New insight into protein-ligand interactions. The case of the D-galactose/D-glucose-binding protein from Escherichia coli.

In this work we have shown that the unfolding-refolding process of the D-galactose/D-glucose-binding protein (GGBP) in the presence of glucose (Glc) induced by the chemical denaturant Gdn-HCI is reversible. In addition, Glc binding does not only stabilize GGBP structure but it also considerably slows down the achievement of the equilibrium between the native protein in GGBP/Glc complex and the unfolded protein. The limiting step of the unfolding-refolding process of the complex GGBP/Glc is the arrangement/de-arrangement of the configuration fit between the protein in the native state and the ligand. The rate of these processes increases/decreases with the increase/decrease of the denaturant concentration. Calcium depletion had a pronounced destabilizing effect on the structure of GGBP but did not affect the stability of GGBP/Glc complex. Unfolding of GGBP/Ca complex is reversible. Only incubation of the unfolded protein at high temperature leads to an irreversible process due to the aggregation of the protein. The amount of protein aggregation is determined by the protein concentration, the temperature and the duration of the incubation.