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  • 1
    ISSN: 1573-5079
    Keywords: cytochrome bf complex ; electron transfers ; kinetics ; quinol oxidation ; Q-cycle
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The kinetics of oxidation and reduction of P700, plastocyanin, cytochrome f and cytochrome b-563 were studied in a reconstituted system consisting of Photosystem I particles, cytochrome bf complex and plastocyanin, all derived from pea leaf chloroplasts. Decyl plastoquinol was the reductant of the bf complex. Turnovers of the system were initiated by laser flashes. The reaction between oxidised P700 and plastocyanin was non-homogeneous in that a second-order rate coefficient of c. 5×10−7 M−1 s−1 applied to 80% of the P700+ and c. 0.7×107 M−1 s−1 to the remainder. In the presence of bf complex, but without quinol, the electron transfer between cytochrome f and oxidised plastocyanin could be described by a second-order rate coefficient of c. 4×107 M−1 s−1 (forward), and c. 1.6×107 M−1 s−1 (reverse). The equilibrium coefficient was thus 2.5. Unexpectedly, there was little reduction of cytochrome f + or plastocyanin+ by electrons from the Rieske centre. With added quinol, reduction of cytochrome b-563 occurred. Concomitantly, electrons appeared in the oxidised species. It was inferred that either the Rieske centre was not involved in the high-potential chain of electron transfer events, or that, only in the presence of quinol, electrons were quickly passed from the Rieske centre to cytochrome f +. Additionally, the presence of quinol altered the equilibrium coefficient for the cyt f/PC interaction from 2.5 to c. 5. The reaction between quinol and the bf complex was describable by a second-order rate coefficient of about 3×106 M−1 s−1. The pattern of the redox reactions around the bf complex could be simulated in detail with a Q-cycle model as previously found for chloroplasts.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1573-5079
    Keywords: electron transfers ; decyl plastoquinol ; plastocyanin ; P700
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Rate-coefficients describing the electron transfer reactions between P700 and plastocyanin, between cytochromef in cytochromebf complexes and plastocyanin, and between decyl plastoquinol and cytochromebf complexes were determined as a function of pH in the range 4–10 from flash-induced absorbancy changes at four wavelengths. The reactions between P700 and plastocyanin, and between cytochromef and plastocyanin were optimised when there was electrostatic interaction between ionised acidic groups in plastocyanin with a pKa of 4.3–4.7 and ionised basic constituents in P700 (assumed to be in the PSI-F subunit) and in cytochromef, with a pKb of 8.9–9.4. The basic groups are thought to be lysine rather than arginine. This mechanism agrees with that inferred from effects of ionic strength changes on rate-coefficients. The relation between the second-order rate-coefficient for decyl plastoquinol oxidation by thebf complex and pH was characterised by a pKa of 6.1. This is interpreted as showing that the anion radical form of that quinol, which has a pKa of 6, and which becomes progressively protonated when pH is changed from 7 to 5, is essential to reduce cytochromeb-563 (low potential) during quinol oxidation. Above pH 9, permanent effects were observed on this rate-coefficient, which were absent in the reactions between P700, plastocyanin and cytochromef.
    Type of Medium: Electronic Resource
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