Plastocyanin reduction potentials

Summary of reduction potentials and rate constants for reactions with plastocyanin PCu(I) and PCu(II) at 25<>C, pH 7-8,... 

Reduction potentials (Eg) for different plastocyanins, the PCu(II)/PCu(I) couple, have been determined by spectrophotometric titration against, e.g. [Fe(CN)e] . At pH 7.5 for higher plant and green algal plastocyanins values are close to 370 mV at 25 °C, 1=0.10 M(NaCl) [1]. Thus French bean gives a value 360 mV and S. obliquus 363 mV [50]. However, A. variabilis gives an... 

Earlier suggestions that the two uncoordinated and invariant residues His35 (inaccessible to solvent and covered by polypeptide) and His83 (remote and 13 A from Cu) are, from effects of [H ] on rate constants (and related pKg values), sites for electron transfer may require some re-examination. Thus, it has been demonstrated in plastocyanin studies [50] that a surface protonation can influence the reduction potential at the active site, in which case its effect is transmitted to all reaction sites. In other words, an effect of protonation on rate constants need not necessarily imply that the reaction occurs at the site of protonation. His35 is thought to be involved in pH-dependent transitions between active and inactive forms of reduced azurin [53]. The proximity of... 

Reduction potentials of the S. obliqms His59 Ru(NH3)5-modified protein have been determined by cyclic voltammetry using as electrode the oxidized surface obtained by polishing the edge plane of pyrolytic graphite [137]. The modified protein responds well at the electrode, whereas the native protein requires multi-eharged cations, e.g. Mg or [Cr(NH3)g] as mediators to give satisfactory reversibility. Separate reduction potentials at 1=0.10 M(NaCl) for native S. obliquus plastocyanin (389 mV) and [Ru(NH3)5 (imidazole)]... 

The kinetics of the reactions of horse cytochrome c(II), M, 12,400, (charge 8+) reduction potential 260 mV, with parsley and French bean plastocyanins PCu(II) (charges — 7 and — 8 respectively), have been studied. As in the case of HIPIP, cytochrome c is not a physiologically relevant protein. It is nevertheless important in assessing different approaches prior to investigating the reactions of physiological redox partners. In the case of the reaction of parsley PCu(II) with cytochrome c(II), the rate constant (25 °C) is 1.5 X 10 s at pH7.6, 1 = 0.10 M(NaCl) [141]. There is no evidence... 

Blue copper proteins, 36 323, 377-378, see also Azurin Plastocyanin active site protonations, 36 396-398 charge, 36 398-401 classification, 36 378-379 comparison with rubredoxin, 36 404 coordinated amino acid spacing, 36 399 cucumber basic protein, 36 390 electron transfer routes, 36 403-404 electron transport, 36 378 EXAFS studies, 36 390-391 functional role, 36 382-383 occurrence, 36 379-382 properties, 36 380 pseudoazurin, 36 389-390 reduction potentials, 36 393-396 self-exchange rate constants, 36 401-403 UV-VIS spectra, 36 391-393 Blue species... 

This complex catalyzes the reaction through the Q cycle (Section 18.3.4). In the first half of the Q cycle, plastoquinol is oxidized to plastoquinone, one electron at a time. The electrons from plastoquinol flow through the Fe-S protein to convert oxidized plastocyanin into its reduced form. Plastocyanin is a small, soluble protein with a single copper ion bound by a cysteine residue, two histidine residues, and a methionine residue in a distorted tetrahedral arrangement (Figure 19.17). This geometry facilitates the interconversion between the Cu2+ and the Cu+ states and sets the reduction potential at an appropriate value relative to that of plastoquinol. Plastocyanin is intensely blue in color in its oxidized form, marking it as a member of the "blue copper protein," or type I copper protein family. 

