Ca 2+ Function in Photosynthetic Oxygen Evolution Studied by Alkali Metal Cations Substitution

Abstract

Effects of adding monovalent alkali metal cations to Ca 2+-depleted photosystem (PS)II membranes on the biochemical and spectroscopic properties of the oxygen-evolving complex were studied. The Ca 2+-dependent oxygen evolution was competitively inhibited by K +, Rb +, and Cs +, the ionic radii of which are larger than the radius of Ca 2+ but not inhibited significantly by Li + and Na +, the ionic radii of which are smaller than that of Ca 2+. Ca 2+-depleted membranes without metal cation supplementation showed normal S 2 multiline electron paramagnetic resonance (EPR) signal and an S 2Q A − thermoluminescence (TL) band with a normal peak temperature after illumination under conditions for single turnover of PSII. Membranes supplemented with Li + or Na + showed properties similar to those of the Ca 2+-depleted membranes, except for a small difference in the TL peak temperatures. The peak temperature of the TL band of membranes supplemented with K +, Rb +, or Cs + was elevated to ∼38°C which coincided with that of Y D +Q A − TL band, and no S 2 EPR signals were detected. The K +-induced high-temperature TL band and the S 2Q A − TL band were interconvertible by the addition of K + or Ca 2+ in the dark. Both the Ca 2+-depleted and the K +-substituted membranes showed the narrow EPR signal corresponding to the S 2Y Z + state at g = 2 by illuminating the membranes under multiple turnover conditions. These results indicate that the ionic radii of the cations occupying Ca 2+-binding site crucially affect the properties of the manganese cluster.