Potassium and sodium ions enhance the activity and thermostability of 1,4-α-glucan branching enzyme from Geobacillus thermoglucosidasius in the presence of glycerol

Metal ions are essential for the performance of metal-dependent proteins and are known to be important for thermophilic proteins containing a large number of charged residues. The performance of thermophilic proteins may be influenced by metal ions through electrostatic interactions between the metal ions and charged residues. In this study, we investigated the effects of metal ions and glycerol on the activity and stability of the thermophilic 1,4-α-glucan branching enzyme (abbreviated GBE; EC 2.4.1.18) from G. thermoglucosidasius STB02. The results indicate that K+ or Na+ enhance the performance of GBE, and that the addition of glycerol further increases the thermostability of GBE. The effects of K+ or Na+ with glycerol on the structure of GBE were further investigated using intrinsic fluorescence spectra and far-UV circular dichroism spectra. The results show that more secondary structural elements are preserved by the addition of K+ or Na+ in the presence of glycerol. The improved maintenance of GBE structural elements after incubation may arise from electrostatic interactions introduced by the added salt, and glycerol provides a hydrophobic environment that strengthens these electrostatic contacts. This provides a useful perspective for understanding the strategy of thermophilic adaptation used by proteins with plenty of charged residues.