3.A.3.1.2
H⁺-, K⁺-ATPase (gastric; H⁺ efflux; K⁺ uptake). Two H₃O⁺ may be transported per ATP hydrolyzed.  Howeve, a cryo-electron microscope structure suggests that 1 H⁺ and 1 K⁺ are transporter per ATP hydrolyzed, providing the energy needed to generate the one million fold H⁺ concentration gradient effected by this enzyme (Abe et al. 2012).  The detailed mechanism has been discussed, and the roles of essential residues have been proposed (Shin et al. 2011).  A number of inhibitors of acid secretion have been identified, and these are of pharmacological importance (Shin et al. 2011). The catalytic alpha subunit has ten transmembrane segments with a cluster of intramembranal carboxylic amino acids located in the middle of TMSs 4, 5, 6 and 8. The beta subunit has one TMS with the N terminus in the cytoplasm. The extracellular domain of the beta subunit contains six or seven N-linked glycosylation sites. N-glycosylation is important for enzyme assembly, maturation and sorting (Shin et al. 2009). The cryo-EM structure with bound BYK99, a high-affinity member of K⁺-competitive, imidazo[1,2-a]pyridine inhibitors, has been solved (Abe et al. 2017).