I2BC, SB2SM, UMR 9198, CEA Saclay, 91191 Gif-sur-Yvette, France.
Department Chemie, Technische Universität München, Lichtenbergstraße 4, D-85748 Garching, Germany.
Biochim Biophys Acta Bioenerg. 2018 May;1859(5):342-356. doi: 10.1016/j.bbabio.2018.02.010. Epub 2018 Feb 28.
In Photosystem II (PSII), the MnCaO-cluster of the active site advances through five sequential oxidation states (S to S) before water is oxidized and O is generated. Here, we have studied the transition between the low spin (LS) and high spin (HS) configurations of S using EPR spectroscopy, quantum chemical calculations using Density Functional Theory (DFT), and time-resolved UV-visible absorption spectroscopy. The EPR experiments show that the equilibrium between S and S is pH dependent, with a pK ≈ 8.3 (n ≈ 4) for the native MnCaO and pK ≈ 7.5 (n ≈ 1) for MnSrO. The DFT results suggest that exchanging Ca with Sr modifies the electronic structure of several titratable groups within the active site, including groups that are not direct ligands to Ca/Sr, e.g., W1/W2, Asp61, His332 and His337. This is consistent with the complex modification of the pK upon the Ca/Sr exchange. EPR also showed that NH addition reversed the effect of high pH, NH-S being present at all pH values studied. Absorption spectroscopy indicates that NH is no longer bound in the STyr state, consistent with EPR data showing minor or no NH-induced modification of S and S. In both Ca-PSII and Sr-PSII, S was capable of advancing to S at low temperature (198 K). This is an experimental demonstration that the S is formed first and advances to Svia the S state without detectable intermediates. We discuss the nature of the changes occurring in the S to S transition which allow the S to S transition to occur below 200 K. This work also provides a protocol for generating S in concentrated samples without the need for saturating flashes.
在光系统 II（PSII）中，活性位点的 MnCaO 簇在水氧化和 O 生成之前经历五个连续的氧化态（S 到 S）。在这里，我们使用电子顺磁共振（EPR）光谱、使用密度泛函理论（DFT）的量子化学计算以及时间分辨紫外可见吸收光谱研究了 S 到 S 低自旋（LS）和高自旋（HS）构型之间的转变。EPR 实验表明，S 和 S 之间的平衡与 pH 值有关，天然 MnCaO 的 pK 值约为 8.3（n 约为 4），而 MnSrO 的 pK 值约为 7.5（n 约为 1）。DFT 结果表明，用 Sr 取代 Ca 会改变活性位点中几个可滴定基团的电子结构，包括不是 Ca/Sr 直接配体的基团，例如 W1/W2、Asp61、His332 和 His337。这与 Ca/Sr 交换时 pK 值的复杂变化一致。EPR 还表明，NH 添加逆转了高 pH 的影响，在所有研究的 pH 值下都存在 NH-S。吸收光谱表明，NH 在 STyr 态不再结合，这与 EPR 数据一致，表明 S 和 S 没有或很少有 NH 诱导的修饰。在 Ca-PSII 和 Sr-PSII 中，S 都能够在低温（198 K）下前进到 S。这是一个实验证明，S 首先形成，并通过 S 态前进到 S，没有检测到中间态。我们讨论了 S 到 S 转变中发生的变化的性质，这些变化允许 S 到 S 转变在 200 K 以下发生。这项工作还提供了一种在不需要饱和闪光的情况下在浓缩样品中产生 S 的方案。
