Abstract
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(2+)- and Sr(2+)-containing PSII in the S(2) 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(2) 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(2+) for Ca(2+) on any of the exchange rate constants. Second, the exchange rate of the slowly-exchanging water (W(s)) in the S(2) state g = 4.1 conformation is much faster than that in the S(2) state g = 2.0 conformation. The second conclusion is consistent with the assignment of W(s) 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.