Blue copper enzymes

Several copper enzymes will be discussed in detail in subsequent sections of this chapter. Information about major classes of copper enzymes, most of which will not be discussed, is collected in Table 5.1 as adapted from Chapter 14 of reference 49. Table 1 of reference 4 describes additional copper proteins such as the blue copper electron transfer proteins stellacyanin, amicyanin, auracyanin, rusticyanin, and so on. Nitrite reductase contains both normal and blue copper enzymes and facilitates the important biological reaction NO) — NO. Solomon s Chemical Reviews article4 contains extensive information on ligand field theory in relation to ground-state electronic properties of copper complexes and the application of... 

Blue copper proteins are a family of metalloproteins that have been found to play an important role in a number of electron-transfer reactions in nature. Solomon and coworkers have studied a range of blue copper enzymes in detail to produce a thorough description of how molecular and electronic structure interact to provide the function of these enzymes (26,158). 

The high-resolution structure of the blue copper enzyme ascorbate oxidase (AO) prompted several laboratories to examine the internal electron flow from the substrate oxidation site [type 1 Cu(II)] to that of dioxygen reduction. Again, both flash photolysis and pulse radiolysis were employed, providing a comparison of their respective features. 

Messerschmidt A (1988) Metal Sites in Small Blue Copper Proteins, Blue Copper Oxidases and Vanadium-Containing Enzymes. 90 37-68 Michel H, see Hemmerich P (1982) 48 93-124... 

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

Type I copper enzymes are called blue proteins because of their intense absorbance (s 3000 M-1 cm- ) in the electronic absorption spectrum around... 

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... 

Copper oxidases are widely distributed in nature, and enzymes from plants, microbes, and mammals have been characterized (104,105). The blue copper oxidases, which include laccases, ascorbate oxidases, and ceruloplasmin, are of particular interest in alkaloid transformations. The principle differences in specificity of these copper oxidases are due to the protein structures as well as to the distribution and environment of copper(II) ions within the enzymes (106). While an in vivo role in metabolism of alkaloids has not been established for these enzymes, copper oxidases have been used in vitro for various alkaloid transformations. 

A blue, copper-containing glycoprotein present in mammalian blood plasma and containing type 1, type 2, and type 3 copper centers. The type 2 and type 3 copper centers are close together, forming a trinuclear copper cluster. Ceruloplasmin has an important role in the transport and storage of copper ions. Thus, it participates in the metabolism of copper-containing enzymes. 

ENZYME CASCADE KINETICS BLUE COPPER PROTEIN Blue shift,... 

Laccase is perhaps the metallo-enzyme most widely used for this aim. Laccases are a family of multicopper ( blue copper ) oxidases widely distributed in nature Many laccases have fungal origin, while others are produced in plants. They contain four Cu(II) ions, and catalyse the one-electron oxidation of four molecules of a reducing substrate with the concomitant four-electron reduction of oxygen to water . In view of their low redox potential, which is in the range of 0.5-0.8 V vs. NHE depending on the fungal source laccases typically oxidize phenols (phenoloxidase activity) or anilines. 

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. 

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. 

J. A Fee Copper Proteins - Systems Containing the Blue" Copper Center. - M.F.Dunn Mechanisms of Zinc Ion Catalysis in Small Molecules and Enzymes. - W. Schneider Kinetics and Mechanism of Metalloporphyrin Formation. - M. Orchin, D. M. Bollinger Hydro gen-Deuterium Exchange in Aromatic Compounds. 

Two types of dissimilatory nitrite reductases catalyze step b of Eq. 18-30. Some bacteria use a copper-containing enzyme, which contains a type 1 (blue) copper bound to a (3 barrel domain of one subunit and a type 2 copper at the catalytic center. The type 1 copper is thought to receive electrons from the small copper-containing carrier pseudoazurin (Chapter... 

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. 

Table 74 Multiple-type Copper Enzymes in Blue Oxidases...

In the discussion of the biochemistry of copper in Section 62.1.8 it was noted that three types of copper exist in copper enzymes. These are type 1 ( blue copper centres) type 2 ( normal copper centres) and type 3 (which occur as coupled pairs). All three classes are present in the blue copper oxidases laccase, ascorbate oxidase and ceruloplasmin. Laccase contains four copper ions per molecule, and the other two contain eight copper ions per molecule. In all cases oxidation of substrate is linked to the four-electron reduction of dioxygen to water. Unlike cytochrome oxidase, these are water-soluble enzymes, and so are convenient systems for studying the problems of multielectron redox reactions. The type 3 pair of copper centres constitutes the 02-reducing sites in these enzymes, and provides a two-electron pathway to peroxide, bypassing the formation of superoxide. Laccase also contains one type 1 and one type 2 centre. While ascorbate oxidase contains eight copper ions per molecule, so far ESR and analysis data have led to the identification of type 1 (two), type 2 (two) and type 3 (four) copper centres. 

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