CALCIUM-BINDING SITE(S) OF PHOTOSYSTEM-II AS PROBED BY LANTHANIDES

Lanthanides and calcium compete for binding sites on the oxidizing side of Photosystem II. When the trivalent ions replace calcium, the oxygen evolving complex is unable to proceed to the higher oxidation states. Oxygen-evolution activity can be reconstituted by removing the lanthanide and adding back calcium. The number of lanthanide atoms bound to PS II preparations with and without the 17 and 23 kDa species is the same. This is taken as evidence that the extrinsic 17 and 23 kDa do not bind lanthanide/calcium ions. A low-temperature EPR study, with both diamagnetic and paramagnetic lanthanide ions, has demonstrated a close interaction between the lanthanides and the tyrosine radical Y(D+).