Three-dimensional structure of an antibody-antigen complex.

We have determined the three-dimensional structure of two crystal forms of an antilysozyme Fab-lysozyme complex by x-ray crystallography. The epitope on lysozyme consists of three sequentially separated subsites, including one long, nearly continuous, site from Gln-41 through Tyr-53 and one from Gly-67 through Pro-70. Antibody residues interacting with lysozyme occur in each of the six complementarity-determining regions and also include one framework residue. Arg-45 and Arg-68 form a ridge on the surface of lysozyme, which binds in a groove on the antibody surface. Otherwise the surface of interaction between the two proteins is relatively flat, although it curls at the edges. The surface of interaction is approximately 26 X 19 A. No water molecules are found in the interface. The positive charge on the two arginines is complemented by the negative charge of Glu-35 and Glu-50 from the heavy chain of the antibody. The backbone structure of the antigen, lysozyme, is mostly unperturbed, although there are some changes in the epitope region, most notably Pro-70. One side chain not in the epitope, Trp-63, undergoes a rotation of approximately 180 degrees about the C beta--C gamma bond. The Fab elbow bends in the two crystal forms differ by 7 degrees.