Calcium-induced stabilization of α-amylase against guanidine hydrochloride denaturation

Guanidine hydrochloride (GdnHCl) denaturation of native and Ca-depleted Bacillus licheniformis alpha-amylase (BLA) was investigated both in the absence and presence of 2 mM calcium chloride (CaCl2) using circular dichroism, fluorescence spectroscopy and biological activity. In both states (Ca-depleted and native form), the protein was denatured to a considerable extent in the absence of 2 mM CaCl2 with concomitant loss of biological activity upon increasing GdnHCl concentration. On the other hand, this effect was significantly reduced when 2 mM CaCl2 was included in the incubation mixture as revealed by a higher relative mean residue ellipticity, higher relative fluorescence intensity, smaller change in emission maximum and lesser reduction in biological activity. Interestingly, using these probes, 2 mM CaCl2 seemed to offer the same degree of stability to Ca-depleted BLA as that observed with native BLA in the absence of 2 mM CaCl2. All these results suggest calcium-induced stabilization of BLA against GdnHCl denaturation.