Methionine

From Free net encyclopedia

---

Current revision

Methionine is an essential nonpolar amino acid, and a lipotropic.

Methionine and cysteine are the only sulfur-containing proteinogenic amino acids. The methionine derivative S-adenosyl methionine (SAM) serves as a methyl donor. Methionine plays a role in cysteine, carnitine and taurine synthesis by the transsulfuration pathway, lecithin production, the synthesis of phosphatidylcholine and other phospholipids. Improper conversion of methionine can lead to atherosclerosis. Methionine is a chelating agent.

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. Methionine can also occur at other positions in the protein.

Foods containing methionine include fruits, meat, vegetables, nuts and legumes. High levels of methionine can be found in spinach, green peas, garlic, some cheeses, corn, brazil nuts, pistachios, cashew nuts, kidney beans, black turtle beans, tofu, and tempeh. Most meat is also a rich source of Methionine including chicken, beef and fish

Contents 1 Biosynthesis 2 Other biochemical pathways 3 See also 4 External links

Biosynthesis

Since methionine is an essential amino acid, it cannot be synthesized in humans. However, in plants and microorganisms, methionine is synthesized from aspartic acid and cysteine. First, aspartic acid is converted to β-aspartyl-semialdehyde, an important intermediate in the biosynthesis of methionine, lysine, and, threonine. Of homoserine by homoserine acyltransferase, puts a good leaving group on homoserine allowing it to react with cysteine to produce cystathionine. Enzymatic cleavage of cystathionine yilds homocysteine, which can then be methylated by folates to give methionine. Both cystathionine-γ-synthase and cystathionine-β-lyase require Pyridoxyl-5'-phosphate as a cofactor, while homocysteine methyltransferase requires Vitamin B12 as a cofactor.

Image:Met biosynthesis.gif

Enzymes invovled in methionine biosynthesis:

1. aspartokinase
2. β-aspartate semialdehyde dehydrogenase
3. homoserine dehydrogenase
4. homoserine acyltransferase
5. cystathionine-γ-synthase
6. cystathionine-β-lyase
7. methionine synthase (in mammals, this step is performed by homocysteine methyltransferase)

Other biochemical pathways

Although mammals cannot synthesize methionine, they can still utilize it in a variety of biochemical pathways:

Image:Met pathway.gif

Methionine is converted to S-adenosylmethionine (SAM) by (1) methionine adenosyltransferase. SAM serves as a methyl-donor in many (2) methyltransferase reactions and is conveted to S-adenosylhomocysteine (SAH). (3) adenosylhomocysteinase converts SAH to homocysteine.

There are two fates of homocysteine. First, methionine can be regenerated from homocysteine via (4) methionine synthase. Alternatively, 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).

See also

• Allantoine
• Formylmethionine

External links

• Foods containing methionine
• Computational Chemistry Wiki

Template:AminoAcidsde:Methionin es:Metionina eo:Metionino fr:Méthionine it:Metionina he:מתיונין lt:Metioninas lb:Methionin nl:Methionine ja:メチオニン pl:Metionina pt:Metionina ru:Метионин fi:Metioniini sv:Metionin zh:蛋氨酸