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S 13-011 His 190 might directly bridge Yz and Mn-cluster : Mechanistic consequences for water oxidation deduced from DFT calculation

Utilizing light energy to oxidize water into oxygen is the unique ability of photosystem II (PSII). This reaction is accomplished through two distinct processes. One is the primary reaction occurring at the reaction center of PSII. This reaction converts the light energy into electrochemical potential energy. The obtained high redox potential P680 is the thermodynamic driving force for water oxidation. The second process is the catalytic water oxidation on the Mn-cluster located at the lumen side of PSII. How to couple these two processes is crucial for the performance of the unique function of PSII. Tyr161 (Yz) and His190 on the D1 protein are key components for this coupling. Experiments have demonstrated that His190 interacts with Yz through a H-bond (Mamedov et al., 1998; Debus, 2001). The turnover of Yz-His190 during water oxidation is shown in scheme 1. Extensive studies have been done on Yz-His190, but the coupling mechanism is still unclear. One major problem is the uncertainty concerning the structural interaction between Yz-His190 and the Mn-cluster, as well as with the substrates. In this paper, we investigate this issue by using quantum chemistry DFT calculations. Combining the new PSII structure data and experimental findings on Yz with our calculations, we suggest that Yz can indeed interact with that His190 in active PSII, and His190 bridging Yz and the Mn-cluster is a promising model for the interaction of Yz and the Mn-cluster. Based on this, a new mechanism for YzHis190 controlling the water oxidation is proposed.