A single K(+)-binding site in the crystal structure of the gastric proton pump

The gastric proton pump (H(+),K(+)-ATPase), a P-type ATPase responsible for gastric acidification, mediates electro-neutral exchange of H(+) and K(+) coupled with ATP hydrolysis, but with an as yet undetermined transport stoichiometry. Here we show crystal structures at a resolution of 2.5 Å of the pump in the E2-P transition state, in which the counter-transporting cation is occluded. We found a single K(+) bound to the cation-binding site of the H(+),K(+)-ATPase, indicating an exchange of 1H(+)/1K(+) per hydrolysis of one ATP molecule. This fulfills the energy requirement for the generation of a six pH unit gradient across the membrane. The structural basis of K(+) recognition is resolved and supported by molecular dynamics simulations, establishing how the H(+),K(+)-ATPase overcomes the energetic challenge to generate an H(+) gradient of more than a million-fold—one of the highest cation gradients known in mammalian tissue—across the membrane.