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.