Department of Chemistry, Umeå University, Linnaeus väg 6 (KBC huset), SE-901 87, Umeå, Sweden.
Phys Chem Chem Phys. 2020 Jun 21;22(23):12894-12908. doi: 10.1039/d0cp01380c. Epub 2020 May 6.
In photosynthesis, dioxygen formation from water is catalyzed by the oxygen evolving complex (OEC) in Photosystem II (PSII) that harbours the MnCa cluster. During catalysis, the OEC cycles through five redox states, S to S. In the S state, the MnCa cluster can exist in two conformations, which are signified by the low-spin (LS) g = 2 EPR multiline signal and the high-spin (HS) g = 4.1 EPR signal. Here, we employed time-resolved membrane inlet mass spectrometry to measure the kinetics of HO/HO exchange between bulk water and the two substrate waters bound at the MnCa cluster in the S, S, and the S states in both Ca-PSII and Sr-PSII core complexes from T. elongatus. We found that the slowly exchanging substrate water exchanges 10 times faster in the S than in the S state, and that the S→ S conversion has at physiological temperature an activation barrier of 17 ± 1 kcal mol. Of the presently suggested S models, our findings are best in agreement with a water exchange pathway involving a S state that has an open cubane structure with a hydroxide bound between Ca and Mn1. We also show that water exchange in the S state is governed by a different equilibrium than in S, and that the exchange of the fast substrate water in the S state is unaffected by Ca/Sr substitution. These findings support that (i) O5 is the slowly exchanging substrate water, with W2 being the only other option, and (ii) either W2 or W3 is the fast exchanging substrate. The three remaining possibilities for O-O bond formation in PSII are discussed.
在光合作用中，水的氧气形成是由光合作用系统 II（PSII）中的氧释放复合物（OEC）催化的，该复合物包含 MnCa 簇。在催化过程中，OEC 经历五个氧化还原状态，从 S 到 S。在 S 状态下，MnCa 簇可以存在两种构象，这两种构象由低自旋（LS）g = 2 EPR 多线信号和高自旋（HS）g = 4.1 EPR 信号表示。在这里，我们采用时间分辨膜入口质谱法测量了在 Ca-PSII 和 Sr-PSII 核心复合物的 S、S 和 S 状态下，MnCa 簇结合的两个基质水中的 HO/HO 在 bulk water 之间的交换动力学。我们发现，在 S 状态下，缓慢交换的基质水的交换速度比 S 状态快 10 倍，并且 S→S 转换在生理温度下具有 17 ± 1 kcal mol 的活化能垒。在目前提出的 S 模型中，我们的发现与一种水交换途径最吻合，该途径涉及一种具有开放的立方烷结构且 Ca 和 Mn1 之间有一个氢氧化物的 S 状态。我们还表明，S 状态下的水交换受不同于 S 状态的平衡控制，并且 S 状态下快速基质水的交换不受 Ca/Sr 取代的影响。这些发现支持以下观点：（i）O5 是缓慢交换的基质水，W2 是唯一的另一种选择，（ii）W2 或 W3 是快速交换的基质水。PSII 中 O-O 键形成的另外三种可能性也进行了讨论。
