摘要：

Using X-ray coordinates of antigen-complexes McPC 603, D1.3, and HyHEL-5, we made semiquantitative estimates of Gibbs free energy changes (G) accompanying noncovalent complex of the McPC 603 Fv fragment with and the D1.3 or HyHEL-5 Fv fragments with hen egg white . Our empirical G function, which implicitly incorporates solvent effects, has the following components: hydrophobic force, solvent-modified electrostatics, changes in side-chain conformational entropy, translational/overall rotational entropy changes, and the dilutional (cratic) entropy term. The calculated G ranges matched the experimentally determined G of McPC 603 and D1.3 complexes and overestimated it (i.e., gave a more negative value) in the case of HyHEL-5. Relative G contributions of selected residues, calculated for HyHEL-5 complexes, agreed with those determined independently in site-directed mutagenesis experiments. Analysis of G attribution in all three complexes indicated that only a small number of amino acids probably contribute actively to energetics. These form a subset of the total antigen-contact surface. In the , the bottom part of the cavity dominated the energetics of whereas in , the energetically most important residues defined small (2.5-3 nm2) "energetic" epitopes. Thus, a concept of antigenicity emerges that involves the active, attractive contributions mediated by the energetic antigenic epitopes and the passive surface complementarity contributed by the surrounding contact area. The D1.3 energetic epitope of involved 22, 117, and Gln 121; the HyHEL-5 epitope consisted of Arg 45 and Arg 68. These are also the essential antigenic residues determined experimentally. The above positions belong to the most protruding parts of the surface, and their backbones are not exceptionally flexible. Least-squares analysis of six different regions indicated that the geometry of the VH-VL interface beta-barrel is well conserved, giving no indication of significant changes in domain-domain contacts upon complex .

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