The Stabilization of Cation Binding and its Relation to Na⁺/K⁺-ATPase Structure and Function

Abstract

During the transport cycle of the sodium pump, monovalent cations are somehow smuggled through the low dielectric medium of the plasma membrane as Na⁺ ions are expelled from the cell and K⁺ ions taken up. These ions also have specific and well-studied effects on the partial catalytic reactions of the Na⁺,K⁺-ATPase activity that accompanies transport. These two aspects of the roles of Na⁺ and K⁺ ions in active transport form the subject of the present article. It is an implicit assumption in most studies that the Na⁺ ions which activate phosphorylation from ATP are indeed the Na⁺ ions transported and the extracellular K⁺ ions which catalyze dephosphorylation are taken up by the cell. The central issues in these two aspects are which parts (amino acid residues) of the protein interact directly with the cation and how are the processes involved in ATP hydrolysis linked to transport. Obviously, this last feature also involves cation recognition by the protein but adds to this the question of “coupling” or “transduction”. These are terms used to connote information transfer between different regions of the protein which are directly involved in cation binding, ATP binding, and phosphorylation.