Retegan Marius, Pantazis Dimitrios A
Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , Germany . Email:
Chem Sci. 2016 Oct 1;7(10):6463-6476. doi: 10.1039/c6sc02340a. Epub 2016 Jul 5.
Methanol has long being used as a substrate analogue to probe access pathways and investigate water delivery at the oxygen-evolving complex (OEC) of photosystem-II. In this contribution we study the interaction of methanol with the OEC by assembling available spectroscopic data into a quantum mechanical treatment that takes into account the local channel architecture of the active site. The effect on the magnetic energy levels of the MnCa cluster in the S state of the catalytic cycle can be explained equally well by two models that involve either methanol binding to the calcium ion of the cluster, or a second-sphere interaction in the vicinity of the "dangler" Mn4 ion. However, consideration of the latest C hyperfine interaction data shows that only one model is fully consistent with experiment. In contrast to previous hypotheses, methanol is not a direct ligand to the OEC, but is situated at the end-point of a water channel associated with the O4 bridge. Its effect on magnetic properties of plant PS-II results from disruption of hydrogen bonding between O4 and proximal channel water molecules, thus enhancing superexchange (antiferromagnetic coupling) between the Mn3 and Mn4 ions. The same interaction mode applies to the dark-stable S state and possibly to all other states of the complex. Comparison of protein sequences from cyanobacteria and plants reveals a channel-altering substitution (D1-Asn87 D1-Ala87) in the proximity of the methanol binding pocket, explaining the species-dependence of the methanol effect. The water channel established as the methanol access pathway is the same that delivers ammonia to the Mn4 ion, supporting the notion that this is the only directly solvent-accessible manganese site of the OEC. The results support the pivot mechanism for water binding at a component of the S state and would be consistent with partial inhibition of water delivery by methanol. Mechanistic implications for enzymatic regulation and catalytic progression are discussed.
长期以来，甲醇一直被用作底物类似物，以探测光合系统II放氧复合体（OEC）的通道通路并研究水的输送。在本论文中，我们通过将现有的光谱数据整合到一个考虑活性位点局部通道结构的量子力学处理方法中，来研究甲醇与OEC的相互作用。催化循环S态中MnCa簇磁能级的变化，可由两种模型很好地解释，一种模型认为甲醇与簇中的钙离子结合，另一种模型认为甲醇与“悬垂”的Mn4离子附近存在二级相互作用。然而，对最新的碳超精细相互作用数据的考量表明，只有一种模型与实验完全一致。与之前的假设不同，甲醇并非OEC的直接配体，而是位于与O4桥相关的水通道末端。它对植物PS-II磁性的影响源于O4与近端通道水分子之间氢键的破坏，从而增强了Mn3和Mn4离子之间的超交换（反铁磁耦合）。相同的相互作用模式适用于暗稳定的S态，可能也适用于该复合体的所有其他状态。对蓝细菌和植物蛋白质序列的比较揭示了甲醇结合口袋附近的一个改变通道的替换（D1-Asn87→D1-Ala87），这解释了甲醇效应的物种依赖性。被确定为甲醇进入途径的水通道，与将氨输送到Mn4离子的通道相同，这支持了该通道是OEC唯一直接可被溶剂接触的锰位点的观点。这些结果支持了S态某一成分上水结合的枢轴机制，并且与甲醇对水输送的部分抑制作用相一致。文中还讨论了对酶调控和催化进程的机制影响。
