|Molecular mass||149.21 g mol-1|
|Melting point||281 °C|
|Density||1.340 g cm-3|
|Disclaimer and references|
Methionine is an α-amino acid present in many proteins and, together with cysteine, is one of two sulfur-containing proteinogenic amino acids. Methionine is the source of sulfur for many compounds, including cysteine (Longe 2005). The derivative S-adenosyl methionine (SAM) is an important methyl donor involved in a variety of biochemical pathways and in the synthesis of epinephrine, choline, and other substances.
The L-isomer of methionine, which is the only form that is involved in protein synthesis, is one of the 20 standard amino acids common in animal proteins and required for normal functioning in humans. Methionine also is classified as an "essential amino acid" since it cannot be synthesized by the human body from other compounds through chemical reactions and thus has to be taken in with the diet.
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 to begin 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. This role of methionine reveals the remarkable unity among living organisms; furthermore, the precision required for a functional protein, involving a particular arrangement of amino acids and specific three-dimensional folding of the protein, reflects on the complex coordination in nature.
Methionine's three letter code is Met, its one letter code is M, and its systematic name is 2-Amino-4-(methylthio)butanoic acid.
In biochemistry, the term amino acid is frequently used to refer specifically to alpha amino acids: those amino acids in which the amino and carboxylate groups are attached to the same carbon, the so-called α–carbon (alpha carbon). The general structure of these alpha amino acids is:
R | H2N-C-COOH | H
where R represents a side chain specific to each amino acid.
Most amino acids occur in two possible optical isomers, called D and L. The L amino acids represent the vast majority of amino acids found in proteins. They are called proteinogenic amino acids. As the name "proteinogenic" (literally, protein building) suggests, these amino acid are encoded by the standard genetic code and participate in the process of protein synthesis. In methionine, only the L-stereoisomer is involved in synthesis of mammalian proteins.
Methionine's chemical formula is CH3-S-(CH2)2)-CH(NH2)-COOH, or in general form C5H11NO2S (IUPAC-IUB 1983).
Like cysteine, methionine contains sulfur, but with a methyl group instead of hydrogen. This methyl group (one carbon and three hydrogen, or CH3) can be activated, and is used in many reactions where a new carbon atom is being added to another molecule. Methionine is classified as nonpolar. Methionine is always the first amino acid to be incorporated into a protein; it is sometimes removed after translation.
As an essential amino acid, methionine is not synthesized in animals, hence it must be ingested as methionine or methionine-containing proteins. High levels of methionine can be found in sesame seeds, Brazil nuts, fish, meat, and some seeds. Most fruit and vegetables contain very little, although peppers and spinach are the best sources. Legumes are not a good source.
In plants and microorganisms, methionine is synthesized via a pathway that uses both of the amino acids 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 (Lehninger 2000).
Enzymes involved in methionine biosynthesis:
S-adenosyl-l-methionine (SAM or SAMe) is a compound that is formed from a reaction of methionine and adenosine triphosphate (ATP). SAM is the main methyl donor in the human body, involved in a variety of biochemical pathways.
Methionine is converted to SAM by (1) methionine adenosyltransferase. SAM then 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.
Racemic methionine (equal portions of L- and D-sterioisomers) can be synthesized from diethyl sodium phthalimidomalonate, (C6H4(CO)2NC(CO2Et)2), by alkylation with chloroethylmethylsulfide, ClCH2CH2SCH3 followed by hydrolysis and decarboxylation (Barger and Weichselvaum 1943).
Methionine and its derivatives fulfill many important functions in the body.
Methionine also plays a role in the synthesis of phosphatidylcholine, and other phospholipids. Improper conversion of methionine can lead to atherosclerosis.
All links retrieved October 21, 2014.
|Major families of biochemicals|
|Peptides | Amino acids | Nucleic acids | Carbohydrates | Nucleotide sugars | Lipids | Terpenes | Carotenoids | Tetrapyrroles | Enzyme cofactors | Steroids | Flavonoids | Alkaloids | Polyketides | Glycosides|
|Analogues of nucleic acids:||The 20 Common Amino Acids||Analogues of nucleic acids:|
|Alanine (dp) | Arginine (dp) | Asparagine (dp) | Aspartic acid (dp) | Cysteine (dp) | Glutamic acid (dp) | Glutamine (dp) | Glycine (dp) | Histidine (dp) | Isoleucine (dp) | Leucine (dp) | Lysine (dp) | Methionine (dp) | Phenylalanine (dp) | Proline (dp) | Serine (dp) | Threonine (dp) | Tryptophan (dp) | Tyrosine (dp) | Valine (dp)|
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