A CALORIMETRIC STUDY OF THE EFFECTS OF GLN(16)-LEU AND TYR(26)-ASP AMINO-ACID SUBSTITUTIONS ON THE STRUCTURE AND STABILITY OF THE CRO REPRESSOR OF PHAGE-LAMBDA

The effects of amino acid substitutions on the thermodynamic parameter s of the cooperative molecular structure formation and stabilization i n the phage lambda Cro repressor were investigated by scanning calorim etry. Gln(16)-Leu and Tyr(26)-Asp substitutions were found to increase protein stability by 32 degrees C relative to the wild-type protein. The values of the denaturation enthalpy for the wild-type and variant Cro protein, as measured calorimetrically at the same temperature, dif fer insignificantly, while the apparent enthalpy is considerably small er for the variant protein. An analysis of the excess heat capacity of the variant protein indicated that this composite parameter measured experimentally can be approximated by a combination of two functions, suggesting the occurrence of two pseudoindependent transitions. The st abilization and redistribution of intramolecular interactions in the r epressor molecule caused by the substitutions result thus in splitting the unified cooperative molecular system into two interacting domains . A plausible mechanism of the domain structure formation is discussed on the basis of the calorimetric evidence obtained.