cu and four His are located in the same plane (Fig. 2-23). The two copper ions are not equivalent and no bridging ligand exist between them. Tyrosinase oxidizes phenols to orthoquinones. O2 is coordinated between two Cu ions as a p-peroxo structure. 

The catalytic properties of laccase differ from tyrosinase in that laccase catalyses the oxidation of diphenols in discrete one-electron steps, whereas a single two-electron step occurs with tyrosinase (5,6). Unlike tyrosinase, laccase has no monophenol o-hydroxylating activity. Both o- and p-diphenols are oxidized by laccase, but tyrosinase oxidizes only the former compounds. Methylhydroquinone is a p-diphenol that is often used to... 

The sequence as it stands today reveals several interesting points. First, the molecule contains no cysteine residues consequently, S-S bonds cannot be formed. Yet optical rotary dispersion studies have indicated that the molecules of parathormone have some degree of tertiary structure that is not covalent in origin. Second, the molecule contains one tyrosine residue, one tryptophan residue, and two methionine residues. These amino acids are in some way involved in activity because treatment with tyrosinase, oxidation of the methionine residues, or the reaction of the tryptophan residues with 2-hydroxy, 5-nitrobenzyl bromide reduces or abolishes activity. 

There is another effective method for the determination of phenol derivatives that makes use of enzymes. The enzyme tyrosinase oxidizes phenol derivatives to benzoquinone derivatives by consuming dissolved oxygen in the solution. Benzoquinone derivatives can be easily reduced electrochemically in the moderately negative potential range. By using an electrode with immobilized tyrosinase, the phenol derivatives can be detected from the response due to the electrochemical reduction of benzoquinone derivatives. 

An interesting combination of enzymatic with non-enzymatic transformation in a one-pot three-step multiple sequence was reported by Waldmann and coworkers [82]. Phenols 125 in the presence of oxygen and enzyme tyrosinase are hydroxylated to catechols 126 which are then oxidized in situ to ortho quinones 127. These intermediates subsequently undergo a Diels-Alder reaction with inverse electron demand by reaction with different dienophiles (Table 4.19) to give endo bicyclic 1,2-diketones 128 and 129 in good yields. 

Table 4.19 Multiple hydroxylation-oxidation Diels-Alder reactions initiated by tyrosinase...

The oxidation of phenol in alcoholic media by a morpholine complex of Cu(II) (as a model for tyrosinase) to give 4,5-dimorpholino-orr/jo-benzoquinone in 64 %... 

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. 

Copper(II) complexes with phenoxo ligands have attracted great interest, in order to develop basic coordination chemistry for their possible use as models for tyrosinase activity (dimeric complexes) and fungal enzyme galactose oxidase (GO) (monomeric complexes). The latter enzyme catalyzes the two-electron oxidation of primary alcohols with dioxygen to yield aldehyde and... 

Encapsulated Cu—chlorophthalocyanines oxidize hexane at C-l using 02 and at C-2 using H202 as oxidants. The dimeric structure of copper acetate is intact when it is incorporated into the zeolite. This is a regioselective aromatic hydroxylation catalyst, which mimics the specificity of the monooxygenase enzyme tyrosinase.82,89 Zeolite NaY catalysts made with a tetranuclear Cu(II) complex were synthesized and characterized.90... 

In 1996, the first successful combination of an enzymatic with a nonenzymatic transformation within a domino process was reported by Waldmann and coworkers [6]. These authors described a reaction in which functionalized bicy-clo[2.2.2]octenediones were produced by a tyrosinase (from Agaricus bisporus) -catalyzed oxidation of para-substituted phenols, followed by a Diels-Alder reaction with an alkene or enol ether as dienophile. Hence, treatment of phenols such as 8-1 and an electron-rich alkene 8-4 in chloroform with tyrosinase in the presence of oxygen led to the bicyclic cycloadducts 8-5 and 8-6 in moderate to good yield (Scheme 8.1). It can be assumed that, in the first step, the phenol 8-1 is hydroxylated by tyrosinase, generating the catechol intermediate 8-2, which is then again oxidized enzy-... 

Cabanes J, Sanchez-Ferrer A, Bru R and Garcia-Carmona F. 1988. Chemical and enzymic oxidation by tyrosinase of 3,4-dihydroxymandelate. Biochem J 256 681—684. 

Jimenez-Atienzar M, Escribano J, Cabanes J, Gandia-Herrero F and Garcia-Carmona F. 2005. Oxidation of the flavonoid eriodictyol by tyrosinase. Plant Physiol Biochem 43(9) 866-873. 

Copper is part of several essential enzymes including tyrosinase (melanin production), dopamine beta-hydroxylase (catecholamine production), copper-zinc superoxide dismutase (free radical detoxification), and cytochrome oxidase and ceruloplasmin (iron conversion) (Aaseth and Norseth 1986). All terrestrial animals contain copper as a constituent of cytochrome c oxidase, monophenol oxidase, plasma monoamine oxidase, and copper protein complexes (Schroeder et al. 1966). Excess copper causes a variety of toxic effects, including altered permeability of cellular membranes. The primary target for free cupric ions in the cellular membranes are thiol groups that reduce cupric (Cu+2) to cuprous (Cu+1) upon simultaneous oxidation to disulfides in the membrane. Cuprous ions are reoxidized to Cu+2 in the presence of molecular oxygen molecular oxygen is thereby converted to the toxic superoxide radical O2, which induces lipoperoxidation (Aaseth and Norseth 1986). 

Copper oxygenases Tyrosinase Fungal, Tyrosine oxidation... 

Polyphenol oxidase occurs within certain mammalian tissues as well as both lower (46,47) and higher (48-55) plants. In mammalian systems, the enzyme as tyrosinase (56) plays a significant role in melanin synthesis. The PPO complex of higher plants consists of a cresolase, a cate-cholase and a laccase. These copper metalloproteins catalyze the one and two electron oxidations of phenols to quinones at the expense of 02. Polyphenol oxidase also occurs in certain fungi where it is involved in the metabolism of certain tree-synthesized phenolic compounds that have been implicated in disease resistance, wound healing, and anti-nutrative modification of plant proteins to discourage herbivory (53,55). This protocol presents the Triton X-114-mediated solubilization of Vida faba chloroplast polyphenol oxidase as performed by Hutcheson and Buchanan (57). 

Menter JM, Moore CL, Willis I, Fisher MS. Melanin markedly accelerates the tyrosinase mediated oxidation of phenohc depigmenters. Photochem Photobiol 1986 43 255. 

---