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 S state in a high spin configuration (S) and in a low spin configuration (S) in both mutants. In contrast to the S in the wild type, the S state in the D1-V185T mutant does not progress to the S state at 198 K. This inability is likely due to alterations in the protonation state and hydrogen-bonding network around the MnCaO cluster. Optical studies show that these mutations significantly affect proton release during the S-to-S transition. While the initial fast proton release associated with Tyr 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 S 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.