metionin

Methionine, sometimes called metionin in some languages, is an essential sulfur-containing amino acid that is incorporated into proteins during translation. It features a sulfur-containing thioether side chain and is classified as one of the standard proteinogenic amino acids. Its systematic name is 2-amino-4-(methylthio)butanoic acid.

It cannot be synthesized in sufficient amounts by humans and other mammals, so it must be obtained from the diet. Methionine is present in animal-derived foods such as meat, fish, eggs, and dairy, as well as in certain plant foods including legumes, nuts, and whole grains, though plant sources may be relatively lower in methionine.

Metabolically, methionine is converted to S-adenosylmethionine (SAM), a universal methyl donor used in countless methylation reactions. After donating its methyl group, SAM becomes S-adenosylhomocysteine and ultimately homocysteine. Homocysteine can be remethylated to methionine (requiring vitamin B12 and folate) or enter the trans-sulfuration pathway to form cysteine, which contributes to glutathione and other molecules.

Methionine also contributes to the initiation of protein synthesis in eukaryotes (the initiator methionyl-tRNA). In bacteria, the initiator amino acid is formylmethionine. In humans, methionine is essential for growth and tissue maintenance.

Industrial and nutritional uses: methionine is supplied as L-methionine in supplements and as DL-methionine in animal feeds. It is used to balance amino acid nutrition in livestock and to support health in methionine-deficient states.