Effect of lanthanides on oxidation of Mn2+ cations via a high-affinity Mn-binding site in photosystem II membranes

Lanthanide cations (La3+ and Tb3+) bind to the Ca-binding site of the oxygen-evolving complex in Ca-depleted PSII membranes and irreversibly inhibit the oxygen evolution. Оn the other hand, EPR measurement of Mn2+ concentration in buffer revealed that lanthanide cations inhibit the light-dependent oxidation of Mn2+ cations via the high-affinity Mn-binding site in Mn-depleted PSII membranes, which suggests that they bind to and inhibit the high-affinity Mn-binding site of the oxygen-evolving complex. The inhibition is irreversible, bound Ln3+ cation could not be washed out from the sample. Calcium ion inhibits oxidation of Mn2+ (5 μM) at very high concentration (tens mM) and the inhibition is reversible. In this work we measured the reduction rate of exogenic electron acceptor 2,6-dichlorophenolindophenol during the oxidation of Mn2+ cations in the Ca-depleted PSII and in the Ca-depleted PSII treated with lanthanides after extraction of Mn cluster from these preparations. We found that irreversible binding of the lanthanide cation to the Ca-binding site in the Ca-depleted PSII membranes leads to a partial inhibition of the high-affinity Mn-binding site. Ln3+ cation irreversibly binds to calcium site of the oxygen-evolving complex of photosystem II. Bound Ln3+ cation inhibits, possibly, through bidentant ligand D1-D170, a high affinity Mn-binding site either before or after extraction of Mn cations from oxygen-evolving complex. [Display omitted] •Ln3+ cations strongly bind to the Ca-binding site in Ca-depleted photosystem II•Ln3+ cations also effectively inhibit the high-affinity Mn-binding site in apo-photosystem II•Mn cations can be extracted from photosystem II without affecting Ln3+ bound to Ca-site•Inhibition of the Mn-site in Ca-depleted photosystem II treated by Ln3+ was found