Bound water molecules and conformational stabilization help mediate an antigen-antibody association (antigen-antibody complex/three-dimensional structure/enthalpy and entropy of association/hydration)

We report the three-dimensional structures, at 1.8-A resolution, of the Fv fragment of the anti-hen egg white lysozyme antibody D1.3 in its free and antigen-bound forms. These structures reveal a role for solvent molecules in stabi- lizing the complex and provide a molecular basis for under- standing the thermodynamic forces which drive the association reaction. Four water molecules are buried and others form a hydrogen-bonded network around the interface, bridging an- tigen and antibody. Comparison of the structures of free and bound Fv fragment of D1.3 reveals that several of the ordered water molecules in the free antibody combining site are re- tained and that additional water molecules link antigen and antibody upon complex formation. This solvation of the com- plex should weaken the hydrophobic effect, and the resulting large number of solvent-mediated hydrogen bonds, in conjunc- tion with direct protein-protein interactions, should generate a significant enthalpic component. Furthermore, a stabilization of the relative mobilities of the antibody heavy- and light-chain variable domains and of that of the third complementarity- determining loop of the heavy chain seen in the complex should generate a negative entropic contribution opposing the enthal- pic and the hydrophobic (solvent entropy) effects. This struc- tural analysis is consistent with measurements of enthalpy and entropy changes by titration calorimetry, which show that enthalpy drives the antigen-antibody reaction. Thus, the main forces stabilizing the complex arise from antigen-antibody hydrogen bonding, van der Waals interactions, enthalpy of hydration, and conformational stabilization rather than sol- vent entropy (hydrophobic) effects.