Cytochrome C oxidase plays a crucial role in cellular respiration and energy generation. It reduces O to water and uses the released free energy to move protons across mitochondrial and bacterial cell membranes adding to the essential electrochemical gradient. Energy storage requires that protons are taken up from the high pH, N-side and released to the low pH, P-side of the membrane. We identify a potential proton exit from a buried cluster of polar residues (the proton loading site) to the P-side of CcO via paths made up of waters and conserved residues. Two water cavities connect the proton exit pathway to the surface only when hydrated. Changing the degree of hydration may control otherwise energetically favorable proton backflow from the P-side.

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