Plastocyanin oxidized

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

M Herv/Es, JM Ortega, JA Navarro, MA De la Rosa and H Bottin (1994) Laserflash kinetic analysis of Synechocystis PCC 6803 cytochrome Cg and plastocyanin oxidation by photosystem I. Biochim Biophys Acta 1184 235-241... 

All were prepared by standard methods. Photosystem I was isolated from spinach in such a way that it retained its site of plastocyanin oxidation (Mullet et al. 1980). Ubiquinone-2 was the kind gift of Eisai Co., Ltd. and plastoquinone-2 was synthesised from geraniol and 2,3 dimethylquinol. 

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... 

Figure 3. Dependence of first-order rate constants ko6, (25 °C) vs. [Co(phen)s3+] for the oxidation of plastocyanin PCu(I). Conditions pH, 7.5 (phos) and I, 0.10 M (NaCl). Key , spinach and A, parsley. (Reproduced from Ref. 10. Copyright 1978, American Chemical Society.)...

Two additional effects with PCu(II) are of interest. Firstly Kg for Pt(NH3)5 + decreases with decreasing pH, 16500 M 1 at pH 5.8 and 6500 M 1 at pH 5.45, which is consistant with protonation at 42-45 influencing the effectiveness of Pt(NH3) 4+. Secondly Pt(NH3)5 + does not have any blocking effect on the reaction of Ananaena variabilis plastocyanin (no negative patch) with Co(phen)33+ as oxidant (16). 

N. Sutin, Brookhaven National Laboratory The 16% decrease in the rate of oxidation of reduced plastocyanin by Co(phen)33 resulting from the attachment of the chromium(III) label seems rather small if the chromium is indeed bound at or near the protein site used for electron transfer to Co(phen)33. ... 

A single-crystal study of oxidized plastocyanin demonstrated that the absence of Cu-S(Met) EXAFS was due to Debye-Waller damping (30). The analysis benefits from the three-fold enhancement of Cu-S(Met) scattering amplitude expected for... 

Table 2. Acid dissociation pK values, 1 = 0.10 M(NaCl), relating to the active site protonation of different plastocyanins, PCu(I), as determined by (a) proton NMR (b) the variation of rate constants (25 °C) with pH for the [FelCN) ] oxidation of PCu(I), 1 = 0.10 M(NaCl), and (c) similar experiments with [Co(phen)3] " as oxidant. The latter is an apparent value only, and is believed to be composite due to reaction occurring at the remote site...

These have been determined for the [Fe(CN) ] oxidation of parsley plastocyanin, and at pH7.5, I=0.10M(NaCl), are AHJ = — 3.3 kcalmol and AS = —47 cal K mol" [39]. To account for the negative enthalpy term it has been suggested that the second-order rate constant is composite incorporating... 

There are surprisingly few examples and considerable care is required. A number of earlier reports have been checked out [90,95,108], and shown to be incorrect with effects certainly not as extensive as claimed. The first plastocyanin example (1978) was the [Co(phen)3] oxidation of parsley PCu(I) [39]. In this study first-order rate constants (k bs) obtained with [Co(phen)3] in > 10-fold excess of PCu(I) ( 10 M) give a non-linear dependence on [Co(phen)3 ] (concentrations of oxidant to 3 x 10 M), Fig. 7. Such behavior can be accounted for by the reaction sequence (2)-(3),... 

Fig. 7. The variation of fiist-order rate constants (25 °C) with oxidant for the [Cotphen),] (reactant in large excess) oxidation of parsley ( ) and spinach ( ) plastocyanin, PCu(I), 1=0.10 M(NaCl) [39]...

Fig. 8. Competitive inhibition of redox inactive complexes (I) on the [Co(phen)j] oxidation of parsley plastocyanin PCu(I) [98]. Second-order rate constants (25 °C), shown as relative values, were determined at pH 5.8 (Mes), 1=0.10 MfNaO) with [Pt(NH3)6] ( ), [(NH3)5CoNHj(NH3)5]s] [Co (III)4]a (A) and [CoflllUg ( ). Full formulae of the latter two complexes are as indicated above...

Binding at the remote site has also been detected in studies on the quenching of the excited states [Cr(phen)3] and [Ru(bipy)3] by French bean plastocyanin [103]. The model adopted allows for electron transfer from the remote and adjacent sites, where at low protein concentrations the adjacent pathway is 10 times faster. At the higher concentrations of protein, up to 4 X 10 M, an interesting feature is the evidence for an adduct in which two PCu(I) molecules are associated with one inorganic complex. The oxidant is believed to be sandwiched between two PCu(I) s. 

Whatever the explanation, the sensitivity of the remote pK to oxidation state of the Cu is of potential importance in relation to the functional role of plastocyanin. Plastocyanin and its physiological electron transport partner cytochrome f are believed to have complementary surfaces which lead to efficient interaction prior to electron transfer. As will be seen below there is substantial evidence for cytochrome f(II) (as reductant) reacting at the remote site of PCu(II). One problem which may be anticipated here is how dissociation of the product... 

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)]... 

More subtle factors that might affect k will be the sites structures, their relative orientation and the nature of the intervening medium. That these are important is obvious if one examines the data for the two copper proteins plastocyanin and azurin. Despite very similar separation of the redox sites and the driving force (Table 5.12), the electron transfer rate constant within plastocyanin is very much the lesser (it may be zero). See Prob. 16. In striking contrast, small oxidants are able to attach to surface patches on plastocyanin which are more favorably disposed with respect to electron transfer to and from the Cu, which is about 14 A distant. It can be assessed that internal electron transfer rate constants are =30s for Co(phen)3+, >5 x 10 s for Ru(NH3)jimid and 3.0 x 10 s for Ru(bpy)3 , Refs. 119 and 129. In the last case the excited state Ru(bpy)3 is believed to bind about 10-12 A from the Cu center. Electron transfer occurs both from this remote site as well as by attack of Ru(bpy)j+ adjacent to the Cu site. At high protein concentration, electron transfer occurs solely through the remote pathway. 

The cytochrome b(6)f complex mediates electron transfer between the PSI and PSII reaction centers by oxidizing hpophUic plastoquinol (PQH2) (see Figure 7.24) and reducing the enzymes plastocyanin or cytochrome Ce. The electronic connection also generates a transmembrane electrochemical proton gradient that can support adenosine triphosphate (ATP) synthesis instead of electron transport. 

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