Thermodynamic Studies on the Interaction of Antibodies with β-Amyloid Peptide

Antibodies against β-amyloid peptides (Aβs) are considered an important therapeutic opportunity in Alzheimer's disease. Despite the vast interest in Aβ no thermodynamic data on the interaction of antibodies with Aβ are available as yet. In the present study we use isothermal titration calorimetry (ITC) and surface plasmon resonance to provide a quantitative thermodynamic analysis of the interaction between soluble monomeric Aβ(1−40) and mouse monoclonal antibodies (mAb). Using four different antibodies directed against the N-terminal, middle, and C-terminal Aβ epitopes, we measured the thermodynamic parameters for the binding to Aβ. Each antibody species was found to have two independent and equal binding sites for Aβ with binding constants in the range of 10⁷ to 10⁸ M^-1. The binding reaction was essentially enthalpy driven with a reaction enthalpy of Δ ≈ −19 to −8 kcal/mol, indicating the formation of tight complexes. The loss in conformational freedom was supported by negative values for the reaction entropy Δ . We also measured the heat capacity change of the 1mAb:2Aβ reaction. Δ was large and negative but could not be explained exclusively by the hydrophobic effect. The free energy of binding was found to be linearly correlated with the size of the epitope.