Copper proteins multicopper oxidases

Figure 12.4 Proposed path for the intracellular transfer of Cu(I) by Atxl. Copper destined for incorporation into the vascular multicopper oxidase Fet3 requires both Ctrl and Ccc2. Cytoplasmic Cu(I)-Atxl, but not apo-Atxl, associates with the amino-terminal domain of Ccc2 and Cu(I) is transferred to the latter. (Inset) A proposed mechanism for the exchange of Cu(I) involving two- and three-coordinate Cu-bridged intermediates. The human homologues of Atxl (Hahl), Ccc2 (Menkes and Wilson s proteins) and Fet3 (ceruloplasmin) are likely to employ similar mechanisms. Reprinted with permission from Pufahl et al., 1997. Copyright (1997) American Association for the Advancement of Science.

The copper centres in the multicopper blue oxidases have been classified into three groups. This classification may be extended to include other copper proteins. 

Figure 5.1 Schematic representations of selected active sites of the copper proteins plastocyanin [56] (type 1, a) galactose oxidase [57] (type 2, b) oxy hemocyanin [58] (type 3, c) ascorbate oxidase [10] (type 4, or multicopper site, d) nitrous oxide reductase [59] (CuA site, e) cytochrome c oxidase [15]...

Some proteins contain more than one copper site, and are therefore among the most complicated and least understood of all. The active site known as type 4 is usually composed of a type 2 and a type 3 active site, together forming a trinuclear cluster. In some cases, such proteins also contain at least one type 1 site and are in this case termed multicopper oxidases, or blue oxidases [3], Representatives of this class are laccase (polyphenol oxidase) [7-9], ascorbate oxidase (Figure 5.Id) [10], and ceruloplasmin [11], which catalyze a range of organic oxidation reactions. 

The multicopper oxidases (laccase, ascorbate oxidase, and ceruloplasmin) catalyze a four-electron reduction of dioxygen to water (285-287). Consistent with the four-electron stoichiometry, the enzymes contain four copper ions. One of the copper ions is type I, causing an intensely blue color of the proteins, thus the enzymes of this family are referred to as blue oxidases. They also contain a monomeric copper site that exhibits normal spectroscopic features, whereas the other two copper... 

What distinguishes multicopper oxidases from other copper proteins is that they contain one each of these three types of copper site (Solomon and Lowery, 1993 Solomon et al., 1996). Not only does this make them excellent models for all copper proteins, but because they have four redox-active metal ions, they also serve as paradigms for other enzymes that couple a one-electron reductant to a four-electron oxidant, most notably cytochrome c oxidase. Indeed, the three copper sites (and four copper atoms) in the multicopper oxidases play essentially equivalent roles in comparison to the two heme groups and two copper atoms in cytochrome c oxidase. 

One also should be aware of the cohort of proteins that provide copper resistance to bacteria that commonly are encoded by an extrachromoso-mal element. Sequence analysis indicates that one of these proteins is a multicopper oxidase since this member of the group contains the copper liganding motifs highlighted in Fig. 1. However, this member also contains a M/S-rich motif that is thought to be essential to the copper trafficking supported by a copper transporter like Ctrlp Sa. cerevisiae) (Dancis et al., 1994) or the CopA (Cha and Cooksey, 1991) and CopB... 

This putative protein product (gil073083) is termed a copA homo-logue although it does not have the CXXC motifs common to the gene products similar to the copA protein from Escherichia coli that is a known copper-translocating ATPase (Rensing et al., 2000). In summary, bacteria also produce multicopper oxidases, and they potentially could be involved in metal metabolism. However, essentially no research has been reported that in any way tests this possible involvement. 

These copper site mutants have been particularly useful in two different types of experiments. First, the T2D protein allows for investigation of the electron transfer to the type 1 Cu(II) in the absence of turnover. This is because in the multicopper oxidase reaction, electron transfer from the type 1 copper—as Cu(I)—to the trinuclear cluster where O2 is reduced requires the type 2 Cu(II) (Solomon and Lowery, 1993 Solomon... 

One recombinant FetSp mutant is unique among multicopper oxidase species and has been particularly informative about the structure of the type 3 binuclear cluster in these species. This is the T1D/T2D double mutant that contains only this type 3 site (Blackburn et al., 2000). EXAFS analysis of this protein contains contributions from electron ejection and scattering from only the type 3 copper atoms and thus provides direct structural information about this cluster. The K-edge XAS spectrum for this mutant in its oxidized and reduced states is shown in Fig. 21. The oxidized sample has a nearly featureless edge with a midpoint energy of 8990 eV typical of tetragonally distorted type 2 Cu(ll) centers, i.e., those with predominantly histidine imidazole coordination. The reduced type 3 cluster exhibited a pronounced shoulder at 8984 eV just below the... 

The second class consists of multidomain blue copper proteins composed of exclusively two or more BCB domains and includes nitrite reductase (Section IV, E), multicopper blue oxidases such as laccase, ascorbate oxidase, ceruloplasmin, and hephaestin (Section VII), and some sequences found in extreme halophilic archaea (see Section V, E). 

The constrained nature of the copper center in BCB domains reduces its reorganization energy, which is considered an important feature for their function in long-range electron transfer processes. They are capable of tunneling electrons, usually over 10- to 12-A distances, intramolecu-larly within the same protein (in the case of multicopper oxidases and nitrite reductases) or intermolecularly between a donor and an acceptor protein (in the case of cupredoxins) in a thermodynamically favorable environment. 

In the area of copper metabolism, four topics are covered bacterial copper transport reviewed by Huat Lu and Sohoz copper P-type ATPases reviewed by Voskoboinik, Camakaris, and Mercer copper chaperones reviewed by Stine Elam et al. and copper metaUoregulation of gene expression reviewed by Winge. An important related topic is the link between copper and iron metabolism. In this area, Kosman has reviewed the multicopper oxidase enzymes, such as FetSp and ceruloplasmin, which catalyze the conversion of iron(II) to iron(III) in preparation for its specific transport by partner transporter proteins. 

Once inside the mucosal cell, iron then has to be transported across the membrane to serum transferrin. This appears to take place via the Iregl transporter protein (also known as ferroportin 1 or MTPl). Iregl is a transmembrane protein located at the basolateral membrane of the cell that has been shown to be involved in iron uptake. Oxidation of Iregl-bound ferrous iron and its release to transferrin is likely to be enhanced by the membrane-bound multicopper ferroxidase hephaestin. This protein is 50% identical to ceruloplasmin, a soluble protein identified as having a possible role in iron loading of transferrin see Copper Proteins Oxidases). Mutation of hephaestin in mice leads to a build up of iron in duodenal cells and overall iron deficiency in the body. ... 

The disturbance of copper excretion, primarily due to a defect in the billiary excretion, is consistent with the biochemical findings in patients with Wilson disease. Urinary copper excretion is increased owing to total body overload of copper. Renal dysfunction includes albuminuria and renal rickets. Incorporation of copper in ceruloplasmin is impaired. Thus, there is a greater proportion of copper bound to albumin and amino acid complexes in the serum. But the overall copper concentration in serum is low. Ceruloplasmin is a multicopper oxidase see Copper Proteins Oxidases) that... 

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