The catalytic subunit of the gastric H(+)/K(+) ATPase pump which transports H(+) ions in exchange for K(+) ions across the apical membrane of parietal cells (PubMed:10722662, PubMed:24188822). Uses ATP as an energy source to pump H(+) ions to the gastric lumen while transporting K(+) ion from the lumen into the cell (PubMed:10722662, PubMed:24188822). Remarkably generates a million-fold proton gradient across the gastric parietal cell membrane, acidifying the gastric juice down to pH 1 (By similarity). Within a transport cycle, the transfer of a H(+) ion across the membrane is coupled to ATP hydrolysis and is associated with a transient phosphorylation that shifts the pump conformation from inward-facing (E1) to outward-facing state (E2). The release of the H(+) ion in the stomach lumen is followed by binding of K(+) ion converting the pump conformation back to the E1 state (PubMed:10722662, PubMed:24188822) (By similarity). ATP + H(+)(in) + H2O + K(+)(out) = ADP + 2 H(+)(out) + K(+)(in) + phosphate The gastric H(+)/K(+) ATPase pump is composed of the catalytic alpha subunit ATP4A and the regulatory beta subunit ATP4B (PubMed:10722662, PubMed:11909858). Interacts (via the P-domain) with ATP4B (via N-terminus); this interaction stabilizes the lumenal-open E2 conformation state and prevents the reverse reaction of the transport cycle (By similarity). Localized in the apical canalicular membrane of parietal cells. Belongs to the cation transport ATPase (P-type) (TC 3.A.3) family. Type IIC subfamily.