Peptide Backbone Cleavage and trans-Amidation via Thioester-to-Imide Acyl Transfer

Cysteine thioesters are involved in a myriad of cen-tral biological transformations because of their unique reactivity. Despite their well-studied proper-ties, we discovered an unexpected trans-amidation reaction of cysteine thioesters that leads to peptide backbone cleavage. S-Acylcysteine-containing pep-tides were found to spontaneously fragment by cleavage of the amide bond in the i-1 position to the acylated cysteine residue at pH 8–10. We pre-sent compelling evidence for a mechanism involv-ing a central reversible thioester-to-imide acyl transfer step. The discovered trans-amidation reac-tion was found to be highly sequence dependent and was found to proceed in peptides containing post-translational modifications (PTMs) such as cysteine S-acetylation and S-palmitoylation as well as in peptide–peptide branched thioesters, mimick-ing class I intein splicing. Thus, the inherent reac-tivity of peptide backbones containing S-acylcysteine residues should represent a starting point for investigation of endogenous protein be-havior and may serve as a foundation for the dis-covery mild new peptide and protein transfor-mations.