Europium- and Dysprosium-Ions as Probes for the Study of Calcium Binding Sites in Photosystem II

Trivalent lanthanide cations are suitable probes for Ca²⁺-binding sites in photosystem II (PS II). PS II membranes prepared from Nicoticinci tabacum, intact and depleted of the extrinsic polypeptides were exposed to lanthanide ions (Dy³⁺ and Eu³⁺). Small concentrations of dysprosium and europium ions enhance oxygen evolution under short saturating flashes. Higher concentrations of the rare earth cations cause the release of the three extrinsic peptides (17, 23 and 33 kDa) and reduce O₂ yield. The reactivation of the PS II membranes, thus depleted of the 33 kDa subunit, by Ca²⁺ ions is not possible. Comparing Eu³⁺ with Dy³⁺ in this effect shows that Eu³⁺ is more effective than Dy³⁺, because a lower Eu³⁺-concentration in comparison to that of Dy³⁺ inactivates O₂-evolution. The differences between europium and dysprosium can be explained by their different ionic radius. Our results suggest the existence of two Ca-binding regions: one with a low affinity for calcium would be located on the contact surface of the 23 and 33 kDa proteins and the second one with a high affinity, should be located close to the Mn-cluster and to tyrosine-161 (Z ). The more tightly-bound calcium would be responsible for the activity of the PS II system.