Activity catecholase

Krebs and co-workers synthesized a series of dinuclear copper(II) complexes as models for catechol oxidase 91 (365) (distorted SP Cu-Cu 2.902 A), (366) (distorted five-coordinate geometry Cu-Cu 3.002A), (367) (distorted SP Cu-Cu 2.995 A), (368) (distorted five-coordinate geometry Cu-Cu 2.938 A), and (369) (distorted SP Cu-Cu 2.874 A). These complexes were characterized by spectroscopic and electrochemical methods. From kinetic analysis, a catalytic order for catecholase activity (aerial oxidation of 3,5 -di - ter t-buty lcatec h o 1) was obtained.326... 

Cabanes J, Chazarra S and Garcia-Carmona F. 1994. Kojic acid, a cosmetic skin whitening agent, is a slow-binding inhibitor of catecholase activity of tyrosinase. J Pharm Pharmacol 46(12) 982—985. 

Alternate Protocol 1 Spectrophotometric Assay of Catecholase Activity C4.1.7... 

These controversial findings inspired numerous subsequent studies on the structure-activity relationship of catalytically active compounds. Very detailed mechanistic studies on the catecholase activity of a series of structurally related dicopper(II) complexes have also been published by Casella and co-workers [37-40], who have grouped together different mechanisms earlier proposed for the catecholase activity of dicopper(II) complexes, as shown in Scheme 5.2. 

Despite its tetranuclear structure in the solid state, the dicopper(II) complex was found to dissociate in solution into dinuclear units at the concentration levels used for catecholase activity studies. Similarly to the copper(II) complex with the ligand [22]py4pz, the present complex also catalyzes the oxidation of the model substrate DTBCH2 in methanol. However, several unexpected observations have been made in the present case. First, the rate-determining step in the catalytic reaction was found to change with the substrate-to-complex ratio. Thus, at low substrate-to-... 

Traverson-Rueda, S. Singleton, V.L. Catecholase Activity in Grape Juice and... 

To distinguish this type of activity from the one mentioned earlier, it is described as cresolase activity, whereas the other is referred to as catecholase activity. For both types of activity, the involvement of copper is essential. Copper has been found as a component of all polyphenolases. The activity of cresolase involves three steps, which can be represented by the following overall equation (Mason 1956) ... 

Tyrosinase is a copper-containing oxidase (Coche-Guerente et al, 2001 Forzani et al, 2000), which possesses the two different activities illustrated in Figure 57.12. In the first step, referred to as the hydroxylase or cresolase activity, molecular oxygen is used to hydroxylate phenol to form catechol. In the second step, known as the catecholase activity, the enzyme oxidizes catechol to o-quinone, which is simultaneously oxidized by oxygen to its original form, with the production of water. The o-quinone is electro-chemically active and can be reduced back to catechol, as illustrated above in Eq. (57.17). 

The dominant feature of tyrosinase is that it has both cresolase and catecholase activity. Laccase has a very clear catecholase activity, but its cresolase activity is not so clear. Mason, Fowlks, and Peterson (109) used 02 to label 3,4-dimethylphenol during the tyrosinase-catalyzed oxidation of this compound and showed that the source of the oxygen introduced into the phenol in the phenolase reaction was molecular oxygen according to Reaction 1. 

The subsequent catecholase activity of tyrosinase requires easy removal of electrons from the phenolic oxygens at the 1 and 2 positions... 

Catechol as an Activator of Tyrosinase. The phenolase activity of tyrosinase has been studied less completely than the catecholase activity, partly because of the lack of a satisfactory assay procedure. The phenolase reaction, however, is characterized by a lag time which can be abolished by adding dihydroxyphenylalanine (DOPA), the immediate product of the hydroxylation reaction 29S4, 102, 117), This phenomenon has been described by several investigators (29-34) and is illustrated in Figure 12, from Pomerantz and Warner (117), using the enzyme from Hamster melanoma. The same phenomenon has been analyzed by Duckworth and Coleman (102) for the mushroom enzyme. In the absence of DOPA, maximum velocity of the hydroxylase reaction is not reached for several minutes. Pomerantz and Warner (117) devised a convenient assay for the phenolase reaction by determining the radio-... 

The dinucleating ligand (58) has been prepared by standard methods via the bis-hydroxymethyl-ation of 4R-substituted phenols. It has been observed that modification of the R substituent induces a drastic effect on the catecholase activity of the dicopper(II) complexes containing (58) in the deprotonated form.85 A similar coordination pattern to those found in metal derivatives of (58) is realized in bis(triazacyclononane) systems bridged by a m-xylyl fragment (59)86 and in (60).87 /)-Xylyl ligands exhibit different coordination chemistry and reactivity 85... 

