Abstract
Photosynthetic water oxidation is performed by the light-driven S-state cycle in the oxygen-evolving complex of photosystem II. While ligation to the Mn cluster from several amino acid residues has been proposed by X-ray crystallographic studies, details of the reaction processes involving these residues are not clarified. In this study, we constructed a mutant of Synechocystis sp. PCC 6803 by substituting glutamine for glutamate at the position 354 of CP43 (CP43-Glu354), and investigated the oxygen-evolution processes using spectroscopic measurements including FTIR analysis. The oxygen-evolving activity of the mutant was approximately 20% of wildtype. The oscillation pattern of flash-dependent delayed luminescence suggested that the transition beyond the S3 state was blocked in the mutant. FTIR difference spectroscopy revealed that CP43-Glu354 functioned as a ligand to the Mn cluster, and changed its coordination structure in the S1-to-S2 transition. Based on these findings, we discuss the reaction processes of oxygen evolution involving CP43-Glu354.
