Isoaspartate

It is a β-amino acid, with the side chain carboxyl moved to the backbone.

Such a change is caused by a chemical reaction in which the nitrogen atom on the N+1 following peptide bond (in black at top right of Figure 1) nucleophilically attacks the γ-carbon of the side chain of an asparagine or aspartic acid residue, forming a succinimide intermediate (in red).

[2] Isoaspartyl formation reactions have been conjectured to be one of the factors that limit the useful lifetime of proteins.

[3] Isoaspartyl formation proceeds much more quickly if the asparagine is followed by a small, flexible residue (such as Gly) that leaves the peptide group open for attack.

L-isoaspartyl methyltransferase repairs isoaspartate and D-aspartate residues by sticking a methyl group onto the side chain carboxyl group in the residue, creating an ester.