Copper plastocyanin

Structure and electron transfer reactivity of the blue copper protein, plastocyanin. A. G. Sykes, Chem. Soc. Rev., 1985,14, 283 (117). 

Blue copper proteins. A typical blue copper redox protein contains a single copper atom in a distorted tetrahedral environment. Copper performs the redox function of the protein by cycling between Cu and Cu. Usually the metal binds to two N atoms and two S atoms through a methionine, a cysteine, and two histidines. An example is plastocyanin, shown in Figure 20-29Z>. As their name implies, these molecules have a beautiful deep blue color that is attributed to photon-induced charge transfer from the sulfur atom of cysteine to the copper cation center. 

In the blue, Type I copper proteins plastocyanin and azurin, the active-site structure comprises the trigonal array [CuN2S] of two histidine ligands and one cysteine ligand about the copper,... 

Copper proteins present interesting problems of structure for the copper(I) oxidation state. They are difficult to probe in detail, and what we do know of them suggests they are rarely regular or predictable.58 In plastocyanin the copper(I) coordination sphere is made up of three strongly... 

The electrons subsequently pass to plastocyanin (PC), which is a copper-containing protein. The Cu-containing redox center of this 10.5 kD monomer cycles between Cu(I) and Cu(II) oxidation states. The structure of PC shows that... 

This discussion of copper-containing enzymes has focused on structure and function information for Type I blue copper proteins azurin and plastocyanin, Type III hemocyanin, and Type II superoxide dismutase s structure and mechanism of activity. Information on spectral properties for some metalloproteins and their model compounds has been included in Tables 5.2, 5.3, and 5.7. One model system for Type I copper proteins39 and one for Type II centers40 have been discussed. Many others can be found in the literature. A more complete discussion, including mechanistic detail, about hemocyanin and tyrosinase model systems has been included. Models for the blue copper oxidases laccase and ascorbate oxidases have not been discussed. Students are referred to the references listed in the reference section for discussion of some other model systems. Many more are to be found in literature searches.50... 

The nature of the ligand donor atom and the stereochemistry at the metal ion can have a profound effect on the redox potential of redox-active metal ions. The standard redox potentials of Cu2+/Cu+, Fe3+/Fe2+, Mn3+/Mn2+, Co3+/Co2+, can be altered by more than 1.0 V by varying such parameters. A simple example of this effect is provided by the couple Cu2+/Cu+. These two forms of copper have quite different coordination geometries, and ligand environments, which are distorted towards the Cu(I) geometry, will raise the redox potential, as we will see later in the case of the electron transfer protein plastocyanin. 

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... 

Plastocyanin from parsley, a copper protein of the chloroplast involved in electron transport during photosynthesis, has been reported to have a fluorescence emission maximum at 315 nm on excitation at 275 nm at pH 7 6 (2°8) gjncc the protein does not contain tryptophan, but does have three tyrosines, and since the maximum wavelength shifts back to 304 nm on lowering the pH to below 2, the fluorescence was attributed to the emission of the phenolate anion in a low-polarity environment. From this, one would have to assume that all three tyrosines are ionized. A closer examination of the reported emission spectrum, however, indicates that two emission bands seem to be present. If a difference emission spectrum is estimated (spectrum at neutral pH minus that at pH 2 in Figure 5 of Ref. 207), a tyrosinate-like emission should be obtained. 

M. T. Graziani, A. L. Agro, G. Rotilio, D. Barra, and B. Mondovi, Parsley plastocyanin. The possible presence of sulfhydryl and tyrosine in the copper environment, Biochemistry 13, 804-809 (1974). 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

Fig. 4. View of the blue copper and a5Rupis59) centers in ruthenated Anabaena variabilis plastocyanin. The edge-edge distance is 11.9 A [39]...

---