Methionine 59364 219551135 2008-06-15T19:55:29Z DumZiBoT 6085301 robot Modifying: [[id:Metionin]] {{NatOrganicBox |image=[[Image:Methionin - Methionine.svg|200px]] [[Image:L-methionine-B-3D-balls.png|180px]] Chemical structure of Methionine |name= (''S'')-2-amino-4-(methylsulfanyl)-butanoic acid |OtherNames = Met, M |PubChem=876 |CAS=63-68-3 |SMILES=CSCC[C@H](N)C(O)=O |C=5 |H=11 |N=1 |O=2 |S=1 |mass=149.21 g/mol }} '''Methionine''' (abbreviated as '''Met''' or '''M''')<ref>{{cite web | author=IUPAC-IUBMB Joint Commission on Biochemical Nomenclature | title=Nomenclature and Symbolism for Amino Acids and Peptides | work=Recommendations on Organic & Biochemical Nomenclature, Symbols & Terminology etc | url=http://www.chem.qmul.ac.uk/iupac/AminoAcid/ | accessdate=2007-05-17}}</ref> is an α-[[amino acid]] with the [[chemical formula]] HO<sub>2</sub>CCH(NH<sub>2</sub>)CH<sub>2</sub>CH<sub>2</sub>SCH<sub>3</sub>. This [[Essential amino acid|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 [[phospholipid]]s. 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 (biology)|translation]] from mRNA. As a consequence, methionine is incorporated into the N-terminal position of all [[protein]]s in [[eukaryote]]s and [[archaea]] during translation, although it is usually removed by [[post-translational modification]]. == Biosynthesis == 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 [[folic acid|folates]] affords methionine. Both [[cystathionine-γ-synthase]] and [[cystathionine-β-lyase]] require [[Pyridoxal-phosphate|Pyridoxyl-5'-phosphate]] as a [[cofactor]], whereas [[homocysteine methyltransferase]] requires [[Cyanocobalamin|Vitamin B12]] as a cofactor.<ref>Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.</ref> Enzymes involved in methionine biosynthesis: # [[aspartokinase]] # β-aspartate semialdehyde [[dehydrogenase]] # homoserine dehydrogenase # homoserine [[acyltransferase]] # cystathionine-γ-[[synthase]] # cystathionine-β-[[lyase]] # [[methionine synthase]] (in mammals, this step is performed by [[homocysteine methyltransferase]]) [[Image:Met biosynthesis.gif|left|500px]] <br style="clear: both;" /> == Other biochemical pathways == Although 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.2.1.1.5, 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). [[Image:Met pathway.gif|Fates of methionine|right|450px]] ==Synthesis== [[Racemic]] methionine can be synthesized from diethyl sodium phthalimidomalonate by alkylation with chloroethylmethylsulfide (ClCH<sub>2</sub>CH<sub>2</sub>SCH<sub>3</sub>) followed by hydrolysis and decarboxylation.<ref>{{OrgSynth | author = Barger, G.; Weichselbaum, T. E. | title = dl-Methionine | | collvol = 2 | collvolpages = 384 | year = 1943 | prep = CV2P0384}}</ref> ==Dietary aspects== High levels of methionine can be found in sesame seeds, Brazil nuts, fish, meats, and some other plant seeds. {{Fact|date=August 2007}} Most fruits and vegetables contain very little of it; however, some have significant amounts, such as spinach, potatoes, and boiled corn.{{Fact|date=August 2007}} Most [[legume]]s, though high in protein, are also low in methionine. DL-methionine is sometimes added as an ingredient to [[pet food]]s.<ref>[http://www.yorkshire-terrier.com/dogfood.htm What's in your dog's food?]</ref> Methionine, cysteine, and soy protein heated in a small amount of water creates a meat-like aroma. == See also == * [[Allantoin]] * [[Formylmethionine]] * [[Paracetamol#Prevention|Paradote]] - A Methionine-Paracetamol preparation that might prevent hepatotoxicity. * [[Photo-reactive amino acid analog|Photo-reactive methionine]] == References == * British National Formulary 55, March 2008; ISBN 978 085369 776 3 {{Reflist}} ==External links== * [http://www.hcusupport.com/diet.htm Foods containing methionine] * [http://www.compchemwiki.org/index.php?title=Methionine Computational Chemistry Wiki] {{AminoAcids}} {{Antidotes}} [[Category:Proteinogenic amino acids]] [[Category:Glucogenic amino acids]] [[Category:Sulfur amino acids]] [[Category:Thioethers]] [[Category:Essential amino acids]] [[ar:ميثيونين]] [[ca:Metionina]] [[cs:Methionin]] [[de:Methionin]] [[es:Metionina]] [[eo:Metionino]] [[fr:Méthionine]] [[ko:메티오닌]] [[hr:Metionin]] [[id:Metionin]] [[it:Metionina]] [[he:מתיונין]] [[lv:Metionīns]] [[lb:Methionin]] [[lt:Metioninas]] [[hu:Metionin]] [[nl:Methionine]] [[ja:メチオニン]] [[no:Metionin]] [[pl:Metionina]] [[pt:Metionina]] [[ru:Метионин]] [[sk:Metionín]] [[sr:Метионин]] [[fi:Metioniini]] [[sv:Metionin]] [[tr:Metiyonin]] [[uk:Метіонін]] [[zh:蛋氨酸]]