Metioniin

Metioniin, known in English as methionine, is an essential sulfur-containing amino acid found in most proteins. It is required for normal growth and tissue repair, and in humans the initiator amino acid for protein synthesis is methionine, carried by the initiator tRNA during translation.

Chemically, methionine has a thioether-containing side chain and exists as L- and D- enantiomers; the L-form is the one incorporated into proteins in human metabolism.

Biochemical roles include serving as a precursor to S-adenosylmethionine (SAM), a universal methyl donor used in a wide range of methylation reactions (including DNA, RNA, proteins, and lipids). After donation of its methyl group, SAM is converted to S-adenosylhomocysteine and then to homocysteine. Homocysteine can be remethylated to methionine via methionine synthase (requiring vitamin B12 and folate) or betaine-homocysteine methyltransferase, or it can enter the transsulfuration pathway to form cysteine via cystathionine beta-synthase (requiring vitamin B6). This methionine cycle links methionine metabolism to cysteine and glutathione synthesis.

Dietary sources include animal products such as meat, fish, eggs, and dairy, as well as plant foods like legumes, nuts, and whole grains. Because it is essential, methionine must be obtained from the diet. In addition to nutrition, methionine is used in supplements and can be considered in the context of specialized animal feeds.

Health considerations: deficiency is uncommon with a balanced diet but impaired methionine metabolism or high homocysteine levels are linked to health risks. Research also explores methionine restriction and its effects on aging and metabolism, with results varying across models.