Kinetics of conformational changes induced by the binding of various metal ions to bovine α-lactalbumin

The kinetic effects of the binding of various metal ions (Ca 2+, Cd 2+, Co 2+, Mg 2+, Mn 2+, Sr 2+ and Zn 2+) to apo bovine α-lactalbumin has been monitored by means of stopped-flow fluorescence spectroscopy. Our results show that the measured rate constant for the binding of metal ions to the Ca 2+-site increases with increasing binding constant. This is, however, not the case for metal ions binding to the Zn 2+-site. The binding experiments performed at different temperatures allowed us to calculate the activation energy for the transition from the metal-free to the metal-loaded state of the protein. These values do not depend on the nature of the metal ion but are correlated with the type of binding site. As a result, we were able to demonstrate that Mg 2+, a metal ion which was thought to bind to the Ca 2+-site, shows the same binding characteristics as Co 2+ and Zn 2+ and therefore most likely interacts with the residues belonging to the Zn 2+-binding site.