Ca-binding to Bacillus licheniformis α-amylase (BLA)

Ca-induced renaturation of Bacillus licheniformis α-amylase in the presence of urea has been employed to determine the binding constants of the ion. The native enzyme is folded at 3 M urea while the Ca-depleted protein is largely unfolded at this denaturant concentration. Refolding of the protein has been monitored by circular dichroism and the titration curves have been analyzed assuming a model of three independent binding sites. The stoichiometry has been taken from X-ray studies. The refolded protein exhibits a secondary structure that is similar but not identical to that of the native protein. The binding constants have been used to construct a phase diagram that illustrates the contribution of Ca-binding to the resistance against urea unfolding.