N-carboxy-L-methionine (S)-2-carboxyamino-4-(methylsulfanyl)butanoic acid 176.0381317138671875 176.2100067138671875 2-carbamic-4-(methylsulfanyl)butanoic acid 2-carbamic-4-(methylthio)butanoic acid 43.98983001708984375 44.009998321533203125 A protein modification that effectively converts an L-methionine residue to N-carboxy-L-methionine. At least three protein crystallographic structures have been reported with this modification. However, no chemical evidence for this modification is provided, there were no reports of this modification before these crystallographic reports, and there is no metabolic explanation for the conversion of a formyl group to a carboxy group. There is confusion in its description, and misnaming is common. This modification is probably a misidentification of N-(dihydroxymethyl)methionine, the hydrated form of N-formylmethionine. See MOD:01446 [JSG]. C 1 H 0 N 0 O 2 S 0 C 6 H 10 N 1 O 3 S 1 Carboxy Carboxylation M MOD:00368 N-carboxy-L-methionine N-carboxymethionine N-term PSI-MOD Unimod:299 hypothetical modified L-methionine residue carboxylated residue