Conformational and phase transitions in lysozyme-thermosensitive polyelectrolyte complexes

Abstract

The interaction of a small globular protein, lysozyme, with a thermosensitive N-isopropylacrylamide-sodium styrene sulfonate copolymer at pH 4.6 was studied by high-sensitivity differential scanning calorimetry. It was shown that, under these conditions, the copolymer and the protein are involved in formation of polyelectrolyte complexes. It was demonstrated that complexation affects the conformational state of lysozyme. One heat capacity peak attributed to protein denaturation or two well-resolved peaks related to denaturation of free and bound proteins were observed in the DSC curves depending on the mixture composition. For both forms of lysozyme, denaturation parameters (temperature and enthalpy) were determined as a function of mixture composition. For mixtures with low lysozyme contents, the above parameters of bound protein denaturation were independent of the mixture composition. At higher protein contents, these parameters increased with a rise in the protein content. The binding isotherm for the protein with the copolymer was obtained from calorimetric data at 64 ± 1°C. An analysis of the isotherm suggests that the native protein is bound to 24 equivalent binding sites of the polymer matrix. It was established that there are nearly 10 charged units of the copolymer per protein molecule in the native conformation. Lysozyme in the unfolded conformation additionally interacts with hydrophobic groups of the copolymer.