Mechanism of Inhibition of the Oxygen-Evolving Complex of Photosystem II by Lanthanide Cations

The interaction of La³⁺ and Tb³⁺ cations with the Ca-binding site of the oxygen-evolving complex of photosystem II samples depleted of calcium has been studied. The binding of cations to the Ca-binding site is irreversible and the bound cations cannot be washed out or replaced by Ca²⁺ cations. The feature of lanthanides to bind strongly to the Ca-binding site was used to investigate whether the bound Ln³⁺ cation has an effect on the high-affinity Mn-binding site of the oxygen-evolving complex. For this purpose, hydroquinone was used to extract manganese cations from the oxygen-evolving complex of the calcium-depleted photosystem II membranes with the Ca-binding site blocked by La³⁺ or Tb³⁺, and the activity of the high-affinity site was then examined using exogenous electron donors (Mn²⁺ + H₂O₂) and 1,5-diphenylcarbazide. It was found that lanthanide cations bound to the Ca-binding site significantly inhibited oxidation of electron donors through the high-affinity Mn-binding site. The mechanism of the observed effect is discussed.