Figure 6.

Schematic representation of irreversible antigen (Ag) binding and catalytic Ag hydrolysis by a nucleophilic antibody (Ab). Specificity is derived from non-covalent epitope–paratope binding. The antibody nucleophile (Nu) attacks the weakly electrophilic peptide bond carbonyl. Covalent immune complex 1 is a resonant stable complex formed prior to expulsion of C-terminal antigen fragment (NH₂-Ag″). Covalent immune complex 2 is an acyl-antibody complex that is hydrolyzed to release the N-terminal antigen fragment (Ag′-X; X is an amino acid) when activated water is available in the antibody combining site. Ag′ and Ag″ are components of the epitope recognized by the antibody.