Type-3 copper centres

Blue or Type 1 copper centres comprise one of the three types of copper found in biological systems. The distinctive properties of this class of protein is an intense... 

This is the copper centre responsible for the deep blue colour of the blue oxidases and the blue electron-transfer proteins. Type 1 copper centres have an intense absorption band near 600 nm,... 

While nitrite reductases in many bacteria are haem proteins, some are copper-containing homotrimers which bind three type I and three type II copper centres The type 1 copper centre serves to transfer electrons from donor proteins to the type 2 centre which has been proposed to be the site of substrate binding. 

Cytochrome c and Cytochrome c Oxidase. - The mitochondrial electron transport chain is the site at which most of the free energy to be obtained from the oxidation of substrates is released and conserved as the energy-rich molecule ATP. In the final stage of this process, CcO, which is supplied with electrons by cyt c, catalyses the four-electron reduction of oxygen to water. Both are haem proteins, with CcO containing two haem and three copper centres, and both exhibit peroxidase-type activity. 

Ceruloplasmin is involved in copper storage and transport as well as in iron mobilisation and oxidation. Among the blue oxidases it is unique since it contains, in addition to the usual motif of a type 1 combined with the trinuclear cluster, two other type 1 coppers. Electron transfer occurs, however, only between five of the six copper ions since one of the type 1 centres is not catalytically relevant due to its too high redox potential. The redox potentials of the centres were determined and possible electron transfer pathways among the copper sites were discussed.101... 

The multicomponent type IV copper proteins are usually coloured (Figure 95) as they generally contain at least one blue type I copper centre, but as there are usually more than one of each type present, the physical properties of the different types I—III environments are difficult to separate and, for ESR-silent type III centres, difficult to detect.1203 No crystallographic data are yet available on a type IV copper protein. 

This centre consists of a pair of Cu11 ions, which are diamagnetic as a result of antiferromagnetic interaction. It is characterized by a strong absorption at around 330 nm with extinction coefficients in the range 3000 to 5000 dm3 moF1 cm-1. The type 3 centre is associated with redox reactions of dioxygen, as it can transfer two electrons and so bypass the formation of reactive superoxide. A number of model systems for type 3 copper have been reported. 

ESR spectra of type 2 Cu suggest the presence of three to four nitrogen ligands, while bound water has also been implicated by proton relaxation rate measurements. A number of anionic inhibitors bind to type 2 Cu. These results suggest that substrates may bind to the type 2 copper centres in oxidases. 

There has been much uncertainty over the molecular weight and copper content. It appears to involve a single polypeptide chain of molecular weight about 130 000 with six or seven copper ions. On balance, there are two type 1, one type 2 and four type 3 copper centres. This is in accord with the fact that 44% of the total copper is paramagnetic and ESR-detectable.977... 

Similarities with respect to the location of cysteine, histidine and methionine residues in the proteins of azurin, plastocyanin and ceruloplasmin indicate that the type 1 centres in ceruloplasmin are similar to those in the other two proteins. The nine-line superhyperfine splitting in the ESR spectrum of the type 2 Cu has been interpreted in terms of four equivalent nitrogen ligands.978 This was observed in a protein from which the type 1 copper was depleted by dialysis against ascorbate. 

The several functions of ceruloplasmin cannot be explained at present. It seems reasonable that this diversity is related to the activity of the copper centres. The general pattern of oxidase activity is probably similar to that of the other blue oxidases, with a type 3 binuclear site serving to bind and reduce dioxygen, with electrons transferred from the type 1 site. The type 2 copper may represent a substrate-binding site. 

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. 

Kinetic studies with laccase have shown that the enzyme must be reduced by the organic substrate before reaction with dioxygen occurs. The first electron from the substrate is accepted by the type 1 Cu2+, and the second by the type 2 Cu2+. The electrons from these reduced sites are then transferred to the type 3 copper pair, which then binds dioxygen with reduction to peroxide. It is possible that the type 2 and type 3 centres are in the same cavity, which only becomes accessible to the solvent when the type 1 Cu+ is oxidized. 

Both copper centres in each subunit appear to be essential for catalytic activity, and cannot be replaced by other metal ions. Two electrons are involved in the redox reaction to give norepinephrine, and it seems reasonable to postulate a bridging site between two Cu1 centres for dioxygen, with production of peroxide and Cu11. However, the copper centres in each subunit are ESR detectable,1404 and may not, therefore, present a type 3 site. In this case dioxygen must coordinate to one copper(I) centre, with electron transfer from the second copper(I) centre. 

Copper ions in the (I) and (II) oxidation states are biologically important. Basically, three different types of copper centre are shown. Blue of type I copper occurs in the blue electron carrying proteins such as stellacyanin, plastocyanin and azurin. There is also non-blue or type II copper and a type III copper centre that is non-detectable by EPR, apparently containing a pair of contiguous copper atoms. Tyrosinase is a type III protein that is not detected EPR because of antiferromagnetism of a pair of copper in it [137],... 

Type 2 centres include superoxide dismutase, discussed above, and cytochrome c oxidase (COX). COX consists of two haem iron centres, cytochrome a and cytochrome a3, and two electronically distinct copper centres, Cua and Cub, which catalyse ... 

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