methionines

Methionine is one of the twenty standard amino acids used to build proteins. It is a sulfur-containing, nonpolar amino acid with a thioether side chain (-CH2-CH2-S-CH3). In most organisms, methionine is encoded by the codon AUG and serves as the initiator amino acid for protein synthesis, delivered by the initiator tRNA Met-tRNAiMet in eukaryotes and archaea; bacteria commonly initiate with formylmethionine.

Beyond its role as a protein constituent, methionine participates in essential metabolic processes through its conversion to S-adenosylmethionine (SAM), the major methyl donor in cells. After SAM donates its methyl group, it becomes S-adenosylhomocysteine and then homocysteine. Homocysteine can be remethylated to methionine via methionine synthase (using 5-methyl-tetrahydrofolate and vitamin B12) or via betaine-homocysteine methyltransferase (using betaine). Alternatively, homocysteine can enter the transsulfuration pathway and be converted to cysteine.

Methionine residues within proteins, referred to as methionines, can be oxidized to methionine sulfoxide by reactive oxygen species; methionine sulfoxide reductases can revert this modification, linking methionine status to redox regulation. The availability of methionine and its metabolism influence protein structure, function, and cellular signaling.

Dietary methionine is essential for adults, who must obtain it from food. Good sources include animal products and some plant proteins. In nutritional planning, methionine is often considered together with cysteine, the other sulfur-containing amino acid, to meet overall sulfur amino acid requirements.