ANALYSIS OF THE REACTION COORDINATE OF PHOTOSYNTHETIC WATER OXIDATIONBY KINETIC MEASUREMENTS OF 355 NM ABSORPTION CHANGES AT DIFFERENT TEMPERATURES IN PHOTOSYSTEM-II PREPARATIONS SUSPENDED IN EITHER H2O OR D2O

Flash-induced absorption changes at 355 nm were measured at different temperatures within the range of 2 degrees C less than or equal to cur ly theta less than or equal to 25 degrees C in dark-adapted PS II core complexes from spinach [O-2 evolution rate: 1500 +/- 100 mu mol of O- 2 (mg of Chi)(-1) h(-1)] that were dissolved either in H2O- or in D2O- containing buffer. Comparative measurements were performed at 20 degre es C in H2O- or D2O-containing suspensions of PS II membrane fragments [O-2 evolution rate: 600 +/- 40 mu mol of O-2 (mg of Chi)(-1) h(-1)]. The results obtained reveal the following: (a) The activation energie s of the individual redox steps in the water oxidizing complex (WOC) a re dependent on tile redox state S-i with E-A(S-1-->S-2) = 14 kJ/mol, E-A(S-2-->S-3) = 35 kJ/mol, and. E-A(S-3-->-->S-0 + O-2) = 21 kJ/mol f or curly theta > 11 degrees C, 67 kJ/mol for curly theta < 11 degrees C in PS II core complexes dissolved in H2O; (b) replacement of exchang eable protons by deuterons causes only minor changes (less than or equ al to 15%) of the activation energies; and (c) the rate constants of t hese reactions in PS II fore complexes are characterized by H/D isotop e ratios, k(i)(H)/k(i)(D), of 1.6, 2.3, and 1.5 for the transitions S- 1-->S-2, S-2-->S3, and S-3-->-->S-0 + O-2, respectively. The correspon ding values of PS II membrane fragments are 1.3, 1.3, and 1.4, Based o n these results and corresponding E-A data reported in the literature for PS II membrane fragments from spinach [Renger, G., & Hanssum, B. ( 1992) FEES Lett. 299, 28-32] and PS II particles from the thermophilic cyanobacterium Synechococcus vulcanus Copeland [Koike, II., Hanssum, B., Inoue, Y., & Renger, G. (1987) Biochim. Biophys. Acta 893, 524-533 ], the reaction coordinate of the redox sequence in the WOC is inferre d to be almost invariant to the evolutionary development from cyanobac teria to higher plants. Furthermore, the rather high activation energy of the S-2-->S-3 transition provides evidence fur a significant struc tural change coupled with this reaction, Implications for the mechanis m of photosynthetic water oxidation are discussed.