Methionine-associated peptide α-amidation is directed both to the N- and the C-terminal amino acids

Peptide-bound methionine may transfer oxidative damage from the thioether side chain to the peptide backbone, catalyzing decomposition in general and alpha-amidation in particular. In the present study, we focused on the reactivity and reaction pathways of peptides. We synthesized model peptides comprising methionine or not and investigated their overall tendency towards decomposition and formation of specific products under conditions mimicking the cooking process at 100 degrees C in buffered solution (pH 6.0) in the presence of redox-active substances such as transition metal ions and reductones. Peptides containing methionine were more susceptible to alpha-amidation under all oxidative conditions, and the products of N-terminus-directed alpha-amidation were quantified. Exemplarily, after incubation in the presence of cupric sulfate, about 2.0 mol-% of the overall decomposition of Z-glycylmethionylglycine accounted for the formation of Z-glycinamide, whereas it was below 0.1 mol-% for Z-glycylalanylglycine. Surprisingly and different from previous observations, C-terminus-directed alpha-amidation was observed for the first time. From Z-glycylmethionylglycine, the respective products were formed in higher amounts than the N-terminus-directed alpha-amidation product Z-glycinamide under all applied oxidation conditions. The preference of electron transfer from the amino nitrogen bound in the peptide bond directed to the C-terminus may be ascribed to a sterically less demanding hexagonal 3-electron-2-center intermediate during methionine-catalyzed alpha-amidation.

Automated summary

Peptide-bound methionine can catalyze the decomposition and alpha-amidation of peptides. Peptides containing methionine are more susceptible to alpha-amidation, and C-terminus-directed alpha-amidation was observed for the first time.