[Calorimetric studies of the effect of amino acid replacements 16Gln-Leu and 26Tyr-Asp on the structural organization and stability of the Cro-repressor from phage lambda].

The scanning calorimetry technique has been used to study the influence of amino acid substitutions on thermodynamic parameters of formation and stabilization of a cooperative structure of Cro repressor of bacteriophage lambda molecule. It is shown that substitutions 16Gln-Leu and 26Tyr-Asp enhances the protein molecule stability by 32 degrees C as compared with the wild-type protein. It is also demonstrated that the denaturation enthalpy of the mutated Cro differs slightly from that of the wild-type protein at the same temperature, while the effective enthalpy value is significantly lower. The analysis of excess heat capacity of the mutated protein shows that this complex function determined experimentally is approximated by two functions, which is indicative of the presence of two quasi-independent transitions. Thus, the stabilization and redistribution of intermolecular interactions evoked by the above substitutions leads to disintegration of the single cooperative repressor molecule in two interacting cooperative domains. The plausible mechanism of formation of such a domain structure is discussed on the basis of the available calorimetric data.