methionine n : a crystalline amino acid containing sulfur; found in most proteins and essential for nutrition
- Spanish: metionina
Methionine (abbreviated as Met or M) is an α-amino acid with the chemical formula HO2CCH(NH2)CH2CH2SCH3. This essential amino acid is classified as nonpolar. Together with cysteine, methionine is one of two sulfur-containing proteinogenic amino acids. Its derivative S-adenosyl methionine (SAM) serves as a methyl donor. Methionine is an intermediate in the biosynthesis of cysteine, carnitine, taurine, lecithin, phosphatidylcholine, and other phospholipids. Improper conversion of methionine can lead to atherosclerosis.
Methionine is one of only two amino acids encoded by a single codon (AUG) in the standard genetic code (tryptophan, encoded by UGG, is the other). The codon AUG is also significant, in that it carries the "Start" message for a ribosome that signals the initiation of protein translation from mRNA. As a consequence, methionine is incorporated into the N-terminal position of all proteins in eukaryotes and archaea during translation, although it is usually removed by post-translational modification.
BiosynthesisAs an essential amino acid, methionine is not synthesized in humans, hence we must ingest methionine or methionine-containing proteins. In plants and microorganisms, methionine is synthesized via a pathway that uses both aspartic acid and cysteine. First, aspartic acid is converted via β-aspartyl-semialdehyde into homoserine, introducing the pair of contiguous methylene groups. Homoserine converts to O-succinyl homoserine, which then reacts with cysteine to produce cystathionine, which is cleaved to yield homocysteine. Subsequent methylation of the thiol group by folates affords methionine. Both cystathionine-γ-synthase and cystathionine-β-lyase require Pyridoxyl-5'-phosphate as a cofactor, whereas homocysteine methyltransferase requires Vitamin B12 as a cofactor.
Enzymes involved in methionine biosynthesis:
Other biochemical pathwaysAlthough mammals cannot synthesize methionine, they can still utilize it in a variety of biochemical pathways:
Methionine is converted to S-adenosylmethionine (SAM) by (1) methionine adenosyltransferase. SAM serves as a methyl-donor in many (2) methyltransferase reactions and is converted to S-adenosylhomocysteine (SAH). (3) adenosylhomocysteinase converts SAH to homocysteine.
There are two fates of homocysteine:
- Methionine can be regenerated from homocysteine via (4) methionine synthase. It can also be remethylated using glycine betaine (NNN-trimethyl glycine) to methionine via the enzyme Betaine-homocysteine methyltransferase (E.C.126.96.36.199, BHMT). BHMT makes up to 1.5% of all the soluble protein of the liver, and recent evidence suggests that it may have a greater influence on methionine and homocysteine homeostasis than Methionine sythase.
- Homocysteine can be converted to cysteine. (5) Cystathionine-β-synthase (a PLP-dependent enzyme) combines homocysteine and serine to produce cystathionine. Instead of degrading cystathionine via cystathionine-β-lyase, as in the biosynthetic pathway, cystathionine is broken down to cysteine and α-ketobutyrate via (6) cystathionine-γ-lyase. (7) α-ketoacid dehydrogenase converts α-ketobutyrate to propionyl-CoA, which is metabolized to succinyl-CoA in a three-step process (see propionyl-CoA for pathway).
SynthesisRacemic methionine can be synthesized from diethyl sodium phthalimidomalonate by alkylation with chloroethylmethylsulfide (ClCH2CH2SCH3) followed by hydrolysis and decarboxylation.
Dietary aspectsHigh levels of methionine can be found in sesame seeds, Brazil nuts, fish, meats, and some other plant seeds. Most fruits and vegetables contain very little of it; however, some have significant amounts, such as spinach, potatoes, and boiled corn. Most legumes, though high in protein, are also low in methionine. DL-methionine is sometimes added as an ingredient to pet foods. Methionine, cysteine, and soy protein heated in a small amount of water creates a meat-like aroma.
- British National Formulary 55, March 2008; ISBN 978 085369 776 3
methionine in Arabic: ميثيونين
methionine in Catalan: Metionina
methionine in Czech: Methionin
methionine in German: Methionin
methionine in Spanish: Metionina
methionine in Esperanto: Metionino
methionine in French: Méthionine
methionine in Korean: 메티오닌
methionine in Croatian: Metionin
methionine in Indonesian: Metionin
methionine in Italian: Metionina
methionine in Hebrew: מתיונין
methionine in Latvian: Metionīns
methionine in Luxembourgish: Methionin
methionine in Lithuanian: Metioninas
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methionine in Dutch: Methionine
methionine in Japanese: メチオニン
methionine in Norwegian: Metionin
methionine in Polish: Metionina
methionine in Portuguese: Metionina
methionine in Russian: Метионин
methionine in Slovak: Metionín
methionine in Serbian: Метионин
methionine in Finnish: Metioniini
methionine in Swedish: Metionin
methionine in Turkish: Metiyonin
methionine in Ukrainian: Метіонін
methionine in Chinese: 蛋氨酸