INTERACTIONS OF PROTEIN ANTIGENS WITH ANTIBODIES

There are now several crystal structures of antibody Fab fragments com plexed to their protein antigens. These include Fab complexes with lys ozyme, two Fab complexes with influenza virus neuraminidase, and three Fab complexes with their anti-idiotype Fabs. The pattern of binding t hat emerges is similar to that found with other protein-protein intera ctions, with good shape complementarity between the interacting surfac es and reasonable juxtapositions of polar residues so as to permit hyd rogen-bond formation. Water molecules have been observed in cavities w ithin the interface and on the periphery, where they often form bridgi ng hydrogen bonds between antibody and antigen. For the most part the antigen is bound in the middle of the antibody combining site with mos t of the six complementarity-determining residues involved in binding, For the most studied antigen, lysozyme, the epitopes for four antibod ies occupy approximate to 45% of the accessible surface area. Some con formational changes have been observed to accompany binding in both th e antibody and the antigen, although most of the information on confor mational change in the latter comes from studies of complexes with sma ll antigens.