Ionic-strength dependence of the conformational change in the unphosphorylated sodium pump

The conformational change in the unphosphorylated sodium pump was studied as a function of ionic strength to learn whether the rate of the reaction is affected. The results corroborate our proposal [Smirnova, I. N., Lin, S.-H., and Faller, L. D. (1995) Biochemistry 34, 8657-8667] that competitive binding of the transported ions to two (or more) equivalent sites regulates a concerted change in protein conformation. An approximately 10-fold increase in ionic strength decreased the intrinsic affinity of the Na+ conformation of the enzyme for both Na+ and K+ roughly 3-fold, decreased the rate of the change from Na+ to K+ conformation by more than half, and increased the rate of the reverse reaction by about an order of magnitude. The logarithm of the first-order rate constant for the change from Na+ to K+ conformation depended inversely upon the square root of the ionic strength with the extrapolated value at zero ionic strength expressed as a second-order rate constant (1.1 x 10(9) M(-1) sec(-1)) approaching the limit for a diffusion-controlled reaction. The first-order rate constant for the change from K+ to Na+ conformation depended directly upon ionic strength and extrapolated to a zero-ionic-strength value (0.002 s(-1)) far below the diffusion limit. The results are compatible with shielding of oppositely charged domains that move through the solvent when the pump cycles between conformations. Electrostatic interactions between domains evidently contribute to the driving force for the change from Na+ to K+ conformation and to the stability of the K+ conformation.