Structural and antigenic features of the synthetic SF23 peptide corresponding to the receptor binding fragment of diphtheria toxin.

The research focuses on the structural and antigenic features of a synthesized peptide, SF23, which corresponds to a fragment of the diphtheria toxin, and its potential application in the development of a synthetic vaccine.

About the SF23 Peptide

• The SF23 is a synthesized peptide that corresponds to the receptor binding fragment (residues 508-530) of the diphtheria toxin. The portion is selected due to its characteristic of being the less mutable B-cell epitope, which means this part is less likely to alter and it recognizes and binds very specific antigens.
• Its structure forms a protruding beta hairpin which was revealed by the analysis of circular dichroism spectra, a technique used to investigate the secondary structure of proteins.

Reactivity of SF23 Peptide

• The study shows that antibodies from the sera of individuals infected by toxigenic Corynebacterium diphtheriae and those immunized by diphtheria toxoid can bind the synthetic SF23 peptide.
• The synthetic SF23 peptide also attracted antibodies from the serum of horses hyperimmunized with diphtheria toxoid, which suggests that the peptide can elicit an immune response.

Potential Application

• Since antibodies against the SF23’s corresponding epitope in the diphtheria toxin should block its interactions with cellular receptor (heparin-binding epidermal growth factor), the SF23 peptide has been suggested as a promising candidate for the development of a synthetic vaccine against diphtheria, an infectious disease caused by Corynebacterium diphtheriae.
• Moreover, fluorescence quenching studies showed the existence of chloride and phosphate binding sites on the SF23 molecule, suggesting that including phosphate containing adjuvants (like aluminum hydroxyphosphate or aluminum hydroxyphosphate sulfate) can potentially boost the SF23’s immunogenic properties, further supporting its potential as a vaccine candidate.