Thermodynamic analysis of the interaction between the 0.5beta Fv fragment and the RP135 peptide antigen derived from the V3 loop of HIV-1 gp120.

The Fv fragment of the 0.5beta monoclonal antibody has recently been constructed, expressed, and purified. It binds with nanomolar affinity to the immunogenic RP135 peptide that is derived from the principal neutralizing determinant of HIV-1 in the third hypervariable region of gp120. Here, we analyzed the temperature-dependence of binding of the 0.5beta Fv fragment to the RP135 peptide and a series of mutants thereof. Our results show that there is almost complete enthalpy-entropy compensation in the effects of mutations in the peptide on binding to the Fv, indicating that the mutations do not change the binding mechanism. There is good correlation, for residues within the antigenic epitope, between mutational effects on DeltaCp and calculated values of DeltaDeltaCp based on the extent of burial of polar and non-polar surface areas of amino acids. The value of DeltaCp for the binding of the 0.5beta Fv fragment to the wild-type RP135 peptide is found to be -5.0 (+/- 0.9) kcal K-1 mol-1 in the presence of 0.1% Tween-20 but only -0.1 (+/- 0.9) kcal K-1 mol-1 in its absence. This result has important implications for the successful application of the structural parameterization approach to predicting changes in heat capacity that accompany binding reactions carried out in the presence of detergent or protein-stabilizing agents.