Role of D1-Glu65 in Proton Transfer during Photosynthetic Water Oxidation in Photosystem II.

Photosynthetic water oxidation takes place at the Mn 4 CaO 5 cluster in photosystem II (PSII) through a light-driven cycle of five intermediates called S states (S 0 -S 4 ). Although the PSII structures have shown the presence of several channels around the Mn 4 CaO 5 cluster leading to the lumen, the pathways for proton release in the individual S-state transitions remain unidentified. Here, we studied the involvement of the so-called Cl channel in proton transfer during water oxidation by examining the effect of the mutation of D1-Glu65, a key residue in this channel, to Ala using Fourier transform infrared difference and time-resolved infrared spectroscopies together with thermoluminescence and delayed luminescence measurements. It was shown that the structure and the redox property of the catalytic site were little affected by the D1-Glu65Ala mutation. In the S 2 → S 3 transition, the efficiency was still high and the transition rate was only moderately retarded in the D1-Glu65Ala mutant. In contrast, the S 3 → S 0 transition was significantly inhibited by this mutation. These results suggest that proton transfer in the S 2 → S 3 transition occurs through multiple pathways including the Cl channel, whereas this channel likely serves as a single pathway for proton exit in the S 3 → S 0 transition.