The structural basis of antigen-antibody recognition.

We have reviewed here the three-dimensional structure of an antigen-Fab complex as determined by X-ray crystallographic studies. The antigen is hen egg-white lysozyme (HEL), a protein whose three-dimensional structure and antigenic properties are well known. The Fab was prepared from a murine monoclonal anti-HEL antibody, IgGl,kappa, obtained by cell-hybridization techniques. The equilibrium association constant for the complex is 4.5 X 10(7) mol-1. The complex was crystallized and its three-dimensional structure was determined at 6-A and 2.8-A resolution. A three-dimensional model of the structure was built based on electron-density maps and the amino-acid sequence [determined from the nucleotide sequence of cDNA clones (M. Verhoeyen, C. Berek, J.M. Jarvis, G. Winter, in preparation)]. The three-dimensional structure of the complex shows that 17 antibody residues make close contacts (less than or equal to 4 A) with 16 antigen residues. Fifteen of the contacting antibody residues belong to the six complementarity-determining regions of the light chain (6 residues) and of the heavy chain (9 residues). The remaining two are located in regions of constant or nearly constant sequence ["framework" regions]. The 16 contacting lysozyme residues form a discontinuous, topographical determinant, since they are widely separated in the linear amino-acid sequence but are brought to relative spatial proximity by the three-dimensional folding of the polypeptide chain. The contacting surfaces are relatively flat, with protruding side chains of antigen and antibody penetrating each other over an area with maximum dimensions of 30 X 20 A. As in several other systems of protein-protein interactions, the contacts are chemically characterized as van der Waals interactions and hydrogen bonds. Detailed analysis of the interactions reveals that the antibody's recognition of the antigen is finely specific and is affected by antigenic variation (as observed in lysozymes from other avian species). The quaternary structure of the complexed Fab is elongated, with the axes of the variable (VH + VL) and constant (CH1 + CL) domains making an angle close to 180 degrees. Comparison of the three-dimensional structure of the complexed lysozyme with that of native lysozyme showed no significant conformational change at the current resolution (2.8 A). Comparison of the Fab moiety of the complex with other Fabs of known three-dimensional structure suggested that upon complexing no conformational change takes place in the tertiary structure of Fab either.(ABSTRACT TRUNCATED AT 400 WORDS)