Chemical nature of the interactions between antibody and antigen

The interactions of antibody with antigen are generally van der Waals interactions involving surface residues, which despite being solvent exposed are relatively hydrophobic in nature. However, most antibody-antigen interactions contain at least one hydrogen bond or salt link and typically contain many. For example, protein antigens form on the order of 15-20 hydrogen bonds and/or salt links with antibody. However, it is impossible to extrapolate from the number of hydrogen bonds and salt links to the strength of the interaction between antibody and antigen. For example, the 26-10-digoxin complex has no hydrogen bonds and no salt links, but its association constant (-1 x 1010 M"1) is as strong as any whose three-dimensional structure is known. In the antibody-protein complexes, some water molecules have been found to be part of the interface between antibody and antigen.

In the antigen-binding site, tyrosines and tryptophans occur more frequently than in the rest of the antibody. These two residues are both capable of forming hydrogen bonds to solvent or antigen, but have

Table 1 Antibody-antigen complexes and their Protein Data Bank (PDB) codes

Antibodya Antigen" PDB code

Complexed Uncomplexed

Antiprotein Antilysozymes

D1.3 Fv

Hen egg lysozyme

1VFB, 1FDL

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