Division of Material Science, Graduate School of Science, Nagoya University , Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
Biochemistry. 2014 May 20;53(19):3131-44. doi: 10.1021/bi500237y. Epub 2014 May 7.
The redox-active tyrosine YZ (D1-Tyr161) in photosystem II (PSII) functions as an immediate electron acceptor of the Mn4Ca cluster, which is the catalytic center of photosynthetic water oxidation. YZ is also located in the hydrogen bond network that connects the Mn4Ca cluster to the lumen and hence is possibly related to the proton transfer process during water oxidation. To understand the role of YZ in the water oxidation mechanism, we have studied the hydrogen bonding interactions of YZ and its photooxidized neutral radical YZ(•) together with the interaction of the coupled His residue, D1-His190, using light-induced Fourier transform infrared (FTIR) difference spectroscopy. The YZ(•)-minus-YZ FTIR difference spectrum of Mn-depleted PSII core complexes exhibited a broad positive feature around 2800 cm(-1), which was absent in the corresponding spectrum of another redox-active tyrosine YD (D2-Tyr160). Analyses by (15)N and H/D substitutions, examination of the pH dependence, and density functional theory and quantum mechanics/molecular mechanics (QM/MM) calculations showed that this band arises from the N-H stretching vibration of the protonated cation of D1-His190 forming a charge-assisted strong hydrogen bond with YZ(•). This result provides strong evidence that the proton released from YZ upon its oxidation is trapped in D1-His190 and a positive charge remains on this His. The broad feature of the ~2800 cm(-1) band reflects a large proton polarizability in the hydrogen bond between YZ(•) and HisH(+). QM/MM calculations further showed that upon YZ oxidation the hydrogen bond network is rearranged and one water molecule moves toward D1-His190. From these data, a novel proton transfer mechanism via YZ(•)-HisH(+) is proposed, in which hopping of the polarizable proton of HisH(+) to this water triggers the transfer of the proton from substrate water to the luminal side. This proton transfer mechanism could be functional in the S2 → S3 transition, which requires proton release before electron transfer because of an excess positive charge on the Mn4Ca cluster.
在光合作用的水氧化过程中，光系统 II（PSII）中的氧化还原活性酪氨酸 YZ（D1-Tyr161）作为 Mn4Ca 簇的直接电子受体起作用，而 Mn4Ca 簇是光合作用水氧化的催化中心。YZ 还位于连接 Mn4Ca 簇与腔的氢键网络中，因此可能与水氧化过程中的质子转移过程有关。为了了解 YZ 在水氧化机制中的作用，我们使用光诱导傅里叶变换红外（FTIR）差谱法研究了 YZ 及其光氧化中性自由基 YZ(•)与偶联 His 残基 D1-His190 的氢键相互作用。Mn 耗尽 PSII 核心复合物的 YZ(•)-减去-YZ FTIR 差谱在 2800cm(-1)左右显示出一个宽的正特征峰，而另一个氧化还原活性酪氨酸 YD（D2-Tyr160）的相应光谱中则没有这个特征峰。通过 (15)N 和 H/D 取代分析、pH 值依赖性研究、密度泛函理论和量子力学/分子力学（QM/MM）计算表明，该带源于 D1-His190 的质子化阳离子的 N-H 伸缩振动，与 YZ(•)形成电荷辅助强氢键。这一结果提供了强有力的证据表明，YZ 氧化时释放的质子被捕获在 D1-His190 中，并且这个 His 上仍然存在正电荷。~2800cm(-1) 带的宽特征反映了 YZ(•)和 HisH(+)之间氢键中的质子极化率较大。QM/MM 计算进一步表明，YZ 氧化后，氢键网络发生重排，一个水分子向 D1-His190 移动。根据这些数据，提出了一种通过 YZ(•)-HisH(+)的新型质子转移机制，其中 HisH(+)的可极化质子的跳跃引发质子从底物水向腔侧的转移。这种质子转移机制可能在 S2→S3 跃迁中起作用，因为 Mn4Ca 簇上的正电荷过剩，所以在电子转移之前需要释放质子。
