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

Abstract

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 S_0–4 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 bands in the mid-infrared region (1600–1200 cm^–1). However, the S₃ to S₀ and S₀ to S₁ spectra and the flash dependence of W₅⁺ decay are not significantly altered in strontium PSII. The observed decrease in mid-infrared intensity is consistent with inhibition of a protein reorganization event, which may be associated with a strontium-induced change in S₃ charge distribution. These data provide evidence that strontium replacement alters the S₂ to S₃ conformational landscape