Methionine

Methionine is an essential sulfur-containing amino acid incorporated into proteins and used to make molecules involved in metabolism. It is encoded by the start codon AUG in nearly all genetic codes.

S-Adenosylmethionine: A molecule made from methionine that donates methyl groups in many cellular reactions. Methionine is converted into this major methyl donor.
Amino acid: An organic molecule with amino and carboxyl groups that serves as a protein building block. Methionine is one of the amino acids incorporated into proteins.
Proteinogenic amino acid: An amino acid that cells use to build proteins during translation. Methionine is one of the standard amino acids specified by the genetic code.
Cysteine: A sulfur-containing amino acid with a reactive thiol side chain. Cysteine’s thiol forms disulfide bonds, unlike methionine’s comparatively stable thioether.
Methionine cycle: A metabolic cycle that regenerates methionine after its methyl group is transferred. It recycles methionine after S-adenosylmethionine donates a methyl group.
Essential amino acid: An amino acid that an organism cannot synthesize in sufficient amounts and must obtain from its diet. Humans must obtain methionine from food.
Dietary reference intake: A set of nutrient intake standards used to assess and plan diets. Its essentiality makes methionine part of dietary protein requirements.
Selenomethionine: A selenium-containing analogue of methionine found in some foods and incorporated into proteins. It can substitute for methionine in proteins while carrying selenium instead of sulfur.
Transsulfuration: A metabolic pathway that converts homocysteine into cysteine through cystathionine. Homocysteine from methionine metabolism can enter this route to form cysteine.
Thioether: An organic functional group in which a sulfur atom bonds to two carbon atoms. Its thioether side chain distinguishes methionine’s sulfur chemistry from cysteine’s.

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