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. Uses ATP as an energy source to pump H(+) ions to the gastric lumen while transporting K(+) ion from the lumen into the cell (By similarity). 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:29618813, PubMed:31436534, PubMed:30143663, PubMed:19387495). ATP + H(+)(in) + H2O + K(+)(out) = ADP + 2 H(+)(out) + K(+)(in) + phosphate Down-regulated by K(+)-competitive acid blockers (P-CABs) such as vonoprazan. The gastric H(+)/K(+) ATPase pump is composed of the catalytic alpha subunit ATP4A and the regulatory beta subunit ATP4B (PubMed:29618813, PubMed:19387495, PubMed:21224846). 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 (PubMed:19387495). 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.