Ionic-strength-dependent transition of hen egg-white lysozyme at low pH to a compact state and its aggregation on thermal denaturation

Equilibrium acid-induced unfolding of hen egg-white lysozyme has been investigated by a combination of optical methods, size-exclusion chromatography, and differential scanning calorimetry. The results showed the presence of a partially folded state of hen egg-white lysozyme at pH 1.5, characterized by a substantial secondary structure, a large solvent exposure of non-polar clusters, and significantly disrupted tertiary structure. A large enthalpy was also associated with the conversion of the acid-unfolded state to a fully unfolded state. Size-exclusion chromatography and 8-anilino-1-naphthalenesulphonic acid-binding studies showed an ionic-strength-induced transition of the partially folded state to a compact conformation. Furthermore, an ionic-strength-dependent aggregation on thermal unfolding of the partially folded intermediate was also observed. These observations provide insights into the possible features responsible for the stabilization of intermediates in the folding of hen egg-white lysozyme.