We report the three-dimensional structures, at 1.8-A resolution, ofthe Fv fragment ofthe anti-hen egg white lysozyme antibody D1.3 in its free and antigen-bound forms.These structures reveal a role for solvent molecules in stabilzing the complex and provide a molecular basis for understanding the thermodynamic forces which drive the association reaction. Four water molecules are buried and others form a hydrogen-bonded network around the interface, bridging antigen 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 retained and that additional water molecules link antigen and antibody upon complex formation. This salvation of the complex should weaken the hydrophobic effect, and the resulting large number of solvent-mediated hydrogen bonds, in conijunction 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 complementaritydetermining loop of the heavy chain seen in the complex should generate a negative entropic contribution opposing the enthalpic and the hydrophobic (solvent entropy) effects. This structural 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 solvent entropy (hydrophobic) effects.X-ray crystallographic studies of several complexes of antigens with specific antibodies have revealed a high degree of complementarity between their interacting surfaces (reviewed in refs. 1 and 2). Water molecules have been identified at the interfaces of the Fab fragment of antibody D1.3 (Fab D1.3)-hen egg-white lysozyme (HEL) (3) and NC41-neuraminidase (4) complexes on the basis of structure determinations at 2.5-A resolution. Unfortunately, at such resolution, which is about the best which has been so far attained with conventional Fab fragments, the certainty with which ordered water molecules can be located is seriously limited (5). We have now determined the three-dimensional structure, at 1.8-A resolution, of the Fv fragment of monoclonal antibody (mAb) D1.3 (6, 7), Fv D1.3, consisting of only the variable domains ofthe heavy (VH) and light (VL) polypeptide chains and that of its complex with HEL, permitting a more detailed description of an antibody combining site in its free and antigen-bound states. These studies reveal both buried and exposed water molecules linking antigen and antibody and contributing to chemical complementarity between their interacting surfaces.An understanding of how antibodies react with antigens must involve the thermodynamics of the binding interaction. We have therefore experimentally de...

The crystal structure of the extracellular portion of the beta chain of a murine T cell antigen receptor (TCR), determined at a resolution of 1.7 angstroms, shows structural homology to immunoglobulins. The structure of the first and second hypervariable loops suggested that, in general, they adopt more restricted sets of conformations in TCR beta chains than those found in immunoglobulins; the third hypervariable loop had certain structural characteristics in common with those of immunoglobulin heavy chain variable domains. The variable and constant domains were in close contact, presumably restricting the flexibility of the beta chain. This may facilitate signal transduction from the TCR to the associated CD3 molecules in the TCR-CD3 complex.

The potential use of monoclonal antibodies in immunological, chemical and clinical applications has stimulated the protein engineering and expression of Fv fragments, which are heterodimers consisting of the light and heavy chain variable domains (VL and VH) of antibodies. Although Fv fragments exhibit antigen binding specificity and association constants similar to their parent antibodies or Fab moieties, similarity in their interactions with antigen at the level of three-dimensional structure has not been investigated. We have determined the high-resolution crystal structure of the genetically engineered FvD1.3 fragment of the anti-hen egg-white lysozyme (HEL) monoclonal antibody D1.3, and of its complex with HEL. On comparison with the crystallographically refined FabD1.3-HEL complex, we find that FvD1.3 and FabD1.3 make, with minor exceptions, very similar contacts with the antigen. Furthermore, a small but systematic rearrangement of the domains of FvD1.3 occurs on binding HEL, bringing the contacting residues closer to the antigen by a mean value of about 0.7 A for VH (aligning on VL) or of 0.5 A for VL (aligning on VH). This is indicative of an induced fit rather than a 'lock and key' fit to the antigen.