Methionine

Methionine (abbreviated as Met or M)^[1] is an α-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 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 Vitamin B12 as a cofactor.^[2]

Enzymes involved 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:

Methionine is converted 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) propionyl-CoA for pathway).

Synthesis

Racemic methionine can be synthesized from diethyl sodium phthalimidomalonate by alkylation with chloroethylmethylsulfide (ClCH₂CH₂SCH₃) followed by hydrolysis and decarboxylation.^[3]

Dietary aspects

High levels of methionine can be found in sesame seeds, Brazil nuts, fish, meats, and some other plant seeds.^{[citation needed]} Most fruit and vegetables contain very little; however, some have significant amounts, such as spinach, potatoes, and boiled corn.^{[citation needed]} DL-methionine is sometimes added as an ingredient to pet foods.^[4] Methionine, cysteine, and soy protein heated in a small amount of water creates a meat-like aroma.

See also

• Allantoin
• Formylmethionine
• Paradote - A Methionine-Paracetamol preparation that might prevent hepatotoxicity.
• Photo-reactive methionine

References