Blue copper proteins stellacyanin

The blue copper protein stellacyanin, with a molecular weight of about 20,000, is obtained from the Japanese lacquer tree Rhus vemicifera. The EPR spectrum is described by roughly axial g and ACu hfs tensors and an unusually small a j value. As shown in Fig. 39 a, only the largest copper hf value A u can be directly determined from the EPR spectrum202. This coupling does not lie along the largest g-principal axis, in contrast to the usual behaviour of square planar copper complexes. 

Table 12.1. Magnetic parameters of the blue copper protein stellacyanin (data from Roberts et al.199) Ai and Qj in MHz)...

Negative values for redox couple entropy have also been obtained for the Cu(II)/Cu(I) reduction, in aqueous medium, of the blue copper proteins stellacyanin, plastocyanin and azurin.14 The decrease in molecular disorder has been attributed in this case to the fact that the charge neutralization of the redox site (from + 1 to 0) favours the formation of hydrogen bonds between the solvent (water) and the copper centre.17... 

Intramolecular Ru(II) to Cu(II) ET rates have been measured in two other blue copper proteins, stellacyanin [42, 43] and azurin [9, 13, 28]. Pseudomonas aeruginosa azurin has been ruthenated at His83 [13] (Fig. 5). The intramolecular Ru(II) to Cu(II) ET rate of 1.9 s was found to be independent of temperature [28]. The Cu reorganization enthalpy was estimated to be < 7 kcal/mol [13, 28], a value confirming that blue copper is structured for efficient ET. Again, a blue copper ET rate is low in comparison with heme protein rates over similar distances (at similar driving forces) (Table 1). 

Bacterial hosts are inappropriate choices for expression of proteins such as the blue copper proteins stellacyanin, laccase, and ceruloplasmin which are extensively glycosylated. In these cases, it may be necessary to employ tissue cultures of appropriate origin to obtain the native protein. In this regard, the amino-terminal half of human serum transferrin, which lacks carbohydrate, has been expressed in high yield in baby hamster kidney cells by Funk et al. [13], while the glycosylated carboxyl-terminus has proved to be more problematic [103]. 

Figure 4. Stimulated echo envelope recordings for a frozen solution sample of the blue copper protein stellacyanin at a field Hq = 3165 Gauss and a frequency 9.175 GHz. The modulation pattern is due to the remote nucleus in a histidyl ligand which coordinates the Cu(II) ions. These results are taken at times t which illustrate the "frequency suppression effect" (see text).

The type-1 blue copper proteins act as electron carriers azurin, plastocyanin, stellacyanin, umecyanin e.g. They are characterized by a rather strong LMCT (ligand to metal charge transfer) band near 600 nm and by small hyperline coupling constants A in EPR. Copper is bound to two imidazole groups of histidine and to two... 

Copper proteins are involved in a variety of biological functions, including electron transport, copper storage and many oxidase activities. A variety of reviews on this topic are available (Sykes, 1985 Chapman, 1991). Several copper proteins are easily identified by their beautiful blue colour and have been labelled blue copper proteins. The blue copper proteins can be divided into two classes, the oxidases (laccase, ascorbate oxidase, ceruloplasmin) and the electron carriers (plastocyanin, stellacyanin, umecyanin, etc.). 

Unknown stellacyanin, umecyanin, cucumber basic blue copper protein... 

Solvent is usually excluded from the blue copper site, which is buried 6 A inside the protein, having only the His ligand from the copperbinding loop exposed to the surface. The phytocyanins, stellacyanin and plantacyanin (cucumber basic protein), are exceptions, in which both His ligands are solvent exposed and the copper ion is only 3 A beneath the protein surface. This situation makes the copper center in this family of blue copper proteins more accessible to low-molecular-weight solutes (see Section V). 

Fig. 3. Geometries of the type 1 copper sites of various blue copper proteins. The trigonal planar geometry is the type 1 site of laccase from Coprinus cinereus (PDB Code 1A65). The trigonal bipyramidal geometry shown is the copper site of azurin from Pseudomonas aeruginosa (PDB Code lAZU). The trigonal pyramidal/distorted tetrahedral sites are of the stellacyanin from Cucumis sativus (PDB Code IJER), NNSO site, and of the plastocyanin from Populus nigra (PDB Code IPLC) NNSS site.

The blue copper proteins azurin, plastocyanin, stellacyanin, and umecyanin incorporate Cu bound to a combination of N/thiolate/thioether ligands. An important feature of these metalloenzymes is the facile copper(II)/(I) couple that these species exhibit, which is linked to the highly strained, asymmetric coordination geometry at the metal center. The synthesis of model complexes for these so-called Type 1 copper proteins has been reviewed. ... 

The reduction potential is central for the function of electron-transfer proteins, since it determines the driving force of the reaction. In particular, it must be poised between the reduction potentials of the donor and acceptor species. Therefore, electron-transfer proteins normally have to modulate the reduction potential of the redox-active group. This is very evident for the blue copper proteins, which show reduction potentials ranging from 184 mV for stellacyanin to 1000 mV for the type 1 copper site in domain 2 of ceruloplasmin [1,110,111]. 

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