Smethylcysteine

S-methylcysteine, also written Smethylcysteine, is a nonprotein amino acid derivative in which a methyl group is attached to the sulfur atom of cysteine, forming a thioether linkage. Its chemical formula is C4H9NO2S and its approximate molecular weight is 135.2 g/mol. Like cysteine, it contains an α-amino group, an α-carboxyl group, and a sulfur-containing moiety, but in S-methylcysteine the thiol is replaced by a thioether (-S-CH3). In biological contexts, it is most often encountered as the L-enantiomer when produced in nature.

Smethylcysteine has been identified in certain plant tissues as a nonprotein amino acid. It is not typically incorporated into proteins during translation. Biosynthesis is thought to involve methylation of the thiol sulfur in L-cysteine via a S-methyltransferase enzyme, though the full pathways are not widely detailed. It may accumulate under specific physiological conditions or as part of broader sulfur metabolism in plants.

The precise physiological role of S-methylcysteine in organisms is not well defined. As a sulfur-containing amino acid derivative, it could participate in sulfur storage, detoxification processes, or serve as a precursor to other sulfur compounds, but it is not considered an essential amino acid for humans. Nutritional and metabolic significance remains a topic of limited and specialized research.

S-methylcysteine is related to other sulfur-containing amino acids and derivatives, such as S-methylcysteine sulfoxide (methiin) and S-allyl-L-cysteine sulfoxide found in Allium vegetables. Distinguishing among these compounds requires careful analytical methods due to structural similarities and overlapping names.