Effective binding of Tb and La cations on the donor side of Mn-depleted photosystem II.

The interaction of Tb and La cations with different photosystem II (PSII) membranes (intact PSII, Ca-depleted PSII (PSII[-Ca]) and Mn-depleted PSII (PSII[-Mn]) membranes) was studied. Although both lanthanide cations (Ln) interact only with Ca-binding site of oxygen-evolving complex (OEC) in PSII and PSII(-Ca) membranes, we found that in PSII(-Mn) membranes both Ln ions tightly bind to another site localized on the oxidizing side of PSII. Binding of Ln cations to this site is not protected by Ca and is accompanied by very effective inhibition of Mn oxidation at the high-affinity (HA) Mn-binding site ([Mn  + HO] couple was used as a donor of electrons). The values of the constant for inhibition of electron transport K are equal to 2.10 ± 0.03 µM for Tb and 8.3 ± 0.4 µM for La, whereas OEC inhibition constant in the native PSII membranes is 323 ± 7 µM for Tb. The value of K for Tb corresponds to K for Mn cations in the reaction of diphenylcarbazide oxidation via HA site (1.5 µM) presented in the literature. Our results suggest that Ln cations bind to the HA Mn-binding site in PSII(-Mn) membranes like Mn or Fe cations. Taking into account the fact that Mn and Fe cations bind to the HA site as trivalent cations after light-induced oxidation and the fact that Mn cation bound to the HA site (Mn4) is also in trivalent state, we can suggest that valency may be important for the interaction of Ln with the HA site.