Equilibrium and kinetic study of sodium- and potassium-induced conformational changes of apo-alpha-lactalbumin.

Equilibrium and kinetics of Na+-and K+-induced conformational changes of apo-alpha-lactalbumin were studied by means of circular dichroism. While apo-alpha-lactalbumin was considerably unfolded in the absence of Na+ or K+ in 20 mM Tris at pH 8.0 and 25 degrees, both the monovalent cations restored the tertiary structure of the protein. Apparent binding constants of Na+ and K+ to the apoprotein were estimated from the equilibria of the Na+- and K+-induced conformational changes. Based on kinetic data of the conformational changes induced by the monovalent cations, binding mechanism of the ions to the apo-protein was examined. Bound alkali-metal ions stabilize the native-like state and an activated state in the unfolding-refolding reaction of the apoprotein.