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 degreesC but required much longer time at 0 degreesC 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 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 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. (C) 2000 Elsevier Science B.V. All rights reserved.