Complex formation of a calcium-dependent antibody: A thermodynamical consideration

The elution of FLAG-fusions (an octapeptide with the sequence DYKDDDDK) from immobilized anti-FLAG antibody M1 cannot be explained by a switch of the equilibrium binding constant to a lower value. To get a further insight into thermodynamics, the binding of anti-FLAG antibody M1 to the FLAG peptide was studied by real-time biosensor technology at seven different temperatures in the range from 5 to 35 °C. Binding studies were performed in the presence and absence of calcium. Thermodynamic parameters such as change in Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were evaluated from the corresponding equilibrium data applying the integrated Van’t Hoff equation. In contrast to similar kinetic data obtained, the contribution of ΔH and ΔS to ΔG in the presence or absence of calcium results in a different conformation of the antibody–antigen complex under binding and non-binding conditions. Therefore, complex dissociation with EDTA must be effected during a transition state of complex formation and dissociation.