Enzymes copper-containing oxidases

Tire most studied of all copper-containing oxidases is cytochrome c oxidase of mitochondria. This multisubunit membrane-embedded enzyme accepts four electrons from cytochrome c and uses them to reduce 02 to 2 H20. It is also a proton pump. Its structure and functions are considered in Chapter 18. However, it is appropriate to mention here that the essential catalytic centers consist of two molecules of heme a (a and a3) and three Cu+ ions. In the fully oxidized enzyme two metal centers, one Cu2+ (of the two-copper center CuA) and one Fe3+ (heme a), can be detected by EPR spectroscopy. The other Cu2+ (CuB) and heme a3 exist as an EPR-silent exchange-coupled pair just as do the two copper ions of hemocyanin and of other type 3 binuclear copper centers. 

The phenomenon of bioelectrical catalysis with direct electron transfer from electrode to enzyme active site was primarily observed in the study of electrochemical oxygen reduction in the presence of a copper-containing oxidase - laccase, adsorbed on electrodes of different origins. This work was developed with peroxidase and hydrogenase application as the working components [2],... 

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). 

BG Mahnstrom, L-E Andreasson, B Reinhammar. Copper-containing oxidases and superoxide dismutase. In The Enzymes, Vol.l2. PD Boyer, ed. New York Academic Press, 1975, pp. 507-579. 

In a search for a possible biochemical means of investigating the mode of action of LSD it was noted that the administration of this drug to animals resulted in an elevation of brain 5-HT levels and a depression of brain catecholamine levels (Barchas and Freedman, 1963 Koenig-Bersin et al., 1970). It therefore seemed of interest to study the effects of LSD on an enzyme which could utilize both NA and 5-HT as substrates since it was considered that such an enzyme could be used as a model for those central receptors with which LSD must interact in order to produce its characteristic central effects. The enzyme which was eventually selected for study in this way was the copper-containing oxidase caeruloplasmin which utilizes both NA and... 

The blue oxidases-related enzymes include phenoxazinone synthase from S. antibioticus This enzyme is a copper-containing oxidase that catalyzes the coupling of 2-aminophenols to form the 2-aminophenoxazinone chromophore. This reaction constitutes the final step in the biosynthesis of the potent antineoplastic agent actinomycin. The crystal structure of the oxidized form phenoxazinone synthase from S. anibioticus has been determined. It has been solved in his hexameric form. One monomer is very similar to Iaccase or ascorbate oxidase but it contains a long loop, which connects two domains and stabilizes the hexameric structure. Bound... 

Among the enzyme systems performing redox reactions oxidations copper containing oxidases perform the irreversible oxidation of phenols. Two main types of phenol oxidases are known, the catechol oxidases and the laccases of which the latter is supposed to be... 

In addition to all of the expected enzyme systems present in leaf tissue, fresh tea leaves contain a high level of polyphenol oxidase that catalyzes the oxidation of the catechins by atmospheric oxygen. Tea polyphenol oxidase exists as series of copper-containing (0.32%) isoenzymes. The major component has a molecular weight of about 144,000.54 The enzyme is concentrated in the leaf epidermis.55 Soil copper deficiency is sometimes responsible for inadequate oxidation during processing.56... 

This discussion of copper-containing enzymes has focused on structure and function information for Type I blue copper proteins azurin and plastocyanin, Type III hemocyanin, and Type II superoxide dismutase s structure and mechanism of activity. Information on spectral properties for some metalloproteins and their model compounds has been included in Tables 5.2, 5.3, and 5.7. One model system for Type I copper proteins39 and one for Type II centers40 have been discussed. Many others can be found in the literature. A more complete discussion, including mechanistic detail, about hemocyanin and tyrosinase model systems has been included. Models for the blue copper oxidases laccase and ascorbate oxidases have not been discussed. Students are referred to the references listed in the reference section for discussion of some other model systems. Many more are to be found in literature searches.50... 

Interest in this class of coordination compounds was sparked and fueled by the discovery that radical cofactors such as tyrosyl radicals play an important role in a rapidly growing number of metalloproteins. Thus, in 1972 Ehrenberg and Reichard (1) discovered that the R2 subunit of ribonucleotide reductase, a non-heme metal-loprotein, contains an uncoordinated, very stable tyrosyl radical in its active site. In contrast, Whittaker and Whittaker (2) showed that the active site of the copper containing enzyme galactose oxidase (GO) contains a radical cofactor where a Cu(II) ion is coordinated to a tyrosyl radical. 

There are a number of excellent sources of information on copper proteins notable among them is the three-volume series Copper Proteins and Copper Enzymes (Lontie, 1984). A review of the state of structural knowledge in 1985 (Adman, 1985) included only the small blue copper proteins. A brief review of extended X-ray absorption fine structure (EXAFS) work on some of these proteins appeared in 1987 (Hasnain and Garner, 1987). A number of new structures have been solved by X-ray diffraction, and the structures of azurin and plastocyanin have been extended to higher resolution. The new structures include two additional type I proteins (pseudoazurin and cucumber basic blue protein), the type III copper protein hemocyanin, and the multi-copper blue oxidase ascorbate oxidase. Results are now available on a copper-containing nitrite reductase and galactose oxidase. 

Amine oxidase catalyses the oxidative deamination of amines to the corresponding aldehyde, hydrogen peroxide and ammonia. The copper containing amine oxidase from pig plasma (PPAO) is one of the better characterised in this class of enzyme. The homogeneously pure enzyme has a molecular weight of 190,000 composed of two subunits with equal molecular weight. The present evidence suggests that copper is essential for catalytic activity and therefore much effort has been made to determine the structure of copper sites... 

Complex IV, cytochrome oxidase. This copper-containing enzyme, which also contains cytochromes a and a3, accumulates electrons, then passes them to 02, reducing it to H20. 

Pyrazole also causes, both in vitro and in vivo, a reduction in the activity of the mixed function oxidase enzyme system (B-79MI10504), and has also been used to inhibit the copper-containing enzyme monoamine oxidase in vitro, as have several related heterocyclic compounds (80MI10507) such as indazole (78), isoxazole (75) and benzothiazole (79). 

Reduction of N20 to N2 by bacteria (Eq. 18-30, step d) is catalyzed by the copper-containing nitrous oxide reductase. The purple enzyme is a dimer of 66-kDa subunits, each containing four atoms of Cu.353 It has spectroscopic properties similar to those of cytochome c oxidase and a dinuclear copper-thiolate center similar to that of CuA in cytochrome c oxidase (p. 1030). 

Two ascorbate radicals can react with each other in a disproportionation reaction to give ascorbate plus dehydroascorbate. However, most cells can reduce the radicals more directly. In many plants this is accomplished by NADH + H+ using a flavoprotein monodehydroascorbate reductase.0 Animal cells may also utilize NADH or may reduce dehydroascorbate with reduced glutathione.CC/ff Plant cells also contain a very active blue copper ascorbate oxidase (Chapter 16, Section D,5), which catalyzes the opposite reaction, formation of dehydroascorbate. A heme ascorbate oxidase has been purified from a fungus. 11 1 Action of these enzymes initiates an oxidative degradation of ascorbate, perhaps through the pathway of Fig. 20-2. 

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