Protein, stellacyanin

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... 

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]. 

There are four other proteins - stellacyanin, rusticyanin, umecyanin and ami-cyanin (Table 3) which have been fairly extensively studied. A crystal structure determination for amicyanin from Thiobacillus versutus is now under way [61]. A number of other type 1 proteins have been identified. These include pseudo-... 

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).

Another interesting blue protein is stellacyanin (FW = 20 000) from the Japanese lacquer tree Rhus vernicifera, in which, with respect to the other cupredoxins, glutamine replaces the methionine ligand.64 Stellacyanin also bears an overall positive charge (p/=9.9). It, therefore, gives a reversible Cu(II)/Cu(I) response at a glassy carbon electrode in aqueous solution (pH 7.6).61 The formal electrode potential of the Cu(II)/Cu(I) reduction (E01 = + 0.18 V vs. NHE) is the lowest among cupredoxins. 

Mavicyanin (Mj = 18,000) is obtained from green squash (Cucurbito pepo medullosa), where it occurs alongside ascorbate oxidase [64]. It has a peak at 600 nm (e 5000 M cm and reduction potential of 285 mV. Further studies on this and the mung bean and rice bran proteins [65, 66] would be of interest. All the above type 1 Cu proteins have an intense blue color and characteristic narrow hyperfine EPR spectrum for the Cu(II) state. Table 3 summarizes the properties of those most studied. There is some variation in reduction potential and position of the main visible absorbance peak. In the case of azurin, for example, the latter is shifted from 597 to 625 nm. Stellacyanin has no methionine and the identity of the fourth ligand is therefore different [75]. The possibility that this is the 0(amide) of Gln97 has been suggested [63b]. It now seems unlikely that the disulfide is involved in coordination. Stellacyanin has 107 amino acids, with carbohydrate attached at three points giving a 40% contribution to the M, of 20,000 [75]. 

Spectra, but, in general, leaves the copper site the most exposed of the four cupredoxins. The sequence of Cbp is quite similar to that of stella-cyanin. Stellacyanin is a plant protein, also of unknown function, having visible spectra characteristic of type I copper, but lacking the methionine ligand found in all other type I proteins. A disulfide bond has been suggested as a potential copper ligand in stellacyanin the Cbp has both a methionine and the disulfide, so that prior to the structure determina-... 

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... 

Stellacyanin from Rhus vemicifera is less well studied. The polypeptide does not contain a methionine residue showing that the ligands of the type 1 site may vary. There is EXAFS evidence for a short Cu—S(Cys) bond,920 while UV, visible and near IR studies confirm similarities with the other blue proteins. It has been suggested that methionine is replaced as a ligand by an —S—S— group. This is based on resonance Raman921 and NMR studies.922... 

There are many instances for azurin and plastocyanin where limiting kinetic behaviour is observed, and attributed to the formation of an adduct between the protein and the inorganic complex followed by electron transfer. Values of the association constants and of the electron-transfer rate constants may then be calculated. This situation has not been observed in the case of stellacyanin, which differs from azurin and plastocyanin in that it has an overall positive charge at pH 7 (of +7 in the case of the reduced protein). The electron-transfer rate constants are often associated with fairly large negative values for the entropy of activation (in the range -84 to -210 J K1 mol-1), which are not expected for electron transfer within a compact assembly. 

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