methionin

Methionin is one of the 20 standard amino acids used to build proteins. It is sulfur-containing and classified as essential in humans, meaning it cannot be synthesized in sufficient amounts and must be obtained from the diet. In humans, methionine exists predominantly in the L-configuration and serves as the initiating amino acid for protein synthesis in eukaryotes and archaea; the N-terminal methionine is often removed after translation.

Biochemical roles: Methionin is the precursor to S-adenosylmethionine (SAM), the universal methyl donor in numerous reactions including DNA, RNA, protein, and lipid methylation. After donation of a methyl group, SAM becomes S-adenosylhomocysteine, which is hydrolyzed to homocysteine. Homocysteine can be remethylated back to methionine (requiring vitamin B12 and folate) or diverted to cysteine via the transsulfuration pathway (requiring vitamin B6). Through these pathways, methionine links to the methionine cycle and sulfur amino acid metabolism.

Dietary sources: Methionin is found in animal-derived foods such as meat, fish, eggs, and dairy, and in plant foods including legumes, nuts, seeds, and some cereals. Plant-based diets may provide methionine but sometimes require attention to total sulfur amino acids and lysine balance; methionine intake can be limiting in some vegan diets.

Related compounds and uses: S-adenosylmethionine (SAMe) is a metabolite formed from methionin and adenosine triphosphate. SAMe is studied as a dietary supplement for various conditions; methionin itself is widely used in protein synthesis.

Safety and requirements: Adequate intake is essential; excessive methionin intake can raise homocysteine levels, which are associated with health risks. The precise requirement varies by age, sex, and physiological state.