Mechanism of cleavage at Asn 148 during the maturation of jack bean concanavalin A.

The maturation of the lectin concanavalin A from the jack bean, Canavalia ensiformis, involves an unusual post-translational cleavage at three internal asparagine residues and the subsequent rearrangement and ligation of two of the resulting fragments. It is presently unclear whether these reactions are enzymatically catalyzed or occur autocatalytically. A specific model for non-enzymatic cleavage has been proposed where the attack of the side chain amide nitrogen atom of asparagine on its alpha-carbonyl carbon cleaves the peptide-bond and leaves a C-terminal succinimide residue. Spontaneous hydrolysis of the succinimide would result in the formation of both terminal asparagine and isoasparagine (aspartic acid alpha-amide) residues. We tested this model by chemically analyzing the C-terminal tryptic peptide of mature concanavalin A that contains the cleavage site residue Asn-148 as its C-terminal residue. We found that only free asparagine was released from this peptide with either elastase or leucine aminopeptidase digestion under conditions that released isoasparagine from a similar synthetic peptide. These results suggest that precursor processing in concanavalin A does not in fact follow a succinimide pathway, although other types of autocatalytic mechanisms remain possible.