The exchange of the fast substrate water in the S2 state of photosystem II is limited by diffusion of bulk water through channels – implications for the water oxidation mechanism,Chemical Science

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The exchange of the fast substrate water in the S2 state of photosystem II is limited by diffusion of bulk water through channels – implications for the water oxidation mechanism
Chemical Science ( IF 7.6 ) Pub Date : 2021-09-01 , DOI: 10.1039/d1sc02265b
Casper de Lichtenberg _{1,

2} , Christopher J Kim ₃ , Petko Chernev ₂ , Richard J Debus ₃ , Johannes Messinger _{1,

2}

Affiliation

The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn₄CaO₅ cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the ‘slow exchanging substrate water (W_s)’, while, in the S₂ state, the terminal water ligands W2 and W3 are both discussed as the ‘fast exchanging substrate water (W_f)’. A critical point for the assignment of W_f is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn₄CaO₅ cluster. In this study, we measured the rates of H₂¹⁶O/H₂¹⁸O substrate water exchange in the S₂ and S₃ states of PSII core complexes from wild-type (WT) Synechocystis sp. PCC 6803, and from two mutants, D1-D61A and D1-E189Q, that are expected to alter water access via the Cl1/O4 channels and the O1 channel, respectively. We found that the exchange rates of W_f and W_s were unaffected by the E189Q mutation (O1 channel), but strongly perturbed by the D61A mutation (Cl1/O4 channel). It is concluded that all channels have restrictions limiting the isotopic equilibration of the inner water pool near the Mn₄CaO₅ cluster, and that D61 participates in one such barrier. In the D61A mutant this barrier is lowered so that W_f exchange occurs more rapidly. This finding removes the main argument against Ca-bound W3 as fast substrate water in the S₂ state, namely the indifference of the rate of W_f exchange towards Ca/Sr substitution.

中文翻译：

光系统 II 的 S2 状态下快速底物水的交换受到大量水通过通道扩散的限制——对水氧化机制的影响

我们呼吸的分子氧是由与光系统 II (PSII) 中的 Mn ₄ CaO ₅簇结合的水源氧物质产生的。目前的研究指出中心氧桥O5为“慢交换底物水(W _s )”，而在S ₂态下，末端水配体W2和W3都被讨论为“快交换底物水(W s )”。 _f )'。 W _f分配的一个关键点是它与大量水的交换是否受到通往 Mn ₄ CaO ₅簇的通道中的障碍的限制。在这项研究中，我们测量了来自野生型（WT）集胞藻的 PSII 核心复合物在 S ₂和 S ₃状态下 H ₂ ¹⁶ O/H ₂ ¹⁸ O 底物水交换的速率。 PCC 6803 以及两个突变体 D1-D61A 和 D1-E189Q 预计将分别改变通过Cl1/O4 通道和 O1 通道的水进入。我们发现W _f和W _s的交换率不受E189Q 突变（O1 通道）的影响，但受到D61A 突变（Cl1/O4 通道）的强烈干扰。结论是所有通道都具有限制Mn ₄ CaO ₅簇附近的内部水池的同位素平衡的限制，并且D61参与了这样的屏障之一。在 D61A 突变体中，该屏障降低，使得 W _f交换发生得更快。这一发现消除了反对Ca结合的W3作为S ₂态的快速底物水的主要论点，即W _f交换速率对Ca/Sr取代的影响无关。

更新日期：2021-09-06

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