|ATC code||V03,QA05, QG04|
|Molar mass||149.21 g mol−1|
|Appearance||White crystalline powder|
281 °C decomp.
|Solubility in water||Soluble|
|Supplementary data page|
|n, εr, etc.|
Solid, liquid, gas
|Spectral data||UV, IR, NMR, MS|
(what is this?) |
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Methionine (pronounced /mɛˈθaɪ.ɵniːn, mɛˈθaɪ.ɵnɪn/; 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 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.
As 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:
Although mammals cannot synthesize methionine, they can still utilize it in a variety of biochemical pathways:
There are two fates of homocysteine: it can be used to regenerate methionine, or to form cysteine.
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.22.214.171.124, 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 synthase.
Homocysteine can be converted to cysteine.
|Sesame seeds flour (low fat)||1.656|
|Soy protein concentrate||0.814|
|Beans, pinto, cooked||0.117|
|Rice, brown, medium-grain, cooked||0.052|
High levels of methionine can be found in sesame seeds, Brazil nuts, fish, meats and some other plant seeds; methionine is also found in cereal grains. Most fruits and vegetables contain very little of it. Most legumes are also low in methionine. The complement of cereal (methionine) and legumes (lysine), providing a complete protein, is a classic combination, found throughout the world, such as in rice and beans, and similar combinations discussed there.
The use of sesame seeds in cuisine, such as Indian cuisine and, especially in the form of tahini, in Arab cuisine, helps provide essential protein in vegetarian and vegan diets. For example, in hummus, sesame seeds are combined with chickpeas.
DL-methionine is the active ingredient in dog supplements to prevent yellow nitrogen burns to grass from their urine. The action is by reducing the pH of the dog's urine.  One example is "Grass Saver" by NaturVet.  There are claims the supplements can cause bladder stone (animal). 
There is a growing body of evidence that shows restricting methionine consumption can increase lifespans in some animals.
A 2005 study showed methionine restriction without energy restriction extends mouse lifespan.