An investigation of the role of calcium ion in the oxygen-evolving complex of photosystem II

Photosynthetic oxygen evolution takes place in Photosystem II (PSII) at an active site containing a tetranuclearmanganese (Mn4) cluster, a redox-active tyrosine (YZ), and Ca2+ and Cl - cofactors. A molecule of O2 is released from one Kok Sn-state cycle, in which the OEC is successively oxidized after absorption of 4 quanta of light. Ca2+ is essential for the oxygen evolution as its depletion causes the loss of the oxygen-evolving activity. Although several models of oxygen evolution have been proposed, the role of Ca2+ in oxygen evolution is still ambiguous. In this study, we investigate the functional role of Ca2+ in oxygen evolution by pH-dependent activity assays of Ca2+-containing and Sr 2+-substituted PSII, and by the effect of Dy3+-, Cu 2+-, and Cd2+-substitution on the S1 to S2 state transition. The significant change of p Ka associated with the protonation step of oxygen evolving reaction by Sr2+-substitution supports the previously proposed Brudvig model that Ca2+ functions as a Lewis acid and a protein residue acting as an essential base directly coordinates to Ca2+. The S2 state multiline EPR signal is observed in Dy3+- and Cd2+-substituted PSII for the first time. This observation indicates that Ca2+ is not required in the S1 to S2 state advancement. The change of power saturation of the S 2 state multiline EPR signal after Dy3+-substitution indicates that Ca2+ is very close to the Mn4 cluster. The effect of the monovalent cations on the Ca2+-site of the OEC is investigated and an effect of membrane stacking resulting in the loss of the O2-evolving activity is proposed. In addition, the addition of ethylene glycol significantly changes the pKa of the deprotonation reaction in the oxygen evolution reaction, which can be ascribed to the change of the dielectric environment and cosolvent effect.; Overall, this thesis presents the investigation of the effects of mono-, di-, tri-valent cations on the oxygen evolution reaction of PSII. The functional role and the location of Ca2+ in the OEC are discussed.