metioniinia

Methionine, often abbreviated Met and designated as an essential sulfur-containing amino acid, is required for protein synthesis in humans and many other organisms. Its side chain features a thioether group, giving it distinct biochemical properties. The L-enantiomer is the biologically active form incorporated into proteins.

As one of the nine essential amino acids, methionine cannot be synthesized by healthy adults and must be obtained from the diet. It is also the universal initiator amino acid for protein synthesis in eukaryotes, and it serves as a key source of methyl groups through its conversion to S-adenosylmethionine (SAM), a major methyl donor in numerous reactions.

Metabolism: After SAM donates methyl groups, it is converted to S-adenosylhomocysteine and then to homocysteine. Homocysteine can be remethylated back to methionine or enter the transsulfuration pathway to form cysteine, a process requiring vitamins B6, B12, and folate as cofactors. Through this network, methionine links protein turnover, methylation reactions, and synthesis of cysteine and glutathione.

Dietary sources: Methionine is present in many protein-rich foods, with animal-derived products typically providing a rich supply. Plant proteins also contain methionine, but its content varies; methionine availability can influence protein quality in vegetarian and vegan diets.

Health considerations: Adequate intake is essential for growth and methylation reactions; deficiency is uncommon with varied diets but may occur in severe malnutrition. Excessive methionine intake can elevate homocysteine levels if remethylation cofactors are limited; methionine supplementation and SAMe are used in some conditions but should be guided by medical advice.