Plastocyanin source

There are a number of excellent sources of information on copper proteins notable among them is the three-volume series Copper Proteins and Copper Enzymes (Lontie, 1984). A review of the state of structural knowledge in 1985 (Adman, 1985) included only the small blue copper proteins. A brief review of extended X-ray absorption fine structure (EXAFS) work on some of these proteins appeared in 1987 (Hasnain and Garner, 1987). A number of new structures have been solved by X-ray diffraction, and the structures of azurin and plastocyanin have been extended to higher resolution. The new structures include two additional type I proteins (pseudoazurin and cucumber basic blue protein), the type III copper protein hemocyanin, and the multi-copper blue oxidase ascorbate oxidase. Results are now available on a copper-containing nitrite reductase and galactose oxidase. 

Basic blue proteins have been isolated from a number of plant sources and have previously been referred to as plantacyanin (Aikazyan and Nalbandyan, 1975,1979 Sakurai et al, 1982). The protein from cucumber has been the most extensively studied of the basic blue proteins and the crystal structure is now available (Guss et al., 1988). The function of this basic blue protein is unknown, however, it is probably not involved in photosynthetic electron transport as it will not replace plastocyanin in that electron transport chain (Adman, 1985). 

Intriguingly, the blue copper sites, especiaUy those with a carbonyl oxygen at the axial coordination position, display high affinity for Zn + ions. Mutants in which the Met is replaced by Gin or Glu preferentiaUy bind Zn + when expressed in heterologous systems, e.g., Escherichia coli. Examples include azurin, amicyanin, nitrite reductase, and possibly also plastocyanin (Diederix et al., 2000 Hibino et al., 1995 Murphy et al., 1995 Nar et al., 1992a Romero et al., 1993). In the case of azurin it has been shown that both wild-type and the Met—Gin mutant have the same affinity for both Zn +and Cu + (Romero ci a/., 1993). In addition, EXAFS studies showed that some preparations of blue copper proteins purihed from their natural sources also contain small fractions of Zn derivatives (DeBeer George, personal communication). 

These mobile electron carriers are relatively low-molecular-weight proteins and are both linked to the PS-1 reaction-center complex by electrostatic forces. Plastocyanin is located on the lumen side and is the electron donor to P700, while ferredoxin is located on the stroma side and receives electrons from the terminal, membrane-bound iron-sulfur proteins FeS-A/B. Water is the ultimate source of electrons for plastocyanin reduction. The electron from water is generated by its oxidation by photosystem 11, and transferred by way of cytochrome/in the cytochrome b(f complex. Reduction of NADP by ferredoxin requires mediation by the enzyme ferredoxin-NADP -reductase orFNR. Figure 1, right is a schematic representation of the relationship of the PS-1 reaction center to the peripheral electron carriers plastocyanin and ferredoxin as well as the protein catalyst ferredoxin-NADP -reductase. 

Fig. 2. Absorption spectrum of plastocyanin. Figure source Katoh (1982) Plastocyanin, in CRC Handbook of Biosoiar Resources, Voi 1, Basic Principles, p 162. CRC Press.

Fig. 4. Stereogram of the negative patch in plastocyanin. The seven residues that form the negative charges are shown. See text for discussion. The molecule Is rotated 60° about the vertical axis from that shown in Fig. 3 (B) to allows better view of the eastern side. Figure source Redinbo, Cascio, Chaukair, Rice, Merchant and Yeates (1993) The 1.5 A crystal structure ofplastocyanin from the green alga Chlamydomonas reinhardtii. Biochemistry 32 10564.

Fig. 5. Flash-induced absorbance change due to P700 photooxidation and dark reduction monitored at 703 nm in broken chloro-plasts in the presence of high concentrations of salt and sorbitol. The accompanying table shows effects of salts and sorbitol on the amplitude of P700 re-reduction by plastocyanin. See text for discussion. Figure source Haehnel, Prdpper and Krause (1980) Evidence forcompiexed piastocyanin as the immediate eiectmn donor of P-700. Biochim Biophys Acta 593 387, 389.

Fig. 7. Flash-induced absorbance changes at 703 nm in PSI-200 particles (a) wild-type PC (b) PC (Tyr-83- Leu) (c) PC (Ala-90->Leu) and (d) PC (Gly-10- Leu). Note change in time scale marked by the vertical dashed line. Figure source Haehnel, Jansen, Cause, KlOsgen, Stahl, Michl, Huvermann. Karas and Herrmann (1994) Electron transfer from plastocyanin to photosystem I. EMBO J 13 1033.

Fig. 8. Re-reduction Kinetics of P700 in the compiexes formed between PSi-200 particies with wild-type and mutant plastocyanins using /V-ethyl-3[3-(dimethylamino)propyl]carbodiimide (EDC) as the cross-linking agent. Nd-YAG laser flashes were used for excitation. The kinetic traces were presented on divided time scales of 0.2 and 2 ms per division, respectively, (a) PSI/wild-type PC (b), (c) and (d) PSI/mutant PC complexes (see legend on the right side for details). Decay time constants r.,sare also listed in the legend at right. Figure and data source Hippier, Reichert, Sutter, Zak, Altschmied, SchrOer, Herrmann and Haehnel (1996) The plastocyanin binding domain of photosystem I. EMBO J 15 6378.

Fig. 9. Room-temperature absorption spectrum of spinach Cytbe in the presence of dithionite (A) reduced-minus-oxidized difference spectra as indicated for spinach Cyt bg compiex measured at room temperature in (A) inset and at low temperature (77 K) in (B) (C) 77 K difference spectra of the isolated (spinach) Cyt bef complex titrated to the potentials indicated. See text for details. Figure source (A) and (B) Hurt and Hauska (1981) A cytochrome flb complex of five polypeptides with plastoquinol-plastocyanin-oxidoreductase activity from spinach chloroplasts. Eur J Biochem 117 594 (C) Hurt and Hauska (1983) Cytochrome /) from isolated cytochrome complexes. Evidence for two spectral forms with different midpoint potentials. FEBS Lett 153 415.

Fig. 10. (A) EPR spectrum of purified spinach Cyt-dsfcompiex. Sampie was reduced with 2 mM piastoquinoi-1 before freezing in iiquid nitrogen (B) EPR spectra of purified Cyt-befcompiex in the reduced state (upper trace) and in the presence of 1 equivaient of DBMiB(iowertrace). Figure source (A) Hurt and Hauska 1981)Acytochromef/b6Complexoffivepolypeptideswithplastoquinol-plastocyanin-oxidoreductase activity from spinach chloroplasts. EurJ Biochem 117 595 (B) Maikin (1982) interaction of photosynthetic eiectron transport inhibitors and the Rieske iron-suifur center in chioropiasts and the cytochrome be-f comolex Biochemistry 21 2947.

Cross reactions of plastocyanins and azurins from various sources with cytochromes can be fitted to the Marcus equation using constant self-exchange rate constants of 6.6 x 10 s for the plastocyanins, 9.9 x 10 s for Ps. 

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