Thermodynamic studies on the interaction of calcium ions with alpha-amylase

The interaction of α-amylase from Bacillus amyloliquefaciens with divalent calcium ion was studied by equilibrium dialysis, isothermal titration microcalorimetry, UV spectrophotometry and temperature scanning spectrophotometry methods at 27°C in Tris buffer solution at pH 7.5. There is a set of 17 binding sites for calcium binding on the enzyme with weak positive cooperativeness in binding. The binding of calcium is exothermic (ΔH=−16 kJ mol−1) with mean dissociation binding constant of 0.55 mM. The binding of calcium caused the more stability of the enzyme against surfactant and thermal denaturation. Moreover, the binding of calcium prevents from the spontaneous decrease in biological activity of α-amylase.