Thermodynamics of the S₂-To-S₃ state transition of the oxygen-evolving complex of photosystem II

The room temperature pump-probe X-ray free electron laser (XFEL) measurements used for serial femtosecond crystallography provide remarkable information about the structures of the catalytic (S-state) intermediates of the oxygen-evolution reaction of photosystem II. However, mixed populations of these intermediates and moderate resolution limit the interpretation of the data from current experiments. The S₃ XFEL structures show extra density near the OEC that may correspond to a water/hydroxide molecule. However, in the latest structure, this additional oxygen is 2.08 Å from the Oϵ2 of D1-E189, which is closer than the sum of the van der Waals radii of the two oxygens. Here, we use Boltzmann statistics and Monte Carlo sampling to provide a model for the S₂-To-S₃ state transition, allowing structural changes and the insertion of an additional water/hydroxide. Based on our model, water/hydroxide addition to the oxygen-evolving complex (OEC) is not thermodynamically favorable in the S₂g = 2 state, but it is in the S₂g = 4.1 redox isomer. Thus, formation of the S₃ state starts by a transition from the S₂g = 2 to the S₂g = 4.1 structure. Then, electrostatic interactions support protonation of D1-H190 and deprotonation of the Ca²⁺-ligated water (W3) with proton loss to the lumen. The W3 hydroxide moves toward Mn4, completing the coordination shell of Mn4 and favoring its oxidation to Mn(iv) in the S₃ state. In addition, binding an additional hydroxide to Mn1 leads to a conformational change of D1-E189 in the S₂g = 4.1 and S₃ structures. In the S₃ state a fraction of D1-E189 release from Mn1 and bind a proton.