text: The ion transport is accomplished by cyclical conformational changes of the enzyme between its two main reaction states, E1 and E2. The cytoplasmic-open E1 and luminal-open E2 states have high affinity for H and K. The expulsion of the proton at 160 mM (pH 0.8) concentration results from movement of lysine 791 into the ion binding site in the E2P configuration.