Figure 4.

Proposed reaction scheme of the legumain ligase activity. The O⁻ of the new C-terminus of processed hCE (Asn39_I) attacks the energy-rich Suc147, resulting in an activated carboxylic acid at Asn39_I (a). Next, the carbamoyl nitrogen of the P1-Asn39_I side chain will attack the electrophilic carbon of the carboxylic anhydride (b), thereby generating Suc39_I (c). The succinimide ring can now be opened via a nucleophilic substitution by the P1′ nitrogen, resulting in the intact P1–P1′ peptide bond (d). Additionally, the Suc147 may be regenerated via a condensation reaction.