Kinetic analysis of the effect on Fab binding of identical substitutions in a peptide and its parent protein

Monoclonal antibody 57P, which was raised against tobacco mosaic virus protein, cross-reacts with a peptide corresponding to residues 134-146 of this protein. Previous studies using peptide variants suggested that the peptide in the antibody combining site adopts a helical configuration that mimics the structure in the protein. In this study, we carried out a detailed comparison of Fab-peptide and Fab-protein interactions. The same five amino acid substitutions were introduced in the peptide (residues 134-151) and the parent protein, and the effect of these substitutions on antibody binding parameters have been measured with a Biacore instrument. Fabs that recognize epitopes located away from the site of mutations were used as indirect probes for the conformational integrity of protein antigens. Their interaction kinetics with all proteins were similar, suggesting that the substitutions had no drastic effect on their conformation. The five substitutions introduced in the peptide and the protein had minor effects on association rate constants (ka) and significant effects on dissociation rate constants (kd) of the antigen-Fab 57P interactions. In four out of five cases, the effect on binding affinity of the substitutions was identical when the epitope was presented in the form of a peptide or a protein antigen, indicating that antibody binding specifity was not affected by epitope presentation. However, ka values were about 10 times larger and kd values about 5 times larger for the peptide-Fab compared to the protein-Fab interaction, suggesting a different binding mechanism. Circular dichroism measurements performed for three of the peptides showed that they were mainly lacking structure in solution. Differences in conformational properties of the peptide and protein antigens in solution and/or in the paratope could explain differences in binding kinetics. Our results demonstrate that the peptides were able to mimic correctly some but not all properties of the protein-Fab 57P interaction and highlight the importance of quantitative analysis of both equilibrium and kinetic binding parameters in the design of synthetic vaccines and drugs.