Figure 5:

The aspartimidylation pathway of Group 13 graspetides catalyzed by O-MT. The original side chain of Asp is colored maroon, while the rest of the peptide is colored teal. The final product of the reaction catalyzed by ATP-grasp enzyme is the graspetide precursor with all ω-ester linkages with Asp in the aspartimidylation site. The O-MT methylates the Asp using SAM as a methyl group donor. The aspartimide forms spontaneously, resulting in CH₃OH leaving the peptide. The aspartimide can be hydrolyzed at the side chain or the backbone, generating Asp (top) or isoAsp (bottom) at the aspartimidylation site. The O-MT recognizes both species, able to re-aspartimidylate the graspetide.