A Tris‐induced change in the midpoint potential of Z, the donor to photosystem II, as determined by the kinetics of the back reaction

Abstract

A new method of measuring the rate of the back reaction from the state Z+ P680 QA-in Tris-washed chloroplasts is described. By using ratios of back reaction rates we demonstrate a Tris-induced change in the equilibrium between Z end P680 and attribute this change to an alteration of the midpoint potential of Z by Tris treatment. We also demonstrate that the previously observed inhibition of the back reaction by ADRY reagents can be localized at Z and understood in terms of electron donation to Z' by ADRY reagents. Tris-washed chloroplasts Photosystem II EPR signal II_ ADR Y reagentBack reaction pH effect

Electron transfer in photosystem II

“…The couple IT/D has been titrated at 0.76 V, suggesting a value of about 1.00V for Z~/Z i [18]. The back reaction of Z~-Q-is about 2 x 10 a times slower than that of P÷Q- [119], so the midpoint potential of P÷/P might be about 0.058 log (2 x 103) = 0.2 V higher than that of Z~./Zi, near 1.2 V. In the excited state (about 1.8 eV above the ground state) the midpoint potential is 1.8 V lower, so the midpoint potential of P÷/P* would be about --0.6 V, close to the value obtained by titration for I/I- [58,86]. Titrations allow only a rough estimate of the operating potential, but apparently they are not too far off in this case.…”

“…The interpretation might apply both for a series and for a parallel arrangement of Z1 and Z2, but adds an interesting possibility to explain the effects of Tris-washing if ZI and Z2 operate in series: if Tris-treatment somehow results in an even larger electrostatic effect on the equilibrium P+Za ~ P Zi ~, the equilibrium concentration of Z~" would be low, explaining its apparent absence in Tris-washed samples and explaining the slow electron transfer from Z2 to P* as well, assuming no effect of Triswashing on Z2. Tris-washing was claimed to increase the equilibrium constant P Z~/P*Z2 by at least two orders of magnitude [119], but that depends on the unknown rate constant of electron transfer from P to Z~, which remained as adjustable parameter in Bouges-Bocquet's model [15].…”

UV‐spectral characterization in Tris‐washed chloroplasts of the redox component D1 which functionally connects the reaction center with the water‐oxidizing enzyme system Y in photosynthesis

“…(ii) The variations at the same pH of the halflifetimes by a factor of up to 5 reported for the reaction between D;I" and Qn under different conditions (see [14,15,20,26]) suggest that the system is more complex than assumed for the analysis of the data in 1261.…”

“…These findings might favor Di being deprotonated. This idea has been questioned on the basis of EPR data which were claimed to reflect the back reaction between QA and Dy [26]. However, for two reasons it seems worthwhile to confirm this conclusion by independent lines of evidence: (3 222 It has to be shown that the equilibrium between P680+Di and P68O(D~)+ is really pHindependent, because the functional groups of P680 and Dr are embedded in protein matrices, which could be modified by pH, thereby giving rise to a relative redox potential shift.…”

A portable multi-flash kinetic fluorimeter for measurement of donor and acceptor reactions of Photosystem 2 in leaves of intact plants under field conditions

“…5, a very similar pattern is observed at 30/xs in intact leaves. In order to demonstrate that the series four pattern was related to the turnover of the oxygen evolving complex, leaves were infused with the reagent CCCP which deactivates the oxygen evolving complex (Renger 1969) by + donating electrons directly to the oxidized Yz (Yerkes 1981, Ghanotakis et al 1982, Yerkes et al 1983). This treatment led to a dephasing of the period four oscillations in a manner dependent upon CCCP concentration and the fre- of 10 min, a control leaf (no additions) was illuminated by approx.…”