Mechanistic aspects of the tyrosinase oxidation of hydroquinone
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Hydroquinone
Catechol
Catechol oxidase
Catechol
Absorbance
Hydroquinone
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Incubation of catechol with mushroom tyrosinase in the presence of N-acetylmethionine resulted in the generation of an adduct. This product was identified to be N-acetylmethionyl catechol, on the basis of spectral characteristics and well-characterized chemical reaction of o-benzoquinone with N-acetylmethionine. Enzyme-catalyzed oxidation of catechol and the subsequent nonenzymatic addition of the resultant quinone to N-acetylmethionine accounted for the observed reaction. That the reaction is not confined to catechol alone, but is of general occurrence, can be demonstrated by the facile generation of similar adducts in incubation mixtures containing N-acetylmethionine, tyrosinase, and different N-acetylmethionines, such as 4-methylcatechol and N-acetyldopamine. Attempts to duplicate the reaction with insect cuticular phenoloxidases were not successful, as the excess N-acetylmethionine used in the reaction inhibited their activity. Nevertheless, occurrence of this nonenzymatic reaction between N-acetylmethionine and mushroom tyrosinase-generated quinones indicates that a similar reaction between enzymatically generated quinones in the cuticle with protein-bound methionine moiety is likely to occur during in vivo quinone tanning as well. Arch. Insect Biochem. Physiol. 38:44–52, 1998. © 1998 Wiley-Liss, Inc.
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Hydroquinone
Catechol oxidase
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Biocatalysis
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Abstract Cytochemical localization of the enzyme phenol oxidase in Neomurraytrema tengra has been studied. Results reveal that the enzyme reacts with substrates such as catechol, hydroquinone, pyrogallol, dopa, doparmine, epinephrine and tyramine, but not with tyrosine and protocatechuic acid. Thus it shows activity with a wide range of phenols, aminated, mono and diphenols and also with deaminated and decarboxylated, di- and polyphenols. The maximum activity of the enzyme occurs between 40°C and 50°C with a pH optimum of 6–6.
Pyrogallol
Catechol
Hydroquinone
Catechol oxidase
Protocatechuic acid
Tyramine
Guaiacol
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Catechol
Hydroxylation
Catechol oxidase
Hydroquinone
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Catechol
Catechol oxidase
Hydroquinone
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Hydroquinone
Catechol
Catechol oxidase
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Phloroglucinol and resorcinol are not substrates for clingstone peach (Prunus persica) polyphenol oxidase, but they react with 4-methyl-o-quinone, produced either enzymatically or nonenzymatically, to give an intense red or red-brown color with a maximal absorption at about 470 nanometers. Several colored products were isolated from an ethyl acetate extract of the reaction by two-dimensional thin layer chromatography. Based on thin layer chromatographic and spectral studies of the enzymatic and nonenzymatic reactions, polyphenol oxidase does not play a role in the reaction between 4-methyl-o-quinone and phloroglucinol, resorcinol, d-catechin, or orcinol. In such reactions, the function of polyphenol oxidase is the formation of 4-methyl-o-quinone which then reacts nonenzymatically with the above phenols. Activation energies of both enzymatic and nonenzymatic reactions were determined.
Phloroglucinol
Resorcinol
Catechol
Hydroquinone
Orcinol
Catechol oxidase
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Abstract Ubiquitous tyrosinase catalyses the aerobic oxidation of phenols to catechols through the binuclear copper centres. Here, inspired by the Fischer indole synthesis, we report an iridium-catalysed tyrosinase-like approach to catechols, employing an oxyacetamide-directed C–H hydroxylation on phenols. This method achieves one-step, redox-neutral synthesis of catechols with diverse substituent groups under mild conditions. Mechanistic studies confirm that the directing group (DG) oxyacetamide acts as the oxygen source. This strategy has been applied to the synthesis of different important catechols with fluorescent property and bioactivity from the corresponding phenols. Finally, our method also provides a convenient route to 18 O-labelled catechols using 18 O-labelled acetic acid.
Catechol
Oxidation reduction
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Incubation of catechol with mushroom tyrosinase in the presence of N-acetylmethionine resulted in the generation of an adduct. This product was identified to be N-acetylmethionyl catechol, on the basis of spectral characteristics and well-characterized chemical reaction of o-benzoquinone with N-acetylmethionine. Enzyme-catalyzed oxidation of catechol and the subsequent nonenzymatic addition of the resultant quinone to N-acetylmethionine accounted for the observed reaction. That the reaction is not confined to catechol alone, but is of general occurrence, can be demonstrated by the facile generation of similar adducts in incubation mixtures containing N-acetylmethionine, tyrosinase, and different N-acetylmethionines, such as 4-methylcatechol and N-acetyldopamine. Attempts to duplicate the reaction with insect cuticular phenoloxidases were not successful, as the excess N-acetylmethionine used in the reaction inhibited their activity. Nevertheless, occurrence of this nonenzymatic reaction between N-acetylmethionine and mushroom tyrosinase-generated quinones indicates that a similar reaction between enzymatically generated quinones in the cuticle with protein-bound methionine moiety is likely to occur during in vivo quinone tanning as well. Arch. Insect Biochem. Physiol. 38:44–52, 1998. © 1998 Wiley-Liss, Inc.
Catechol
Hydroquinone
Catechol oxidase
Moiety
Biocatalysis
Benzoquinone
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Tyrosinase oxidizes tyrosine to dopaquinone, which undergoes nonenzymatic reactions leading to precursors of melanin pigments. Cyclization of dopaquinone gives cyclodopa, which participates in redox exchange with dopaquinone to give the eumelanin precursor dopachrome plus dopa. The indirect formation of the catechol (dopa) from the phenol (tyrosine) leads to unusual enzyme kinetics. Using a combination of enzyme oximetry, pulse radiolysis, and chemical oxidation, the study of structurally modified dopaquinones provides firm evidence of nonenzymatic catechol formation during tyrosinase oxidation of phenols and reveals significant differences in their modes of reaction.
Catechol
Catechol oxidase
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