NMR study of the complexes between a synthetic peptide derived from the B subunit of cholera toxin and three monoclonal antibodies against it

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Abstract
The contact interactions between a synthetic peptide and three different anti-peptide monoclonal antibodies have been studied by nuclear magnetic resonance (NMR). The synthetic peptide is CTP3 (residues 50-64 of the B subunit of cholera toxin) suggested as a possible epitope for synthetic vaccine against cholera. The hybridoma cell lines TE33 and TE32 derived after immunization with CTP3 produce antibodies cross-reactive with the native toxin. The cell line TE34 produces anti-CTP3 antibodies that do not bind the toxin. Selective deuteriation of the antibodies has been used to simplify the proton NMR spectra and to assign resonances to specific types of amino acids. The difference spectra between the proton NMR spectrum of the peptide-Fab complex and that of Fab indicate that the combining site structures of TE32 and TE33 are very similar but differ considerably from the combining site structure of TE34. By magnetization transfer experiments with selectively deuteriated Fab fragment of the antibody, we have found that in TE32 and TE33 the histidine residue of the peptide is buried in a hydrophobic pocket of the antibody combining site, formed by a tryptophan and two tyrosine residues. The hydrophobic nature of the pocket is further demonstrated by the lack of any pH titration effect on the chemical shift of the C4H of the bound peptide histidine. In contrast, for TE34 we have found only one tyrosine residue in contact with the histidine of the peptide. No magnetization transfer was observed between the tryptophan residues of TE34 and the histidine residue of the peptide. The involvement of a tryptophan residue in the combining site of TE33 and TE32 is further demonstrated by the considerable quenching of antibody fluorescence observed upon the binding of the peptide. Such quenching is not observed in TE34. The observed differences between these two types of monoclonal antibodes may be relevant to the complementarity of their respective binding sites to the conformation of the CTP3 peptide in the native toxin.