Our goal is to learn how the gastric proton pump works in molecular detail. It has been demonstrated that the adenosine 5'-triphosphatase (ATPase) isolated in the microsomal fraction from gastric mucosae is necessary and sufficient for coupling the energy released during ATP hydrolysis to the transport of protons in exchange for K+ ions. However, neither the molecular biology, nor the underlying chemistry of the reaction is understood. This proposal focuses on the number of nucleotide sites and their function, the identity of the subunits and quaternary structure of the enzyme, the role of Mg2+ in catalysis and transport, and the involvement of a protein conformational change in the energetics of phosphoenzyme formation and physical transport of ions. The strategy proposed to learn the number of nucleotide sites and their function is to build up a library of site-selective, covalent modifications of the H,K-ATPase. The effect of each reagent on the reactions catalyzed by the enzyme will be studied and the site of covalent modification pinpointed by sequencing the labeled peptide. This approach will also help to resolve the issue of subunit heterogeneity. Evidence for interaction between subunits will be obtained by studying their thermal denaturation. Mg2+ is the only cofactor required for formation of an acylphosphate enzyme intermediate. Therefore it must play a key role in the catalytic mechanism. The number of Mg2+ sites will be determined, and which ones are involved in catalysis will be established. The distance from the nucleotide site(s) to the K+ activation and transport sites will be measured. The thermodynamics of ligand-binding will be studied to learn whether a protein conformational change is triggered that could explain catalysis and transport. This work has implications for the treatment of ulcer disease. Substituted benzimidazoles have been found to inhibit gastric acid secretion in vivo and several are currently undergoing clinical trials for the treatment of peptic ulcers. The target of these drugs is the H,K-ATPase, but how they work is unknown. More detailed knowledge of the mechanism of catalysis and transport is needed both to understand how this new class of existing drugs inhibits acid secretion, and to design more effective inhibitors of the gastric proton pump in the future.

Agency
National Institute of Health (NIH)
Institute
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Type
Research Project (R01)
Project #
1R01DK036873-01
Application #
3235425
Study Section
Physiological Chemistry Study Section (PC)
Project Start
1986-09-29
Project End
1989-08-31
Budget Start
1986-09-29
Budget End
1987-08-31
Support Year
1
Fiscal Year
1986
Total Cost
Indirect Cost
Name
University of California Los Angeles
Department
Type
Schools of Medicine
DUNS #
119132785
City
Los Angeles
State
CA
Country
United States
Zip Code
90095
Smirnova, I N; Lin, S H; Faller, L D (1995) An equivalent site mechanism for Na+ and K+ binding to sodium pump and control of the conformational change reported by fluorescein 5'-isothiocyanate modification. Biochemistry 34:8657-67
Smirnova, I N; Faller, L D (1995) Mechanism of the conformational change in sodium pump reported by eosin. Biochemistry 34:13159-69
Smirnova, I N; Faller, L D (1993) Mechanism of K+ interaction with fluorescein 5'-isothiocyanate-modified Na+,K(+)-ATPase. J Biol Chem 268:16120-3
Phan, B C; Faller, L D; Reisler, E (1993) Kinetic and equilibrium analysis of the interactions of actomyosin subfragment-1.ADP with beryllium fluoride. Biochemistry 32:7712-9
Lin, S H; Faller, L D (1993) Time resolution of fluorescence changes observed in titrations of fluorescein 5'-isothiocyanate-modified Na,K-ATPase with monovalent cations. Biochemistry 32:13917-24
Smirnova, I N; Faller, L D (1993) Role of Mg2+ ions in the conformational change reported by fluorescein 5'-isothiocyanate modification of Na+,K(+)-ATPase. Biochemistry 32:5967-77
Faller, L D; Diaz, R A; Scheiner-Bobis, G et al. (1991) Temperature dependence of the rates of conformational changes reported by fluorescein 5'-isothiocyanate modification of H+,K(+)- and Na+,K(+)-ATPases. Biochemistry 30:3503-10
Faller, L D (1990) Binding of the fluorescent substrate analogue 2',3'-O-(2,4,6-trinitrophenylcyclohexadienylidene)adenosine 5'-triphosphate to the gastric H+,K(+)-ATPase: evidence for cofactor-induced conformational changes in the enzyme. Biochemistry 29:3179-86
Faller, L D; Diaz, R A (1989) Evidence from 18O exchange measurements for steps involving a weak acid and a slow chemical transformation in the mechanism of phosphorylation of the gastric H+, K+-ATPase by inorganic phosphate. Biochemistry 28:6908-14
Faller, L D (1989) Competitive binding of ATP and the fluorescent substrate analogue 2',3'-O-(2,4,6-trinitrophenylcyclohexadienylidine) adenosine 5'-triphosphate to the gastric H+,K+-ATPase: evidence for two classes of nucleotide sites. Biochemistry 28:6771-8