BOUND WATER-MOLECULES AND CONFORMATIONAL STABILIZATION HELP MEDIATE AN ANTIGEN-ANTIBODY ASSOCIATION

We report the three-dimensional structures, at 1.8-angstrom 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 fo r solvent molecules in stabilizing the complex and provide a molecular basis for understanding the thermodynamic forces which drive the asso ciation reaction. Four water molecules are buried and others form a hy drogen-bonded network around the interface, bridging antigen and antib ody. Comparison of the structures of free and bound Fv fragment of D1. 3 reveals that several of the ordered water molecules in the free anti body combining site are retained and that additional water molecules l ink antigen and antibody upon complex formation. This solvation of the complex should weaken the hydrophobic effect, and tbe resulting large number of solvent-mediated hydrogen bonds, in conjunction 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 t he third complementarity-determining loop of the heavy chain seen in t he complex should generate a negative entropic contribution opposing t he enthalpic 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 comp lex arise from antigen-antibody hydrogen bonding, can der Waals intera ctions, enthalpy of hydration, and conformational stabilization rather than solvent entropy (hydrophobic) effects.