Antigen mobility in the combining site of an anti-peptide antibody

The interaction between a high-affinity antibody, raised against a peptide incorporating the loop region of hen egg lysozyme (residues 57-84), and a peptide antigen corresponding to this sequence, has been probed by proton NMR. The two-dimensional correlated spectroscopy spectrum of the antibody-antigen complex shows sharp, well-resolved resonances from at least half of the bound peptide residues, indicating that the peptide retains considerable mobility when bound to the antibody. The strongly immobilized residues (which include Arg-61, Trp-62, Trp-63, and Ile-78) do not correspond to a contiguous region in the sequence of the peptide. Examination of the crystal structure of the protein shows that these residues, although remote in sequence, are grouped together in the protein structure, forming a hydrophobic projection on the surface of the molecule. The antibody binds hen egg lysozyme with only a 10-fold lower affinity than the peptide antigen. We propose that the peptide could bind to the antibody in a conformation that brings these groups together in a manner related to that found in the native protein, accounting for the high crossreactivity.