Mechanism of inhibition of the oxygen-evolving complex of photosystem II by lanthanide cations

Abstract

The process of 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+ cation. A feature of lanthanides to bind strongly to the Ca-binding site has been used to investigate if the bound Ln3+ cation has an effect on the high-affinity Mn-binding site of the oxygen-evolving complex. Therefore, in this work, hydroquinone was used for the extraction of 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-di-phenylcarbazide. It was found that lanthanide cation bound to the Ca-binding site can significantly inhibit the oxidation rates of electron donors through the high-affinity Mn-binding site. The mechanism of the observed effect is discussed.