Ca2+ function in photosynthetic oxygen evolution studied by alkali metal cations substitution

Effects of adding monovalent alkali metal cations to Ca2+-depleted photosystem (PS)II membranes on the biochemical and spectroscopic properties of the oxygen-evolving complex were studied. The Ca2+-dependent oxygen evolution was competitively inhibited by K+, Rb+, and Cs+, the ionic radii of which are larger than the radius of Ca2+ but not inhibited significantly by Li+ and Na+, the ionic radii of which are smaller than that of Ca2+. Ca2+-depleted membranes without metal cation supplementation showed normal S-2 multiline electron paramagnetic resonance (EPR) signal and an S(2)Q(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 Ca2+-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 similar to 38 degreesC 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(2)Q(A)(-) TL band were interconvertible by the addition of K+ or Ca2+ in the dark. Both the Ca2+-depleted and the K+-substituted membranes showed the narrow EPR signal corresponding to the S2YZ+ state at g=2 by illuminating the membranes under multiple turnover conditions. These results indicate that the ionic radii of the cations occupying Ca2+-binding site crucially affect the properties of the manganese cluster.