Effects of lanthanide substitution at Ca 2+-site on the properties of the oxygen evolving center of Photosystem II

Abstract

Functional calcium present in a photosynthetic oxygen evolving center (OEC) was replaced by lanthanides. To this end, sample membranes depleted of Ca 2+ as well as 16 and 24 kDa extrinsic proteins were prepared and the effects of lanthanides substitution on OEC were studied. The lanthanides inhibited Ca 2+-dependent restoration of oxygen evolution but the presence of Ca 2+ during the treatment protected OEC from this inhibition, which occurred within 1 min at 20°C but required much longer time at 0°C. Kinetic analysis suggests that lanthanides function as a mixed-type competitor for Ca 2+. Lanthanides with ionic radii smaller than Ca 2+ show higher affinity for the Ca 2+ site than those with larger radii. A lanthanide-substituted OEC displayed a thermoluminescence (TL) band arising from S 2 Q A − charge recombination, indicating that the Mn cluster is oxidized to the S 2 state. However, the peak temperature of the TL band varied depending on lanthanide species. The results indicate that the oxidation potential of the Mn cluster is modified in various ways in a substituted OEC. Furthermore, the threshold temperature for the S 1 to S 2 transition in the lanthanide-substituted OEC was markedly upshifted to the temperature coincident with that found in Ca 2+-depleted but 24 kDa protein preserved OEC. Changes in the OEC induced by the binding of lanthanides to the Ca 2+-site are discussed based on these results.