Studies on the reaction coordinates of the water oxidase in PS II membrane fragments from spinach

Abstract

The temperature dependency of the rate constants of the univalent redox steps Y_z ^oxS _i → Y_zS _i+1 (i = 0,1,2) and Y_z ^oxS₃ → (Y_zS₄) → Y_zS_o + 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 Y_z ^oxS _i → Y_zS _i+1 with i = 0,1 and 2, respectively. The reaction Y_z ^oxS₃ → (Y_zS₄) → Y_zS₀ + 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.