Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada.
PLoS One. 2013 Jul 30;8(7):e68421. doi: 10.1371/journal.pone.0068421. Print 2013.
Photosystem II (PSII) of photosynthesis has the unique ability to photochemically oxidize water. Recently an engineered bacterioferritin photochemical 'reaction centre' (BFR-RC) using a zinc chlorin pigment (ZnCe6) in place of its native heme has been shown to photo-oxidize bound manganese ions through a tyrosine residue, thus mimicking two of the key reactions on the electron donor side of PSII. To understand the mechanism of tyrosine oxidation in BFR-RCs, and explore the possibility of water oxidation in such a system we have built an atomic-level model of the BFR-RC using ONIOM methodology. We studied the influence of axial ligands and carboxyl groups on the oxidation potential of ZnCe6 using DFT theory, and finally calculated the shift of the redox potential of ZnCe6 in the BFR-RC protein using the multi-conformational molecular mechanics-Poisson-Boltzmann approach. According to our calculations, the redox potential for the first oxidation of ZnCe6 in the BRF-RC protein is only 0.57 V, too low to oxidize tyrosine. We suggest that the observed tyrosine oxidation in BRF-RC could be driven by the ZnCe6 di-cation. In order to increase the efficiency of tyrosine oxidation, and ultimately oxidize water, the first potential of ZnCe6 would have to attain a value in excess of 0.8 V. We discuss the possibilities for modifying the BFR-RC to achieve this goal.
光合作用的光系统 II（PSII）具有独特的光化学氧化水的能力。最近，一种使用锌卟啉色素（ZnCe6）代替其天然血红素的工程菌铁蛋白光化学“反应中心”（BFR-RC）已被证明可以通过酪氨酸残基光氧化结合的锰离子，从而模拟 PSII 电子供体侧的两个关键反应。为了了解 BFR-RC 中酪氨酸氧化的机制，并探索在这种系统中进行水氧化的可能性，我们使用 ONIOM 方法构建了 BFR-RC 的原子水平模型。我们使用 DFT 理论研究了轴向配体和羧基对 ZnCe6 氧化电位的影响，最后使用多构象分子力学-泊松-玻尔兹曼方法计算了 BFR-RC 蛋白中 ZnCe6 氧化还原电位的变化。根据我们的计算，BFR-RC 蛋白中 ZnCe6 的第一次氧化的氧化还原电位仅为 0.57 V，太低以至于无法氧化酪氨酸。我们认为，在 BFR-RC 中观察到的酪氨酸氧化可能是由 ZnCe6 二价阳离子驱动的。为了提高酪氨酸氧化的效率，并最终氧化水，ZnCe6 的第一电势必须达到超过 0.8 V 的值。我们讨论了修饰 BFR-RC 以实现这一目标的可能性。
