Proton and Water Transfer Pathways in the S → S Transition of the Water-Oxidizing Complex in Photosystem II: Time-Resolved Infrared Analysis of the Effects of D1-N298A Mutation and NO Substitution.

Photosynthetic water oxidation is performed through a light-driven cycle of five intermediates (S-S states) in photosystem II (PSII). The S → S transition, which involves concerted water and proton transfer, is a key process for understanding the water oxidation mechanism. Here, to identify the water and proton transfer pathways during the S → S transition, we examined the effects of D1-N298A mutation and NO substitution for Cl, which perturbed the O1 and Cl channels, respectively, on the S → S kinetics using time-resolved infrared spectroscopy. The S → S transition was retarded both upon NO substitution and upon D1-N298A mutation, whereas it was unaffected by further NO substitution in N298A PSII. The H/D kinetic isotope effect in N298A PSII was relatively small, revealing that water transfer is a rate-limiting step in this mutant. From these results, it was suggested that during the S → S transition, water delivery and proton release occur through the O1 and Cl channels, respectively.