Oxidase, catechol

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... 

Klabunde T, Eicken C, Sacchettini JC and Krebs B. 1998. Crystal structure of a plant catechol oxidase containing a dicopper center. Nat Struct Biol 5 1084-1090. 

There has been enormous activity in the field of copper(I)-dioxygen chemistry in the last 25 years, with our information coming from both biochemical-biophysical studies and to a very important extent from coordination chemistry. This has resulted in the structural and spectroscopic characterization of a large number of copper dioxygen complexes, some of which are represented in Figure 14.2. The complex F, first characterized in a synthetic system was subsequently established to be present in oxy-haemocyanin, and is found in derivatives of tyrosinase and catechol oxidase, implying its involvement in aromatic hydroxylations in both enzymes and chemical systems. 

Catechol oxidase and tyrosinase are among the enzymes that have Type 3 dinuclear centres. However, the prototype of this class of proteins is the invertebrate oxygen transport protein, haemocyanin (Figure 14.5), for which structures of the oxy and deoxy forms have been determined at high resolution and confirm, as predicted from model compounds, that the... 

Figure 14.6 The active site of sweet potato catechol oxidase. (From Bento et al., 2006. With kind permission of Springer Science and Business Media.)...

Granata, A., Monzani, E. and Casella, L. (2004), Mechanistic insight into the catechol oxidase activity by a biomimetic dinuclear copper complex, J. Biol. Inorg. Chem., 9, 903-913. 

Transition metal complexes with o-dioxolene ligands constitute one of the most intriguing classes of complexes as far as their electrochemical behaviour is concerned, in that, as already mentioned in Chapter 5, Section 1, such ligands are able to shuttle through the oxidation states o-benzoquinone)o-benzosemiquinone/catecholate illustrated in Scheme 250 (a process carried out in nature by the dicopper (I I)-based enzyme catechol oxidase through a single two-electron step see Chapter 9, Section 1.2). 

It is interesting to underline that there is another (plant) enzyme which possesses a coordinatively similar dicopper environment catechol oxidase.11 As already mentioned in Chapter 6, Section 3, such an ubiquitous enzyme catalyses the two-electron oxidation by molecular dioxygen of catechols to the corresponding quinones (the so-generated quinones in turn polymerize to form brown polyphenolic catechol melanins, which protect damaged plants from pathogens or insects). 

This copper-dependent enzyme [EC 1.14.18.1] (also known as tyrosinase, phenolase, monophenol oxidase, and cresolase) catalyzes the reaction of L-tyrosine with L-dopa and dioxygen to produce L-dopa, dopaquinone, and water. This classification actually represents a set of copper proteins that also catalyze the reaction of catechol oxidase [EC 1.10.3.1] if only 1,2-benzenediols are available as substrates. 

PHENOL HYDROXYLASE CATECHOL 1,2-DIOXYGENASE CATECHOL 2,3-DIOXYGENASE CATECHOL O-METHYLTRANSFERASE CATECHOL OXIDASE... 

Copper-dependent enzymes, ASCORBATE OXIDASE CATECHOL OXIDASE FERROXIDASE GAACTOSE OXIDASE ACCASE... 

The titre of antibodies in laccase antisera was 1 16 as measured by the immunodiffusion technique of Ouchterlony (21). These antibodies were effective inhibitors of catechol oxidase activity, with 100/il of antisera reducing activity by 95%, in an assay mixture of 3ml of 0.1M catechol in... 

Fig.1 Structures of the active sites of galactose oxidase, catechol oxidase, and ascorbate oxidase metalloenzymes...

Hemocyanin [30,31], tyrosinase [32] and catechol oxidase (2) [33] comprise this class of proteins. Their active sites are very similar and contain a dicopper core in which both Cu ions are ligated by three N-bound histidine residues. All three proteins are capable of binding dioxygen reversibly at ambient conditions. However, whereas hemocyanin is responsible for O2 transport in certain mollusks and arthropods, catechol oxidase and tyrosinase are enzymes that have vital catalytic functions in a variety of natural systems, namely the oxidation of phenolic substrates to catechols (Scheme 1) (tyrosinase) and the oxidation of catechols to o-quinones (tyrosinase and catechol oxidase). Antiferromagnetic coupling of the two Cu ions in the oxy state of these metalloproteins leads to ESR-silent behavior. Structural insight from X-ray crystallography is now available for all three enzymes, but details... 

