Methionine (Met)

Methionine is an α-amino acid with the chemical formula HO₂CCH(NH₂)CH₂CH₂SCH₃. This essential amino acid is classified as nonpolar.This amino acid is also used by plants for synthesis of ethylene. The process is known as the Yang Cycle or the methionine cycle.

Methionine is one of only two amino acids encoded by a single codon (AUG) in the standard genetic code (tryptophan, encoded by UGG, is the other). The codon AUG is also the "Start" message for a ribosome that signals the initiation of protein translation from mRNA. As a consequence, methionine is incorporated into the N-terminal position of all proteins in eukaryotes and archaea during translation, although it is usually removed by post-translational modification. In bacteria, the derivative N-formylmethionine is used as the initial amino acid.