Analysis of the Reaction Coordinate of Photosynthetic Water Oxidation by Kinetic Measurements of 355 nm Absorption Changes at Different Temperatures in Photosystem II Preparations Suspended in Either H2O or D2O

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Abstract
Flash-induced absorption changes at 355 nm were measured at different temperatures within the range of 2 °C ≤ ϑ ≤ 25 °C in dark-adapted PS II core complexes from spinach [O2 evolution rate: 1500 ± 100 μmol of O2 (mg of Chl)-1 h-1] that were dissolved either in H2O- or in D2O-containing buffer. Comparative measurements were performed at 20 °C in H2O- or D2O-containing suspensions of PS II membrane fragments [O2 evolution rate: 600 ± 40 μmol of O2 (mg of Chl)-1 h-1]. The results obtained reveal the following: (a) The activation energies of the individual redox steps in the water oxidizing complex (WOC) are dependent on the redox state Si with EA(S1→S2) = 14 kJ/mol, EA(S2→S3) = 35 kJ/mol, and EA(S3→→S0 + O2) = 21 kJ/mol for ϑ > 11 °C, 67 kJ/mol for ϑ < 11 °C in PS II core complexes dissolved in H2O; (b) replacement of exchangeable protons by deuterons causes only minor changes (≤15%) of the activation energies; and (c) the rate constants of these reactions in PS II core complexes are characterized by H/D isotope ratios, ki(H)/ki(D), of 1.6, 2.3, and 1.5 for the transitions S1 → S2, S2 → S3, and S3 →→ S0 + O2, respectively. The corresponding values of PS II membrane fragments are 1.3, 1.3, and 1.4. Based on these results and corresponding EA data reported in the literature for PS II membrane fragments from spinach [Renger, G., & Hanssum, B. (1992) FEBS Lett. 299, 28−32] and PS II particles from the thermophilic cyanobacterium Synechococcus vulcanus Copeland [Koike, H., Hanssum, B., Inoue, Y., & Renger, G. (1987) Biochim. Biophys. Acta 893, 524−533], the reaction coordinate of the redox sequence in the WOC is inferred to be almost invariant to the evolutionary development from cyanobacteria to higher plants. Furthermore, the rather high activation energy of the S2 → S3 transition provides evidence for a significant structural change coupled with this reaction. Implications for the mechanism of photosynthetic water oxidation are discussed.