Blue copper proteins spectroscopic properties

Copper proteins have been classified according to their spectroscopic properties (Malkin and Malmstrbm, 1970 Fee, 1975) as type I, II, or III. Type I blue copper proteins are characterized by an extraordinarily intense absorption near 600 nm and by unusually small hyperfine coupling constants for the paramagnetic [oxidized Cu(II)] form of the protein. 

The plastocyanins are blue copper proteins found in the chloroplasts of higher plants and algae where they mediate electron transport between cytochrome f and P-700 (Barber, 1983 Haehnel, 1984, 1986 Cramer etal., 1985 Sykes, 1985 Andersen et al., 1987). Plastocyanins each contain one copper bound by a single polypeptide chain of molecular weight around 10500 (Sykes, 1985). The spectroscopic properties of the copper are those of a typical blue site. The properties of the plastocyanins have been the subject of detailed reviews (Sykes, 1985 Haehnel, 1986 Chapman, 1991). 

Halocyanin and sulfocyanin are archaebacterial cupre-doxins that are attached to peripheral membranes through a lipid anchor at their N-terminus. Halocyanin was the first cupredoxin purified from an archaeon, haloalkaliphilic Natronobacterium pharaonis These cells live in high pH (around 10-11) and in extreme salinity (30%) environments. The presence of the blue copper protein, sulfocyanin in Sul-folobus acidocaldarius was first predicted from its gene sequence. It has been subsequently purified as a recombinant protein and shown to bind a single copper ion with spectroscopic properties typical for a blue copper site. ... 

A combination of spectroscopic methods (EPR, light absorption and CD) was used in order to obtain information on the copper binding site of bovine hemocuprein (erythrocuprein), which has been considered to be intermediate in its spectroscopic properties between the blue copper proteins and complexes of copper with model peptides (94). 

Phytocyanins are a family of blue copper proteins found exclusively in plants.They are further classified into three subfamilies plantacyanins (PNC), stellacyanins (STC), and uclacya-nins (UCC)." Spectroscopic properties of uclacyanins are closer to those of blue copper centers and thus they were covered in Section 8.4.2.3. The function of phytocyanins has not been determined conclusively. 

When the prediction or interpretation of spectroscopic properties and reactivities of metalloenzymes is the aim, QM-based methods - and DFT in particular - have proven to be the methods of choice [498]. Some of the recent results in the area of blue-copper proteins have been discussed above, and the methods used to interpret and predict spectroscopic parameters are provided in Chapter 10. Here, we concentrate on computational studies related to the understanding of the reactivity of... 

The copper(II) atoms present in copper metalloproteins have been classified according to their spectroscopic properties. Type-1, as found in blue copper proteins such as... 

The earlier sections of this chapter have been concerned mostly with the multiconfigurational description of the electronic structure of compounds involving a transition metal. In this final section, our goal is sKghtly different. We shall show an example where the approach has been used to study the bonding and spectroscopic properties in an important class of transition-metal compounds, the so called blue copper proteins. The emphasis will now be on the results obtained and the comparison of different quantum-chemical methods. Only a brief summary can be given here and the reader is referred to the original literature for more details. 

Copper sites in proteins have traditionally been classified into three classes blue type 1 sites (present in the blue copper proteins), normal type 2 sites (tetragonal mononuclear copper sites), and type 3 (spin-coupled pairs of copper ions). The type 1 sites have been further classified as axial or rhombic depending on their EPR (and other spectroscopic) characteristics." Plastocyanin (axial) and nitrite reductase (rhombic) are typical examples of type 1 proteins. Proteins with properties intermediate between those of type 1 and type 2 sites have been termed type 1.5." ... 

This article has reviewed some recent theoretical work on the relation between structural and spectroscopic properties of the blue copper proteins. The intention has been to illustrate how contemporary quantum chemical methods can be used to study and solve a non-trivial biochemical problem. The accuracy of the methods we use today is such that once the answer has been obtained we can with confidence use them to explain the chemical properties. It is, for example, clear that the DFT calculations of the geometry preferences of the four-coordinated Cu(ll) ion explain the structural properties of the active site without invoking the rest of the protein more... 

The cucumber basic blue protein is made up of a single polypeptide chain of 96 amino acids containing a single copper ion with a molecular weight of 10000 (Adman, 1985). The amino acid sequence (Bergman et al., 1977) is very similar to those of stellacyanin and umecyanin. The copper in cucumber blue protein displays the characteristic spectroscopic properties of a typical blue copper site (Table 5-5). 

Based on spectroscopic properties, mainly electron paramagnetic resonance (EPR), the active sites of copper proteins have been classified into three groups, types I, II, and III. This nomenclature was originally applied to blue oxidases to distinguish the four copper ions contained in these proteins. The original classification has been extended to the copper sites of other proteins. The recent increase in structural information on the copper sites in proteins has, however, revealed greater diversity in the type of copper site. For instance, the type III and type II sites in ascorbate oxidase are in close proximity, forming a trinuclear site, in which all three copper ions are essential for the reactivity. Some proteins, once believed to contain a copper site with normal spectroscopic properties, and thus referred as type II, have been shown to contain copper coordinated by an unusual side chain. Therefore, in this review, new nomenclature is used to classify the copper sites more precisely with respect to their structural features and spectroscopic properties. The definitions are as follows ... 

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