The low spin - high spin equilibrium in the S2-state of the water oxidizing enzyme
The low spin - high spin equilibrium in the S2-state of the water oxidizing enzyme
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DOI：
10.1016/j.bbabio.2018.02.010
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发表时间：
2018-05-01
影响因子：
4.3
通讯作者：
Rutherford, A. William
中科院分区：
文献类型：
--
作者：
Boussac, Alain;Ugur, Ilke;Rutherford, A. William
在光系统II （PSII）中，活性位点的mn4cao5簇在水被氧化和生成O-2之前经历了5个连续的氧化态（S-0到S-4）。本文利用EPR光谱、密度泛函理论（DFT）的量子化学计算和时间分辨紫外-可见吸收光谱研究了S-2的低自旋（LS）和高自旋（HS）构型之间的跃迁。EPR实验表明，S-2（LS）和S-2（HS）之间的平衡与pH有关，原生Mn4CaO5的pK(a)近似为8.3 (n近似为4)，Mn4SrO5的pK(a)近似为7.5 （n = 1）。DFT结果表明，Ca与Sr交换改变了活性位点内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(a)的复合修饰一致。EPR还表明，NH3的加入逆转了高pH的影响，在所有pH值下都存在NH3- s2ls。吸收光谱表明NH3不再束缚在S(3)Tyr(z)态，这与EPR数据一致，表明NH3诱导的S-3和S-0的修饰很少或没有。在Ca-PSII和Sr-PSII中，S-2（HS）在低温（198 K）下都能向S-3转变。这是一个实验证明，S-2（LS）首先形成，并通过S-2（HS）状态进入S-3，没有检测到中间产物。我们讨论了S-2（LS）到S-2（HS）转变过程中发生的变化的性质，这些变化允许S-2（HS）到S-3的转变发生在200 K以下。这项工作还提供了一种在浓缩样品中产生S-3而不需要饱和闪光的方案。
In Photosystem II (PSII), the Mn4CaO5-cluster of the active site advances through five sequential oxidation states (S-0 to S-4) before water is oxidized and O-2 is generated. Here, we have studied the transition between the low spin (LS) and high spin (HS) configurations of S-2 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-2(LS) and S-2(HS) is pH dependent, with a pK(a) approximate to 8.3 (n approximate to 4) for the native Mn4CaO5 and 7.5 (n = 1) for Mn4SrO5. 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(a) upon the Ca/Sr exchange. EPR also showed that NH3 addition reversed the effect of high pH, NH3-S2LS being present at all pH values studied. Absorption spectroscopy indicates that NH3 is no longer bound in the S(3)Tyr(z) state, consistent with EPR data showing minor or no NH3-induced modification of S-3 and S-0. In both Ca-PSII and Sr-PSII, S-2(HS) was capable of advancing to S-3 at low temperature (198 K). This is an experimental demonstration that the S-2(LS) is formed first and advances to S-3 via the S-2(HS) state without detectable intermediates. We discuss the nature of the changes occurring in the S-2(LS) to S-2(HS) transition which allow the S-2(HS) to S-3 transition to occur below 200 K. This work also provides a protocol for generating S-3 in concentrated samples without the need for saturating flashes.