Polyphenol catecholase activity

Tyrosinase or polyphenol oxidase (EC 1.14.18.1) is a bifunctional, copper-containing enzyme widely distributed on the phylogenetic tree. This enzyme uses molecular oxygen to catalyze the oxidation of monophenols to their corresponding o-diphenols (cresolase activity) as well as their subsequent oxidation to o-quinones (catecholase activity). The o-quinones thus generated polymerize to form melanin, through a series of subsequent enzymatic and nonenzymatic reactions [1-3]. 

Further validation of the mechanism proposed for the catecholase activity of the dicopper complexes [Cu2(L66)]" , [Cu2(L55)], and [Cu2(EBA)]" " (Scheme 17) has been obtained investigating the inhibitory effect of kojic acid [5-hydroxy-2-(hydroxymethyl)-y-pyrone] (154). This fungal metabolite is one of the most efficient inhibitors of mushroom tyrosinase and other polyphenol oxidases (160,161). When the catalytic oxidation of DTBCH2 was studied in the presence of kojic acid, strong competitive inhibition was observed in the steps exhibiting substrate concentration dependence,... 

Polyphenol oxidase catalyzes two reactions first the hydroxylation of a monophenol to o-diphenol (EC 1.14.18.1, monophenol monooxygenase) followed by an oxidation to o-quinone (EC 1.10.3.1, o-diphenol oxygen oxidoreductase). Both activities are also known as cresolase and catecholase activity. At its active site, polyphenol oxidase contains two Cu ions with two histidine residues each in the ligand field. In an ordered mechanism (cf. 2.5.1.2.1) the enzyme first binds oxygen and later monophenol with participation of the intermediates shown in Fig. 2.8. The Cu ions change their valency (Cu Cu ). The newly formed complex ([] in Fig. 2.8) has a strongly polarized... 

Melanins are widely distributed in biopigments in bacteria, fungi, plants, and animals. Melanins are heterogeneous polyphenol-like biopolymers with a complex structure and color varying from yellow to black [210]. They are biosynthesized by a combination of enzymatic and chemical reactions which are initiated by the oxidation of tyrosine. Tyrosinase catalyzes the hydroxylation (monophenolase activity) of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) and the oxidation (diphenolase or catecholase activity) of DOPA to the corresponding -quinone [211] as shown in eq. (28.)... 

Perez-Gilabert M and Garcia-Carmona F. 2000. Characterization of catecholase and cresolase activities of eggplant polyphenol oxidase. J Agric Food Chem 48(3) 695-700. 

The more stable activity toward polyphenols was correspondingly named catecholase. These terms, which bear the suffix -ase used to denote enzymes, have not been applied to specific catalysts. They are used merely to describe aspects of the activity of phenol oxidases. 

The relation between oxidation of monophenols and polyphenols has been investigated extensively by Nelson and his students. The nature of the reactions studied is still not clear, but many interesting aspects have been explored. As in the case of other workers, it was found that there is a tendency for the ability to oxidize monophenols (cresolase) to be lost on purification, while polyphenol oxidation (catecholase) is more stable. The ratio of catecholase to cresolase activity of purified preparations appears to vary with electrophoretic mobility, and it has been suggested that a peptide component is lost from the enzyme, and that the purified preparations represent partially degraded enzyme. ... 

Flqure 9.31 Reactions catalysed by polyphenol oxidases with creasolase, catecholase and laccase activities. 

Tyrosinase is an enzyme complex (phenolase, polyphenol oxidase are other names which have been used for this enzyme), which catalyses of the ortho hydroxylation of monohydric phenols. The enzyme, which should not be confused with L-tyrosine hydroxylase mentioned above, contains Cu (I) and catalyses two distinct reactions—the hydroxylation of monohydric phenols to o-diphenols (cresolase activity) and the oxidation of o-diphenols to o-quinones (catecholase or catechol oxidase activity) . Most enzymes of this type, which are widely distributed in both the plant and animal kingdoms, exhibit both cataljrtic functions. Thus typically, the conversion of L-tyrosine (5) to L-dopa (15) and dopaquinone (36) which occurs in melanin biosynthesis is catalysed by an enzyme of the tyrosinase category. The two activities appear, in the majority of cases, to be functions of the same enzyme. However, certain o-diphenol oxidases such as those from tea , sweet potato and tobacco have been reported to show no capacity to catalyse the hydroxylation reaction but this is most probably due to destruction of the cresolase activity during purification. 

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