Thermodynamic analysis of additivity between the heavy and light chains in affinity maturation of an antibody

In a recent article published in Molecular Immunology [Furukawa, K., Shimizu, T., Murakami, A., Kono, R., Nakagawa, M., Sagawa, T., Yamato, I., Azuma, T., 2007. Strategy for affinity maturation of an antibody with high evolvability to (4-hydroxy-3-nitrophenyl) acetyl hapten. Mol. Immunol. 44, 2436-2445], the authors measure thermodynamic parameters of the antigen-antibody interaction for a set of antibodies using an isothermal titration calorimetry to quantitatively assess the contribution of amino acid replacements to an increase in affinity during antibody maturation. One of the findings in their study is that the binding free energy change elicited by mutations in the heavy and light chains is additive. In this letter, we report our analysis of their results in terms of equilibrium thermodynamics to show that enthalpy-entropy compensation is responsible for the additivity.