New insights into the involvement of residue D1/V185 in photosystem II function in Synechocystis 6803 and Thermosynechococcus vestitus

The effects of D1-V185T and D1-V185N mutations in Photosystem II (PSII) from Thermosynechococcus vestitus (formerly T. elongatus) and Synechocystis 6803, respectively, were studied using both EPR and optical kinetics. EPR spectroscopy reveals the presence of a mixture of a S₂ state in a high spin configuration (S₂^HS) and in a low spin configuration (S₂^LS) in both mutants. In contrast to the S₂^HS in the wild type, the S₂^HS state in the D1-V185T mutant does not progress to the S₃ state at 198 K. This inability is likely due to alterations in the protonation state and hydrogen-bonding network around the Mn₄CaO₅ cluster. Optical studies show that these mutations significantly affect proton release during the S₃-to-S₀ transition. While the initial fast proton release associated with Tyr_Z^● formation remains unaffected within the resolution of our measurements, the second, and slower, proton release is delayed, suggesting that the mutations disrupt the hydrogen-bonding interactions necessary for efficient deprotonation of substrate water (O6). This disruption in proton transfer also correlates with slower water exchange in the S₃ state, likely due to non-native hydrogen bonds introduced by the threonine or asparagine side chains at position 185. These findings point to a critical role of D1-V185 in regulating both proton transfer dynamics and water binding, underscoring a complex interplay between structural and functional changes in PSII.