DL-метионин

Описание

Methionine is an α-amino acid with the chemical formula HO₂CCH(NH₂)CH₂CH₂SCH₃. This essential amino acid is classified as nonpolar. This amino-acid is coded by the initiation codon AUG which indicates mRNA's coding region where translation into protein begins.

Химические свойства

White crystalline powder

Вхождение

High levels of methionine can be found in eggs, 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. Racemic methionine is sometimes added as an ingredient to pet foods.

Использование

DL-Methionine is sometimes given as a supplement to dogs; it helps keep dogs from damaging grass by reducing the pH of the urine.
Methionine is allowed as a supplement to organic poultry feed under the US certified organic program.

Определение

ChEBI: A sulfur-containing amino acid that is butyric acid bearing an amino substituent at position 2 and a methylthio substituent at position 4.

Подготовка

By addition of methanethiol to acrolein; by chemical conversion of methylthiopropionic aldehyde.

Биосинтез

As an essential amino acid, methionine is not synthesized de novo in humans, who must ingest methionine or methioninecontaining 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.

Биологические функции

Together with cysteine, methionine is one of two sulfurcontaining 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.
This amino acid is also used by plants for synthesis of ethylene. The process is known as the Yang Cycle or the methionine cycle.
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 most common eukaryote "Start" message for a ribosome that signals the initiation of protein translation from mRNA when the AUG codon is in a Kozak consensus sequence. As a consequence, methionine is often incorporated into the N-terminal position of proteins in eukaryotes and archaea during translation, although it can be removed by post-translational modification. In bacteria, the derivative Nformylmethionine is used as the initial amino acid.

Общее описание

DL-Methionine is an essential amino acid containing sulphur. Methionine consists of an asymmetric carbon and exists as D (dextrogyre) and L (levogyre) optical isomers. The L-methionine is considered as biologically active. The racemic mixture of D and L-isomers forms DL-methionine, which is the commercially available methionine.

Профиль безопасности

Moderately toxic by ingestion and other routes. An experimental teratogen. Experimental reproductive effects. When heated to decomposition it emits toxic fumes of SOx and NOx. See also 1-METHIONINE.

Методы очистки

Crystallise it from hot water or EtOH. Also purify it by dissolving it in H2O and passing through an Amberlite IR-120 resin (NH4+ form). The eluate is concentrated and then passed through Amberlite IR-4B resin, and this eluate is evaporated to dryness. The residue is washed with EtOH, then Me2CO, dried and recrystallised from aqueous EtOH (colourless plates) [Baddiley & Jamieson J Chem Soc 4283 1954]. [Greenstein & Winitz The Chemistry of the Amino Acids J. Wiley, Vol 3 p 2125 1961, Beilstein 4 IV 3190.]