Given that the reduction potentials for plastocyanin and ferredoxin are +0.37 V and -0.45 V, respectively, the standard free energy for the oxidation of reduced plastocyanin by oxidized ferredoxin is +18.9 kcal mob (+79.1 kJ mofi). This uphill reaction is driven by the absorption of a 700-nm photon which has an energy of 40.9 kcal mofi (171 kJ mob )... 

Moreover, other effects are as important as the ligands. The dielectric properties of the protein matrix are very different from those of water. It has often been argued that it behaves as a medium with a low dielectric constant (around 4 compared to 80 in water) [47,123,124]. Figure 11 shows that this gives rise to a very prominent change in the reduction potential of a blue-copper site [45]. It increases by 0.8-1 V as the site is moved from water solution to the centre of a protein with a radius of 1.5 nm (like plastocyanin) or 3.0 nm (like an azurin tet-ramer). It can also be seen that it is not necessary to move the site to the centre of the protein to get a full effect. Already at the surface of the protein, 80% of the maximum effect is seen, and when the site is 0.5 nm from the surface (as is typi-... 

The reduction potentials of different plastocyanins increase as the pH is decreased below 7 (Fig. 8), due to protonation of His 87 at the Cu(I) active site and resultant redox inactivation. Much of this information has been obtained from rate constants (18, 57, 58). The reduction potential of the basic A. variabilis plastocyanin is noticeably smaller than for plastocyanins from higher plant and green algal... 

Fk 8. Variation of reduction potential with pH for the spinach, parsley, and Ana-baena variabilis plastocyanins, the PCu(II)/PCu(I) couple. 

Although the Cu(II) form of T. versutus amicyanin is stable down to pH 4, the reduced form begins to denature below pH 6.4 (63). An active site protonation of the Cu(I) protein similar to that of plastocyanin, but with pKa 7.18, affects the reduction potential. 

Olsson, M.H.M., Hong, G.Y., Warshel, A. Frozen density functional free energy simulations of redox proteins Computational studies of the reduction potential of plastocyanin and rusticyanin. J. Am. Chem. Soc. 2003,125, 5025-39. 

Given that the reduction potentials for plastocyanin and ferredoxin are + 0.37 V and —0.45 V, respectively, the standard free energy for this reaction... 

Battistuzzi G, Borsari M, Loschi L, Menziani MC, De Rienzo F, Sola M (2001) Control of metalloprotein reduction potential the role of electrostatic and solvation effects probed on plastocyanin mutants. Biochemistry 40 6422-6430... 

The measurement of reduction potential is an important step in establishing the order of chain elements. Reduction potentials vary with relative concentrations of the two redox species, of course, with an order of magnitude increase in ratio of reduced to oxidized form leading to a 60-mV increase in reduction potential in one electron reactions. Differences of a few tens of millivolts between the standard reduction potentials of cytochrome / and plastocyanin in green plant photosynthesis are not enough to establish their relative order since relative concentrations could reverse the actual reduction potentials. Still, a cytochrome c with a standard reduction potential of -(-50 mV is unlikely to donate electrons to... 

The conserved hydrogen bond network in cupredoxins is believed to play a role in maintaining the geometry of the copper center and in determining the relative constraints of the Cu and Cir sites, which in turn influences the relative stability of the two states of the protein and, ultimately, the reduction potential. " Indeed, mutations of the Asn in the extra-loop H-bond (i.e., the Asn next to the N-terminal His ligand) in both azurin and plastocyanin resulted in reduced stability and, in the case of azurin, an increase of the reduction potential from 286 mV for wild-type azurin to 396 mV for the variant protein. In rusticyanin, the Asn residue in the extra-loop H-bond is replaced with a Ser residue.Mutation of this Ser in rusticyanin resulted in reduced stability and a decrease of reduction potential by 110 mV. Based on these results, the authors... 

Blue copper proteins transfer electrons between various biological systems, e.g., between the two photosystems in photosynthesis (plastocyanin). They are characterized by a number of unusual properties, viz., a bright blue color, an unusually high reduction potential, and distinctive... 

---