So-called blue multinuclear copper oxidase enzymes, such as laccase and ascorbate oxidase, catalyze the stepwise oxidation of organic substrates (most likely in successive one-electron steps) in tandem with the four-electron reduction of O2 to water, i.e. no oxygen atom(s) from O2 are incorporated into the substrate (Eq. 4) [15]. Catechol oxidase, containing a type 3 center, mediates a two-electron substrate oxidation (o-diphenols to o-chinones), and turnover of two substrate molecules is coupled to the reduction of O2 to water [34,35]. The non-blue copper oxidases, e.g. galactose oxidase and amine oxidases [27,56-59], perform similar oxidation catalysis at a mononuclear type 2 Cu site, but H2O2 is produced from O2 instead of H2O, in a two-electron reduction. 

While only tyrosinase catalyzes the ortho-hydroxylation of phenol moieties, both tyrosinase and catechol oxidase mediate the subsequent oxidation of the resulting catechols to the corresponding quinones. Various mono- and dinu-clear copper coordination compounds have been investigated as biomimetic catalysts for catechol oxidation [21,194], in most cases using 3,5-di-tert-butylcatechol (DTBC) as the substrate (Eq. 16). The low redox potential of DTBC makes it easy to oxidize, and its bulky tert-butyl groups prevent un-... 

Scheme 15 Possible reaction pathways in the catalytic cycle of dicopper(II) catechol oxidase model systems [182]...

Allan, A.C. and Walker, J.R.L. 1988. The selective inhibition of catechol oxidases by salicylhydroxamic acid. Phytochemistry 27 3075-3076. 

This class includes enzymes that use diphenols or related compounds as electron donors and oxygen as the acceptor, thereby forming the oxidized donor and water. Members include catechol oxidase (E.C. 1.10.3.1), laccase (E.C. 1.10.3.2), and o-aminophenol oxidase (E.C. 1.10.3.4). Laccase is also known as / -diphenoloxidase. whereas catechol oxidase is also known as diphenoloxidase, phenoloxidase, polyphenoloxidase, o-diphenolase, phenolase and tyrosinase. Many of these names are also used in reference to a different enzyme, monophenol monooxygenase (E.C. 1.14.18.1). This enzyme will be discussed further in Section 1.8.2.2. 

The phenol oxidases probably play no important role in the elimination of phenolic pressor amines, in spite of the importance that has been attached to the oxidation of the catechol nucleus in the past. The names phenolase and cresolase, polyphenol oxidase, and catechol oxidase serve to identify the enzyme with its mono- or diphenolic substrate, but they usually occur together and are difficultly separated. The enzymes have been purified and their characteristics have been described (56, 104, 106, 156). Beyer (21), Alles (5), and Randall and Hitchings (129) have described the relationship of structure of the phenolic pressor amines to the rate of oxidation of their nucleus in the presence of these enzymes. 

As stated by Beyer, it now does appear that both noradrenaline and adrenaline are implicated in the humoral mediation of adrenergic nerve impulses. The hypothesis that adrenoxine as produced by any action of a catechol oxidase in the body acts under appropriate conditions as the vasodilator substance presently appears to be very doubtful, although some of the evidence along this line presented by Bacq (1) and by Heirman and Bacq (7) appeared to be reliable. Shortly after their reports appeared Carroll Handley and the author in the pharmacology laboratory of the University of California Medical School tried to confirm the apparent reversal of net vasomotor effects of catechol oxidase oxidation of adrenaline solutions but failed to observe any effects beyond those that could be ascribed to the destruction of a part of the adrenaline activity. 

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