3.E.2.2.1
Cyanobacterial photosystem II in thylakoid membranes (Sarcina et al. 2006). Photosynthetic water oxidation is catalyzed by the Mn₄CaO₅ cluster of photosystem II (PSII) with linear progression through five S-state intermediates (S₀ to S₄). To reveal the mechanism of water oxidation, Suga et al. 2019 analyzed structures of PSII of Thermosynechococcus vulcanus in the S₁, S₂, and S₃ states by x-ray free-electron laser serial crystallography. No insertion of water was found in S₂, but flipping of D1 Glu¹⁸⁹ upon transition to S₃ led to the opening of a water channel and provided a space for incorporation of an additional oxygen ligand, resulting in an open cubane Mn₄CaO₆ cluster with an oxyl/oxo bridge. Structural changes of PSII between the different S states revealed cooperative action of substrate water access, proton release, and dioxygen formation in photosynthetic water oxidation (Suga et al. 2019). The oxygen-evolving complex (OEC) is located at a node of five water channels involved in proton release, balancing the net charge of the OEC, and inlet of substrate water (Shen 2015). The PsbJ protein is required for photosystem II activity in centers lacking the PsbO and PsbV lumenal subunits (Choo et al. 2021). The mechanisms by which protons move into the buried active sites of Photosystem II (PSII), bacterial RCs (bRCs) and through the proton pumps, Bacteriorhodopsin (bR), Complex I and Cytochrome c oxidase (CcO), have been reviewed by Kaur et al. 2021. These proteins all use water filled proton transfer paths. The proton pumps, that move protons uphill from low to high concentration compartments, also utilize Proton Loading Sites (PLS), that transiently load and unload protons, and gates, which block backflow of the protons. PLS and gates must be synchronized so PLS proton affinity is high when the gate opens to the side with few protons and low when the path is open to the high concentration side. Proton transfer paths have different design features (Kaur et al. 2021). Photosystem II (PSII) utilizes light energy to split water, and the electrons extracted from water are transferred to Q_B, a plastoquinone molecule bound to the D1 subunit of PSII (Kamada et al. 2023). Many artificial electron acceptors (AEAs) with molecular structures similar to that of plastoquinone can accept electrons from PSII. Kamada et al. 2023 solved the crystal structure of PSII treated with three different AEAs, 2,5-dibromo-1,4-benzoquinone, 2,6-dichloro-1,4-benzoquinone, and 2-phenyl-1,4-benzoquinone, at 1.95 to 2.10 Å resolution. The results showed that all AEAs substitute for Q_B and are bound to the Q_B-binding site (Q_B site) to receive electrons, but their binding strengths are different, resulting in differences in their efficiencies to accept electrons. The acceptor 2-phenyl-1,4-benzoquinone binds most weakly to the Q_B site and showed the highest oxygen-evolving activity, implying a reverse relationship between the binding strength and oxygen-evolving activity. A novel quinone-binding site, designated the Q_D site, was discovered, which is located in the vicinity of Q_B site and close to the Q_C site, a binding site reported previously. This Q_D site is expected to play a role as a channel or a storage site for quinones to be transported to the Q_B site (Kamada et al. 2023). Flv3A facilitates O₂ photoreduction and affects H₂ photoproduction independently of Flv1A in diazotrophic Anabaena filaments (Santana-Sánchez et al. 2023).