The gastric HK-ATPase: structure, function, and inhibition

Abstract

The gastric H,K-ATPase, a member of the P 2 -type ATPase family, is the integral membrane protein responsible for gastric acid secretion. It is an α,β-heterodimeric enzyme that exchanges cytoplasmic hydronium with extracellular potassium. The catalytic α subunit has ten transmembrane segments with a cluster of intramembranal carboxylic amino acids located in the middle of the transmembrane segments TM4, TM5,TM6, and TM8. Comparison to the known structure of the SERCA pump, mutagenesis, and molecular modeling has identified these as constituents of the ion binding domain. The β subunit has one transmembrane segment with N terminus in cytoplasmic region. The extracellular domain of the β subunit contains six or seven Nlinked glycosylation sites. N-glycosylation is important for the enzyme assembly, maturation, and sorting. The enzyme pumps acid by a series of conformational changes from an E 1 (ion site in) to an E 2 (ion site out) configuration following binding of MgATP and phosphorylation. Several experimental observations support the hypothesis that expulsion of the proton at 160 mM (pH 0.8) results from movement of lysine 791 into the ion binding site in the E 2 P configuration. Potassium access from the lumen depends on activation of a K and Cl conductance via a KCNQ1/KCNE2 complex and Clic6. K movement through the luminal channel in E 2 P is proposed to displace the lysine along with dephosphorylation to return the enzyme to the E 1 configuration. This enzyme is inhibited by the unique proton pump inhibitor class of drug, allowing therapy of acid-related diseases.

Arginine substitution of a cysteine in transmembrane helix M8 converts Na + ,K + -ATPase to an electroneutral pump similar to H + ,K + -ATPase

“…Crystal structures of the Na + ,K + -ATPase show that two Na + ions bind at sites that overlap substantially with the K + sites (so-called sites I and II), whereas the third Na + ion is bound at a distinct Na + -specific site (site III) (1,4,5). Two isoforms of H + ,K + -ATPase, with different expression profiles, perform electroneutral H + ,K + -exchange by a mechanism basically similar to the Na + ,K + -ATPase mechanism (6,7). The nongastric H + ,K + -ATPase participates in K + reabsorption in the kidney and colon, whereas the better characterized gastric H + ,K + -ATPase is responsible for acid secretion into the lumen of the stomach and has been proposed to have two different stoichiometries:…”

“…1H + :1K + :1ATP at low luminal pH and 2H + :2K + :1ATP at neutral pH (6)(7)(8). A low-resolution crystal structure of the H + ,K + -ATPase obtained at acidic pH in the presence of the K + congener Rb + shows that one bound Rb + ion occupies a site equivalent to site II of the Na + ,K + -ATPase, whereas the occupancy of site I is very low (8).…”

Cryo-EM structure of gastric H + ,K + -ATPase with a single occupied cation-binding site

“…The reported free energy for ATP hydrolysis of the gastric secretory membrane [−13 kcal/mol (4)] provides sufficient energy to achieve a maximum change in pH (ΔpH) of 4.7 units when two H + ions are transported per hydrolysis of one ATP molecule; thus, the generation of pH 1 (ΔpH > 6 units in the stomach) with a 2H + /2K + / ATP stoichiometry is thermodynamically impossible. Therefore, according to the most widely held hypothesis, the stoichiometry of transported cations per ATP changes from 2 to 1 as the luminal pH decreases (5,6), or the stoichiometry simply remains 1 independent of pH (4), although direct evidence for this is lacking. To address this issue, we attempted to capture H + , K + -ATPase in a transition state of dephosphorylation with bound counter ion(s) in it, to characterize its structural and functional properties.…”

Synthesis and characterization of antibacterial 2-(pyridin-3-yl)-1H-benzo[d]imidazoles and 2-(pyridin-3-yl)-3H-imidazo[4,5-b]pyridine derivatives

“…Because this type of structure is known to have a wide range of biological activity, for example antibacterial [19], antiviral [20], antimicrobial [21], antiulcer proton-pump inhibiting [22,23], and anticancer [24] activity, and because, in the pharmaceutical sciences, new drugs are usually discovered on the basis of molecular modification of lead compounds or already established pharmacophores, we synthesized fourteen novel derivatives of 2-(pyridin-3-yl)-1H-benzo[d]imidazole and 2-(pyridin-3-yl)-3H-imidazo [4,5-b]pyridine.…”