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]. 

The biosynthetic pathway for melanin formation, operating in insects, animals, and plants, has largely been elucidated by Raper [15], Mason [16], and Lerner et al. [17]. The first two steps in the pathway are the hydroxylation of monophenol to o-diphenol (monophenolase or cresolase activity) and the oxidation of diphenol to o-quinones (diphenolase or catecholase activity), both using molecular oxygen followed by a series of nonenzymatic steps resulting in the formation of melanin [15,18,19]. The whole pathway for melanin biosynthesis is shown in Scheme 1. 

By using lineweaver-Burk plots the authors found that four xanthates exhibited different patterns of mixed, competitive, or uncompetitive inhibition. For the cresolase activity, 1 and 2 demonstrated uncompetitive inhibition but 3 and 4 exhibited competitive inhibition [43]. For the catecholase activity, 1 and 2 showed mixed inhibition but 3 and 4 showed competitive inhibition against tyrosinase [43]. The xanthates (compoimds 1, 2, 3 and 4) have been classified as potent inhibitors against tyrosinase due to their Ki values of 13.8,... 

Figure 16.3-4. Oxidation of phenols catalyzed by tyrosinase displaying so-called creolase and catecholase activities.

While hemocyanins can only be found in two phyla, tyrosinases (EC 1.14.18.1) may be found in almost all types of organisms - from bacteria to mammals [251]. Tyrosinase, a monooxygenase, is responsible for the production of melanin and related pigments. It catalyses the o-hydroxylation of monophenols to o-diphenols (monophenolase activity), as well as the two-electron oxidation of o-diphenols to o-quinones (catecholase activity) [252,253]. 

The catalytic mechanism of tyrosinase was first studied in detail by Solomon et al Solomon proposed a mechanism for both the cresolase and catecholase activities of tyrosinase (Figure 25). This mechanism suggests the oxy state to be the starting point of cresolase activity (inner circle). This state is present in the resting form of tyrosinase in a proportion of about 15% (85% met state). A monophenol substrate binds to the oxy state and is monooxygenated to o-diphenol. This diphenol subsequently binds to the copper center of met tyrosinase in a... 

Figure 25 Mechanism of cresolase and catecholase activity of tyrosinase and catechoi oxidase deveioped on the basis of an initiai proposai by Soiomon and coworkers and inciuding more recent resuits.Reproduced from C. Gerdemann C. Eicken B. Krebs, Acc. Chem. Res. 2002, 35, 183-191, with permission from American Chemicai Society.

The mechanism of catecholase activity (outer circle) starts from the oxy and met states. A diphenol substrate binds to the met state (for example), followed by the oxidation of the substrate to the first quinone and the formation of the reduced state of the enzyme. Binding of dioxygen leads to the oxy state, which is subsequently attacked by the second diphenol molecule. Oxidation to the second quinone forms the met state again and closes the catalytic cycle. 

Alternative reaction mechanisms include a radical mechanism proposed by Kitajima and Morooka and a mechanism involving a Cu(III) intermediate based on measurements of model compounds. On the basis of the crystal structure of the catechol oxidase-PTU inhibitor complex, monodentate binding of the substrate was suggested for catechol oxidase. A radical mechanism, as proposed for the weak catecholase activity found in Octopus vulgaris hemocyanin, is also possible for catechol oxidase due to the strong structural relationship between catechol oxidase from /. batatas and odg hemocyanin as described above. 

Dioxygen activation is also accomplished at the dinuclear copper-active sites in tyrosinases and catechol oxidases. Tyrosinases (EC 1.14.18.1) are widely distributed throughout bacteria, fungi, plants, and animals, catalyzing the ortho-hydroxylation of phenols to catechols (phenolase activity. Equation (1)) and the oxidation of catechols to o-quinones (catecholase activity. Equation (2)). 

In this chapter, the dioxygen activation mechanism at the dinuclear copper-active sites of tyrosinase and catechol oxidase has been surveyed. In both enzymes, a (ji-rfirf -peToxo) dicopper(II) complex has been detected and characterized as a common reactive intermediate by several spectroscopic methods. In spite of longstanding efforts in the enzymological studies, mechanistic details of the enzymatic reactions (phenolase and catecholase activities) still remain ambiguous. On the other hand, recent developments in the model chemistry have provided a great deal of information about the structure and physicochemical properties as well as the reactivity of the peroxo intermediate and have advanced our understanding of the enzymatic reactions. 

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