title: Kinetics of K(+) occlusion by the phosphoenzyme of the Na(+),K(+)-ATPase.
abstract: Investigations of K(+)-occlusion by the phosphoenzyme of Na(+),K(+)-ATPase from shark rectal gland and pig kidney by stopped-flow fluorimetry reveal major differences in the kinetics of the two enzymes. In the case of the pig enzyme, a single K(+)-occlusion step could be resolved with a rate constant of 342 (± 26) s⁻¹. However, in the case of the shark enzyme, two consecutive K(+)-occlusions were detected with rate constants of 391 (± 19) s⁻¹ and 48 (± 2) s⁻¹ at 24°C and pH 7.4. A conformational change of the phosphoenzyme associated with K(+)-occlusion is, thus, the major rate-determining step of the shark enzyme under saturating concentrations of all substrates, whereas for the pig enzyme the major rate-determining step under the same conditions is the E2 → E1 transition and its associated K(+) deocclusion and release to the cytoplasm. The differences in rate constants of the K(+) occlusion reactions of the two enzymes are paralleled by compensating changes to the rate constant for the E2 → E1 transition, which explains why the differences in the enzymes' kinetic behaviors have not previously been identified.
authors.affiliation: School of Chemistry, University of Sydney, Sydney, New South Wales, Australia.
authors.forename: Sian L
authors.initials: SL
authors.lastname: Myers
authors.forename: Flemming
authors.initials: F
authors.lastname: Cornelius
authors.forename: Hans-Jürgen
authors.initials: HJ
authors.lastname: Apell
authors.forename: Ronald J
authors.initials: RJ
authors.lastname: Clarke
mesh_terms: D000818:Animals; D005453:Fluorescence; D007700:Kinetics; D010750:Phosphoproteins / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D012754:Sharks; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D034421:Sus scrofa
chemical_list: D010750:Phosphoproteins; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium