The effect of calcium binding on the unfolding barrier: A kinetic study on homologous α-amylases

Extreme thermostabilities of proteins can be achieved by binding co-factors to the protein structures. For various α-amylases protein stabilization upon calcium binding is a well-known phenomenon. In the present study the mechanism of stabilization of three homologous α-amylases was investigated by measuring the unfolding kinetics with CD spectroscopy. For this purpose thermal unfolding kinetics of calcium saturated and calcium depleted enzymes were analyzed by means of Eyring-plots. The free energy change between the native and the transition state which characterized the unfolding barrier height was found to be proportional to the number of calcium ions bound to the protein structures. For the most thermostable α-amylases calcium binding caused a significant increase in the enthalpy change, which was partly compensated by increased entropy changes.