Decarboxylation mechanism of the N-terminal glutamyl moiety in gamma-glutamic acid and methionine containing peptides

The reaction of hydroxyl radicals with gamma-glutamyl-methionine and gamma-glutamyl-glycyl-methionyl-glycine at neutral pH results in similar N-terminal decarboxylation efficiency. The underlying mechanism involves an intramolecular proton transfer from the protonated N-terminal amino group of the glutamyl moiety to an initially formed hydroxy sulphuranyl radical at the methionine residue. This process leads to the formation of a three-electron bonded [> S therefore NH2](+)-peptide intermediate subsequently decomposing into CO2 and an alpha-amino radical of the structure H2N-CH.-CH2-CH2-C(=O)-NH-peptide. This radical has been identified via reduction of a moderately good electron acceptor such as p-nitroacetophenone (PNAP). The arrangement within a sterically favourable 5-membered ring, as observed with methionine, is not a necessary prerequisite for the formation of [> S therefore NH2](+)-type intermediate. Mechanistically, the formation of CO, and an a amino radical suggests the occurrence of an intramolecular electron transfer from the carboxylate group to the electron-deficient center at the nitrogen within the S therefore N-bond followed by homolytic bond breakage of the carbon-carboxylate bond. The decarboxylation benefits in particular from stabilization of the arising carbon-centered radical by a free lone pair from the alpha-amino group. This process seems to occur well in larger peptide structures provided they contain an N-terminal carboxyl group alpha to an amino function and they are flexible enough to allow the protonated amino function to interact with the hydroxyl sulphuranyl radical at the methionine residue.