SUBSTITUTION OF LANTHANIDES AT THE CALCIUM SITE(S) IN PHOTOSYSTEM-II AFFECTS ELECTRON-TRANSPORT FROM TYROSINE-Z TO P680+

Substitution of lanthanides for Ca2+ in Photosystem II results in an inhibition of oxygen-evolution activity. In addition to blocking electron transport from the Mn-complex to TyrZ+, the presence of the lanthanides also affects electron transport from TyrZ to P680+. This latter effect depends on two factors: (i) The ionic radius of the trivalent lanthanide ion; lanthanide ions with an ionic radius smaller than that of Ca2+, exert a more pronounced inhibitory effect. (ii) The pH of the system; although the presence of lanthanides has a strong influence on the reduction of P680+ by TyrZ at pH 6.0, no such effect is observed at pH 7.5. A pH titration has attributed the pH effect to an amino acid residue with a pK of 6.5.