STUDIES ON THE REACTION COORDINATES OF THE WATER OXIDASE IN PS-II MEMBRANE-FRAGMENTS FROM SPINACH

The temperature dependence of the rate constants of the univalent redox steps YzoxSi --> YzSi + 1 (i = 0,1,2) and YzoxS3 --> (YzS4) --> YzSo + O2 in the water oxidase was investigated by measuring time resolved absorption changes at 355 nm induced by a laser flash train in dark adapted PS II membrane fragments from spinach. Activation energies of 5.0, 12.0 and 36.0 kJ/mol were obtained for the reactions YzoxSi --> Y2Si + 1 with i = 0,1 and 2, respectively. The reaction YzoxS3 --> (YzS4) --> YzS0 + O2 exhibits a temperature dependence with a characteristic break point at 279 K with activation energies of 20 kJ/mol (T > 279 K) and 46 kJ/mol (T < 279 K). Evaluation of the data within the framework of the classical Marcus theory of nonadiabatic electron transfer [(1985) Biochim. Biophys. Acta 811, 265 322] leads to the conclusion that the S2 oxidation to S3 is coupled with significant structural changes. Furthermore, the water oxidase in S3 is inferred to attain two different conformational states with populations that markedly change at a characteristic transition temperature.