Fe(II) cations bind with high efficiency and specificity at the high-affinity (HA), Mn-binding site (termed the “blocking effect” since Fe blocks further electron donation to the site) of the oxygen-evolving complex (OEC) in Mn-depleted, photosystem II (PSII) membrane fragments (Semin et al. in Biochemistry 41:5854, 2002). Furthermore, Fe(II) cations can substitute for 1 or 2Mn cations (pH dependent) in Ca-depleted PSII membranes (Semin et al. in Journal of Bioenergetics and Biomembranes 48:227, 2016; Semin et al. in Journal of Photochemistry and Photobiology B 178:192, 2018). In the current study, we examined the effect of Ca²⁺ cations on the interaction of Fe(II) ions with Mn-depleted [PSII(-Mn)] and Ca-depleted [PSII(-Ca)] photosystem II membranes. We found that Ca²⁺ cations (about 50 mM) inhibit the light-dependent oxidation of Fe(II) (5 µM) by about 25% in PSII(-Mn) membranes, whereas inhibition of the blocking process is greater at about 40%. Blocking of the HA site by Fe cations also decreases the rate of charge recombination between Q_A⁻ and Y_Z^•+ from t_1/2 = 30 ms to 46 ms. However, Ca²⁺ does not affect the rate during the blocking process. An Fe(II) cation (20 µM) replaces 1Mn cation in the Mn₄CaO₅ catalytic cluster of PSII(-Ca) membranes at pH 5.7 but 2 Mn cations at pH 6.5. In the presence of Ca²⁺ (10 mM) during the substitution process, Fe(II) is not able to extract Mn at pH 5.7 and extracts only 1Mn at pH 6.5 (instead of two without Ca²⁺). Measurements of fluorescence induction kinetics support these observations. Inhibition of Mn substitution with Fe(II) cations in the OEC only occurs with Ca²⁺ and Sr²⁺ cations, which are also able to restore oxygen evolution in PSII(-Ca) samples. Nonactive cations like La³⁺, Ni²⁺, Cd²⁺, and Mg²⁺ have no influence on the replacement of Mn with Fe. These results show that the location and/or ligand composition of one Mn cation in the Mn₄CaO₅ cluster is strongly affected by calcium depletion or rebinding and that bound calcium affects the redox potential of the extractable Mn4 cation in the OEC, making it resistant to reduction.

Fe(II) 阳离子在氧释放复合物 (OEC) 的高亲和力 (HA) Mn 结合位点（称为“阻断效应”，因为 Fe 阻止进一步向该位点提供电子）处以高效率和特异性结合在 Mn 耗尽的光系统 II (PSII) 膜片段中（Semin 等人在 Biochemistry 41:5854, 2002）。此外，Fe(II) 阳离子可以替代 Ca 耗尽的 PSII 膜中的 1 或 2Mn 阳离子（取决于 pH 值）（Semin 等人，Journal of Bioenergetics and Biomembranes 48:227, 2016；Semin 等人，Journal of Photochemistry and光生物学 B 178:192, 2018)。在当前的研究中，我们研究了 Ca ²⁺阳离子对 Fe(II) 离子与 Mn 耗尽的 [PSII(-Mn)] 和 Ca 耗尽的 [PSII(-Ca)] 光系统 II 膜相互作用的影响。我们发现 Ca ²⁺阳离子（约 50 mM）在 PSII(-Mn) 膜中抑制 Fe(II) (5 µM) 的光依赖性氧化约 25%，而阻断过程的抑制作用更大，约为 40%。由铁阳离子HA位点的阻断也减小之间的电荷重组率Q_甲^-和ÿ _ž ^•+从吨_1/2  = 30毫秒至46毫秒。然而，Ca ²⁺在阻塞过程中不影响速率。Fe(II) 阳离子 (20 µM)在 pH 5.7 时取代PSII(-Ca) 膜的 Mn ₄ CaO ₅催化簇中的1Mn 阳离子，但在 pH 6.5 时取代2 Mn 阳离子。在 Ca ²⁺存在下(10 mM) 在取代过程中，Fe(II) 在 pH 5.7 时无法提取 Mn，在 pH 6.5 时仅提取 1 Mn（而不是在没有 Ca ^{2+ 的}情况下提取 2 个）。荧光诱导动力学的测量支持这些观察结果。OEC 中 Fe(II) 阳离子对 Mn 取代的抑制仅发生在 Ca ²⁺和 Sr ²⁺阳离子中，它们也能够恢复 PSII(-Ca) 样品中的析氧。La ³⁺、Ni ²⁺、Cd ²⁺和Mg ²⁺等非活性阳离子对Mn 被Fe 置换没有影响。这些结果表明 Mn ₄ CaO ₅中一种 Mn 阳离子的位置和/或配体组成 簇受到钙消耗或重新结合的强烈影响，并且结合的钙会影响 OEC 中可提取的 Mn4 阳离子的氧化还原电位，使其抗还原。