The gastric proton pump, H+,K+-ATPase is a membrane protein (P-type ATPase) responsible for acidifying the gastric juice up to pH 1, and is thus an important drug target for treating gastric acid-related diseases. I will present the crystal structures of the H+,K+-ATPase in complex with two acid blockers in the luminal-open E2P state. These drugs have partially overlapped, but clearly distinguishable, binding modes, which are defined in the middle of a conduit running from the gastric lumen to the cation-binding site. The crystal structures also revealed the unusual configuration of the cation-binding site that enables the extrusion of a single proton into the pH1 solution of the stomach. We also show crystal structures of E2-P transition state, in which the counter-transporting K+ is occluded. We found a single K+ bound to the cation-binding site of H+,K+-ATPase, indicating an exchange of 1H+/1K+ per hydrolysis of one ATP molecule. The structural and biochemical analysis establishes how the gastric pump overcomes the energetic challenge to generate an H+ gradient of more than a million-fold - the highest cation gradient known in any mammalian tissue – across the membrane. -Dr. Kazuhiro Abe