Structural Relevance of Ca²⁺ to the Oxygen-evolving Mn cluster: Effects of Sr²⁺ Substitution on the S-state Cycle in Spinach and Cyanobacteria

Abstract

Oxygen evolution in photosytem II (PSII) is performed at the Mn cluster through a light-driven cycle of five intermediates (S₀-S₄). Although Ca²⁺ is known to be an essential cofactor for oxygen evolution, the structural relevance of Ca²⁺ to the Mn cluster remains unclarified. In this study, we have investigated the role of Ca²⁺ in oxygen evolution by detecting FTIR spectra of the Mn cluster in PSII preparations of spinach and the cyanobacterium Thermosynechocuccus elongatus in which Ca²⁺ is replaced with Sr²⁺. In FTIR spectra of S-state transitions, several bands sensitive to Sr²⁺ substitution were found in the symmetric COO^- stretching region, indicating the presence of carboxylate ligands to the Mn cluster affected by Sr²⁺ substitution. The changes were prominent in the S₁->S₂ and S₃->S₀ transitions and similar between spinach and T. elongatus. These results indicate that Ca²⁺ directly interacts with the Mn cluster and is deeply involved in its reactions.