Kinetic modeling of substrate-water exchange in Photosystem II.

We derive a model that provides an exact solution to the substrate-water exchange kinetics in a double-conformation system and use this model to interpret recently published data for Ca- and Sr-containing PSII in the S state, in which the g = 2.0 and g = 4.1 conformations coexist. The component concentrations derived from the kinetic model provide an analytic description of the substrate-water exchange kinetics, allowing us to more accurately interpret the results. Based on this model and the previously reported data on the S state g = 2.0 conformation, we obtain the substrate-water exchange rates of the g = 4.1 conformation and the conformational change rates. Two conclusions are made from the analyses. First, contrary to previous reports, there is no significant effect of substituting Sr for Ca on any of the exchange rate constants. Second, the exchange rate of the slowly-exchanging water (W) in the S state g = 4.1 conformation is much faster than that in the S state g = 2.0 conformation. The second conclusion is consistent with the assignment of W to W1 or W2 bound as terminal ligands to Mn4; Mn4 has been proposed to undergo an oxidation state change from Mn(IV) in the g = 2.0 conformation to Mn(III) in the g = 4.1 conformation.