Abstract
Substitution of lanthanides for Ca 2+ 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 Ca 2+, 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 p K of 6.5.
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