methionine

Methionine is an essential sulfur-containing amino acid that is one of the 20 standard amino acids used to build proteins. Its side chain features a thioether group (-CH2-CH2-S-CH3), giving it the chemical formula C5H11NO2S. In humans, methionine cannot be synthesized in sufficient quantities and must be obtained from the diet. It is also the initiating amino acid for protein synthesis in eukaryotes, incorporated as methionyl-tRNA during the start of translation; in bacteria and organelles, the initiating form is formylmethionine.

Metabolic roles of methionine are diverse. It is a precursor to S-adenosylmethionine (SAM), a universal methyl donor involved in a wide range of methylation reactions, including those affecting DNA, RNA, proteins, and lipids. After donating a methyl group, SAM becomes S-adenosylhomocysteine and then homocysteine. Homocysteine can be remethylated to methionine (requiring 5-methyl-tetrahydrofolate and vitamin B12) or enter the transsulfuration pathway to form cysteine. Through these pathways, methionine links the methionine cycle with cysteine and glutathione biosynthesis, and it interacts with folate and vitamin B12 metabolism. In many plant-based diets, methionine can be relatively limiting, influencing overall protein quality.

Dietary sources and practical use: Common sources include meat, fish, eggs, and dairy, with plant foods such as soy, sesame, and Brazil nuts contributing as well. In animal nutrition, methionine and its form DL-methionine are widely used as feed additives to support growth and protein production. Deficiency is uncommon with a balanced diet but can affect growth and nitrogen balance, and excess methionine can influence homocysteine metabolism.