Modulation of the Na,K-ATPase by Magnesium Ions

Abstract

Since the beginning of investigations of the Na,K-ATPase, it has been well-known that Mg is an essential cofactor for activation of enzymatic ATP hydrolysis without being transported through the cell membrane. Moreover, experimental evidence has been collected through the years that shows that Mg ions have a regulatory effect on ion transport by interacting with the cytoplasmic side of the ion pump. Our experiments allowed us to reveal the underlying mechanism. Mg is able to bind to a site outside the membrane domain of the protein's α subunit, close to the entrance of the access channel to the ion-binding sites, thus modifying the local concentration of the ions in the electrolyte, of which Na, K, and H are of physiological interest. The decrease in the concentration of these cations can be explained by electrostatic interaction and estimated by the Debye-Hückel theory. This effect provokes the observed apparent reduction of the binding affinity of the binding sites of the Na,K-ATPase in the presence of various Mg concentrations. The presence of the bound Mg, however, does not affect the reaction kinetics of the transport function of the ion pump. Therefore, stopped-flow experiments could be performed to gain the first insight into the Na binding kinetics on the cytoplasmic side by Mg concentration jump experiments.

Effect of lansoprazole on human sperm motility, sperm viability, seminal nitric oxide production, and seminal calcium chelation

“…Sodium-potassium adenosine triphosphatase (Na + /K + -ATPase), also known as sodium-potassium pump, is an enzyme that is found in the plasma membrane of almost all animal cells( 32 34 ). Na + /K + -ATPase actively pumps sodium (out of cell) and potassium (into cell) against their concentration gradients to regulate the cell volume, maintain the cell resting potential, enhance cellular transport, affect signal transduction, and control neural activity states( 35 ). It has been shown that Na + /K + -ATPase is expressed in the midpiece of the spermatozoa and has been found to be very important for sperm motility( 36 ).…”

Low salinity‐induced alterations in epithelial ultrastructure, Na+/K+‐ATPase immunolocalization and enzyme kinetic characteristics in the gills of the thinstripe hermit crab, Clibanarius vittatus (Anomura, Diogenidae)

“…The inhibition seen with excess Mg 2+ , likely a result of Mg 2+ binding to a second inhibitory site (Pedemonte & Beaugé, 1983) corroborates the notion of multiple binding sites. Mg 2+ can access the channel connecting the binding sites of the transported ions with the extracellular aqueous phase when the enzyme is in the E 2 conformation, modifying local ion concentrations in the electrolyte, but not affecting pump reaction kinetics (Apell, Hizler, & Schreiber, 2017).…”

Binding of Potassium Ions Inside the Access Channel at the Cytoplasmic Side of Na+,K+-ATPase

“…It has been suggested that this ion has a specific binding site in the protein, different from the enzymatic site, and its presence leads to an apparent decrease of the sodium and potassium affinity of binding sites. Magnesium ions reduce binding of sodium ions, which can be caused by both allosteric and electrostatic actions on the Na + ,K + -ATPase [41]. The quantitative estimates made earlier on the basis of competition of sodium and magnesium ions in the cytoplasmic access channel of Na + ,K + -ATPase, showed that the dissociation constant of Na + in the binding site increased by approximately six times in the presence of magnesium ions [27].…”

“…The similarity of the effect of both ions species in sign and magnitude is in favor of the electrostatic hypothesis of the magnesium effect on sodium and potassium binding in the E 1 conformation. The presence of Mg 2+ both in the double layer close to the membrane surface and bound to the protein near the entrance to the access channel [41] is expected to create a potential barrier for cations on the way from the solution to the binding sites. Then the decrease of cooperativity can be explained by a rate-limitation of the ion flux from the solution to the binding sites since ions have to jump over the barrier one by one.…”