Methionine

It and cysteine are the only sulfur containing amino acids that are coded for by DNA (Homocysteine is an amino acid and contains sulfur, but is a product of S-adenosylmethionine 1 carbon metabolism and is not coded for by DNA). Methionine is a methyl donor as S-adenosyl methionine[?] (SAM). It is incorporated into the N-terminal position of all proteins (though it may be removed by post-translational modification). It plays a role in cysteine, carnitine[?] and taurine synthesis by the transsulfuration pathway[?], lecithin[?] production, the synthesis of phosphatidylcholine[?] and other phospholipids. Improper conversion of methionine can lead to atherosclerosis. Methionine is a chelating agent.

Most amino acids have a redundant genetic code; several different nucleotide sequences code for the same amino acid, giving a degree of error-correction to the genetic sequence, usually with the third base in the codon being non-specific. Methionine is one of only two amino acids with a single code, AGT. (Tryptophan, coded by AGG, is the other)