Hydrogen-potassium adenosine triphosphatase (H⁺, K⁺-ATPase) belongs to a family of P-type cation-transporting ATPases that also includes Ca²⁺-ATPase and Na⁺, K⁺-ATPase. H⁺, K⁺-ATPase is responsible for acid secretion into the stomach and catalyzes electroneutral exchange of cytoplasmic hydrogen ions and external potassium ions coupled with ATP hydrolysis. The enzyme is found in gastric parietal cells where it is located in cytoplasmic vesicles or apical plasma membranes of the secretory canaliculus. H⁺, K⁺-ATPase is a heterodimer consisting of a high molecular weight catalytic a subunit and a smaller but heavily glycosylated b subunit. The a subunit of H⁺, K⁺-ATPase has 10 transmembrane domains and contains sites for ATP binding and its acylphosphorylation, binding sites of proton pump inhibitors, and sites responsible for ion recognition.