Ion Selectivity of the Cytoplasmic Binding Sites of the Na,K-ATPase: I. Sodium Binding is Associated with a Conformational Rearrangement

Ion Selectivity of the Cytoplasmic Binding Sites of the Na,K-ATPase: I. Sodium Binding is... To investigate Na+ binding to the ion-binding sites presented on the cytoplasmic side of the Na,K-ATPase, equilibrium Na+-titration experiments were performed using two fluorescent dyes, RH421 and FITC, to detect protein-specific actions. Fluorescence changes upon addition of Na+ in the presence of various Mg2+ concentrations were similar and could be fitted with a Hill function. The half-saturating concentrations and Hill coefficients determined were almost identical. As RH421 responds to binding of a Na+ ion to the third neutral site whereas FITC monitors conformational changes in the ATP-binding site or its environment, this result implies that electrogenic binding of the third Na+ ion is the trigger for a structural rearrangement of the ATP-binding moiety. This enables enzyme phosphorylation, which is accompanied by a fast occlusion of the Na+ ions and followed by the conformational transition E1/E2 of the protein. The coordinated action both at the ion and the nucleotide binding sites allows for the first time a detailed formulation of the mechanism of enzyme phosphorylation that occurs only when three Na+ ions are bound. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Journal of Membrane Biology Springer Journals

Ion Selectivity of the Cytoplasmic Binding Sites of the Na,K-ATPase: I. Sodium Binding is Associated with a Conformational Rearrangement

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Publisher
Springer-Verlag
Copyright
Copyright © Inc. by 1999 Springer-Verlag New York
Subject
Life Sciences; Biochemistry, general; Human Physiology
ISSN
0022-2631
eISSN
1432-1424
D.O.I.
10.1007/s002329900511
Publisher site
See Article on Publisher Site

Abstract

To investigate Na+ binding to the ion-binding sites presented on the cytoplasmic side of the Na,K-ATPase, equilibrium Na+-titration experiments were performed using two fluorescent dyes, RH421 and FITC, to detect protein-specific actions. Fluorescence changes upon addition of Na+ in the presence of various Mg2+ concentrations were similar and could be fitted with a Hill function. The half-saturating concentrations and Hill coefficients determined were almost identical. As RH421 responds to binding of a Na+ ion to the third neutral site whereas FITC monitors conformational changes in the ATP-binding site or its environment, this result implies that electrogenic binding of the third Na+ ion is the trigger for a structural rearrangement of the ATP-binding moiety. This enables enzyme phosphorylation, which is accompanied by a fast occlusion of the Na+ ions and followed by the conformational transition E1/E2 of the protein. The coordinated action both at the ion and the nucleotide binding sites allows for the first time a detailed formulation of the mechanism of enzyme phosphorylation that occurs only when three Na+ ions are bound.

Journal

The Journal of Membrane BiologySpringer Journals

Published: Apr 1, 1999

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