Calcium, Strontium, and Protein Dynamics during the S2 to S3 Transition in the Photosynthetic Oxygen-Evolving Cycle

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Photosystem II (PSII) catalyzes the oxidation of water at a Mn4CaO5 cluster. The mechanism of water oxidation requires four sequential photooxidation events and cycles the OEC through the S0–4 states. Oxygen is released during a thermal transition from S4 to S0, and S1 is the dark stable state. Calcium is required for activity, and, of substituted cations, only strontium supports activity but at a lower steady-state rate. The S1 to S2 transition corresponds to a Mn oxidation reaction. Previously, we used divalent ion substitution to provide evidence that calcium activates water and that an internal water cluster (W5+) is protonated during the S1 to S2 transition. For the next transition, S2 to S3, either a Mn or a ligand oxidation event has been proposed. Here, we use strontium reconstitution and reaction-induced FT-IR spectroscopy to study this transition. We show that strontium substitution has a dramatic effect on the infrared spectrum of the S2 to S3 transition, reducing the intensity of all spectral ...