Department of Biophysics, Faculty of Biology, Lomonosov Moscow State University, Moscow, Russia, 119234.
Laboratory, BioEnergy Sciences and Technology Directorate, National Renewable Energy, Golden, CO, 80401, USA.
Photosynth Res. 2021 Feb;147(2):229-237. doi: 10.1007/s11120-020-00813-z. Epub 2021 Feb 2.
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 Ca cations on the interaction of Fe(II) ions with Mn-depleted [PSII(-Mn)] and Ca-depleted [PSII(-Ca)] photosystem II membranes. We found that Ca 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 and Y from t = 30 ms to 46 ms. However, Ca does not affect the rate during the blocking process. An Fe(II) cation (20 µM) replaces 1Mn cation in the MnCaO catalytic cluster of PSII(-Ca) membranes at pH 5.7 but 2 Mn cations at pH 6.5. In the presence of Ca (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 Ca). Measurements of fluorescence induction kinetics support these observations. Inhibition of Mn substitution with Fe(II) cations in the OEC only occurs with Ca and Sr cations, which are also able to restore oxygen evolution in PSII(-Ca) samples. Nonactive cations like La, Ni, Cd, and Mg 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 MnCaO 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.
亚铁（II）阳离子与高亲和力（HA）、锰结合位点（称为“阻断效应”，因为 Fe 阻断了该位点进一步向其供电子）结合效率高、特异性强，该位点存在于去锰、光系统 II（PSII）膜片段中的放氧复合物（OEC）中（Semin 等人，生物化学 41:5854, 2002）。此外，亚铁（II）阳离子可以在去钙 PSII 膜中替代 1 或 2 个锰阳离子（取决于 pH）（Semin 等人，生物能量学和生物膜杂志 48:227, 2016；Semin 等人，光化学和光生物学杂志 B 178:192, 2018）。在本研究中，我们研究了钙离子对亚铁（II）离子与去锰（PSII(-Mn)）和去钙（PSII(-Ca)）光系统 II 膜相互作用的影响。我们发现，钙离子（约 50 mM）在 PSII(-Mn)膜中抑制约 25%的 Fe(II)（5 µM）的光依赖性氧化，而对阻断过程的抑制作用更大，约为 40%。Fe 阳离子对 HA 位点的阻断也会使 Q 和 Y 之间的电荷复合速率从 30 ms 增加到 46 ms。然而，在阻断过程中，Ca 并不影响该速率。在 pH 5.7 时，Fe(II) 阳离子取代 PSII(-Ca)膜中 MnCaO 催化簇中的 1 个锰原子，但在 pH 6.5 时取代 2 个锰原子。在取代过程中存在 Ca（10 mM）时，Fe(II) 不能在 pH 5.7 时提取 Mn，而只能在 pH 6.5 时提取 1 个 Mn（而没有 Ca 时则提取 2 个）。荧光诱导动力学的测量结果支持这些观察结果。OEC 中 Mn 被 Fe(II) 阳离子取代的抑制作用仅发生在能够恢复 PSII(-Ca)样品中氧气释放的 Ca 和 Sr 阳离子中。非活性阳离子如 La、Ni、Cd 和 Mg 对 Mn 与 Fe 的取代没有影响。这些结果表明，MnCaO 簇中一个 Mn 阳离子的位置和/或配体组成受到钙缺失或重新结合的强烈影响，结合钙会影响 OEC 中可提取 Mn4 阳离子的氧化还原电位，使其不易还原。
