| Summary: | Photosystem II (PSII) uses water as the terminal electron donor, producing oxygen in the Mn<sub>4</sub>CaO<sub>5</sub> oxygen evolving complex (OEC), while cytochrome c oxidase (CcO) reduces O<sub>2</sub> to water in its heme⁻Cu binuclear center (BNC). Each protein is oriented in the membrane to add to the proton gradient. The OEC, which releases protons, is located near the P-side (positive, at low-pH) of the membrane. In contrast, the BNC is in the middle of CcO, so the protons needed for O<sub>2</sub> reduction must be transferred from the N-side (negative, at high pH). In addition, CcO pumps protons from N- to P-side, coupled to the O<sub>2</sub> reduction chemistry, to store additional energy. Thus, proton transfers are directly coupled to the OEC and BNC redox chemistry, as well as needed for CcO proton pumping. The simulations that study the changes in proton affinity of the redox active sites and the surrounding protein at different states of the reaction cycle, as well as the changes in hydration that modulate proton transfer paths, are described.
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