Selective ion binding by human lysozyme studied by the statistical mechanical integral equation theory

Selective ion-binding by human lysozyme and its mutants is probed with the three-dimensional interaction site model theory, which is the statistical mechanical integral equation theory. The three-dimensional distribution of ions as well as water molecules was calculated for aqueous solutions of three different electrolytes CaCl₂, NaCl and KCl, and for four different mutants of the human lysozyme: wild type, Q86D, A92D, Q86D/A92D that have been studied experimentally. For the wild type of the protein in the aqueous solutions of all the electrolytes studied, there are no distributions observed for the ions inside the active site. The A92D and Q86D/A92D mutants show a large peak of Na⁺ in the recognition site. Especially, holo-Q86D/A92D, one of the mutants, shows conspicuous peak of Ca²⁺. These behaviors are in accord with the experimental results.