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Understanding the water oxidation mechanism in Photosystem II (PSII) stimulates the design of biomimetic artificial systems that can convert solar energy into hydrogen fuel efficiently. The Sr^2+-substituted PSII is active but slower than with the native Ca²⁺ containing PSII as an oxygen evolving catalyst. Here, we use Density Functional Theory (DFT) to compare the energetics of the S₂ to S₃ transition in the Mn₄O₅Ca²⁺ and Mn₄O₅Sr²⁺ clusters. The calculations show that deprotonation of the water bound to Ca²⁺ (W3), required for the S₂ to S₃ transition, is energetically more favorable in Mn₄O₅Ca²⁺ than Mn₄O₅Sr²⁺. In addition, we have calculated the pK_a of the water that bridges Mn4 and the Ca²⁺/Sr²⁺ in the S₂ state using continuum electrostatics. The calculations show that the pK_a is higher by 4 pH units in the Mn₄O₅Sr²⁺cluster.