Proton and Water Transfer Pathways in the S<sub>2</sub> → S<sub>3</sub> Transition of the Water-Oxidizing Complex in Photosystem II: Time-Resolved Infrared Analysis of the Effects of D1-N298A Mutation and NO<sub>3</sub><sup>–</sup> Substitution
Yasutada Okamoto, Yuichiro Shimada, Ryo Nagao, Takumi Noguchi
Abstract
Photosynthetic water oxidation is performed through a light-driven cycle of five intermediates (S0–S4 states) in photosystem II (PSII). The S2 → S3 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 S2 → S3 transition, we examined the effects of D1-N298A mutation and NO3– substitution for Cl–, which perturbed the O1 and Cl channels, respectively, on the S2 → S3 kinetics using time-resolved infrared spectroscopy. The S2 → S3 transition was retarded both upon NO3– substitution and upon D1-N298A mutation, whereas it was unaffected by further NO3– 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 S2 → S3 transition, water delivery and proton release occur through the O1 and Cl channels, respectively.