Effects of lanthanide substitution at Ca2+-site on the properties of the oxygen evolving center of photosystem II.

Functional calcium present in a photosynthetic oxygen evolving center (OEC) was replaced by lanthanides. To this end, sample membranes depleted of Ca2+ as well as 16 and 24 kDa extrinsic proteins were prepared and the effects of lanthanides substitution on OEC were studied. The lanthanides inhibited Ca2+-dependent restoration of oxygen evolution but the presence of Ca2+ during the treatment protected OEC from this inhibition, which occurred within 1 min at 20 degrees C but required much longer time at 0 degrees C. Kinetic analysis suggests that lanthanides function as a mixed-type competitor for Ca2+. Lanthanides with ionic radii smaller than Ca2+ show higher affinity for the Ca2+ site than those with larger radii. A lanthanide-substituted OEC displayed a thermoluminescence (TL) band arising from S2Q(A)- charge recombination, indicating that the Mn cluster is oxidized to the S2 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 S1 to S2 transition in the lanthanide-substituted OEC was markedly upshifted to the temperature coincident with that found in Ca2+-depleted but 24 kDa protein preserved OEC. Changes in the OEC induced by the binding of lanthanides to the Ca2+-site are discussed based on these results.