EUROPIUM-IONS AND DYSPROSIUM-IONS AS PROBES FOR THE STUDY OF CALCIUM-BINDING SITES IN PHOTOSYSTEM-II

Trivalent lanthanide cations are suitable probes for Ca2+-binding site s in photosystem II (PS II). PS II membranes prepared from Nicotiana t abacum, intact and depleted of the extrinsic polypeptides were exposed to lanthanide ions (Dy3+ and Eu3+). Small concentrations of dysprosiu m and europium ions enhance oxygen evolution under short saturating fl ashes. Higher concentrations of the rare earth cations cause the relea se of the three extrinsic peptides (17, 23 and 33 kDa) and reduce O-2 yield. The reactivation of the PS II membranes, thus depleted of the 3 3 kDa subunit, by Ca2+ ions is not possible. Comparing Eu3+ With Dy3in this effect shows that Eu3+ is more effective than Dy3+, because a lower Eu3+-concentration in comparison to that of Dy3+ inactivates O-2 -evolution. The differences between europium and dysprosium can be exp lained by their different ionic radius. Our results suggest the existe nce of two Ca-binding regions: one with a low affinity for calcium wou ld 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 M n-cluster and to tyrosine-161 (Z). The more tightly-bound calcium woul d be responsible for the activity of the PS II system.