Methionine

Abbrev. M Met
Full Name Methionine
Side chain type
hydrophobic
Mass
149.21
pl
5.74
pK1(α-COOH)
2.13
pK2(α-+NH3)
9.28

Side chain -CH2CH2SCH3
Hydro- phobic
yes
Polar
no
Charged
no
Small
no
Tiny
no
Aromatic or Aliphatic
no
van der Waals volume
124
Codon
AUG
Occurrence in proteins (%)
2.3

Remarks: Essential for humans. Always the first amino acid to be incorporated into a protein; sometimes removed after translation. Like cysteine, contains sulfur, but with a methyl group instead of hydrogen. This methyl group can be activated, and is used in many reactions where a new carbon atom is being added to another molecule.

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

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