During photosynthesis, the photosystem II (PSII) enzyme catalyzes the light-driven oxidation of two water molecules, abstracting four electrons and four protons while releasing O 2 as a byproduct. One of the open-questions in the field is how protons are strategically removed from the active site, a Mn 4 CaO 5 cluster called the oxygen-evolving complex (OEC), during the reaction cycle via conserved water channels, and how proton transfer contributes to O–O bond formation energetics. Site-directed mutagenesis has been used to study the reaction mechanism with one of the most influential mutations being the Val185Asn substitution on the D1 subunit, which substantially slows O 2 release kinetics without abolishing catalytic activity. We have investigated the structural basis of the D1-Val185Asn substitution by determining a 1.99 Å-resolution cryo-EM structure. We observe that Asn185 orients away from the OEC and donates an H-bond to Cl1, a conserved chloride ion. We furthermore observe an alternative D2-Glu312-facing conformation of the D2-Lys317 side chain in the Cl1 water channel, a conformation that is consistent with recent models for proton transfer. These changes also produce perturbations to the Cl1 channel hydrogen-bonding network. Overall, these finding provide new insight into proton transfer in the Cl1 channel and its effect on the water oxidation reaction mechanism.
Flesher et al. (Sun,) studied this question.