Methionine

Methionine

Abbreviated: Met or M

Molecular Formula: HO₂CCH(NH₂)CH₂CH₂SCH₃

pKa: 2.28(-COOH), 9.21 (α-NH₂)

Physiological roles:

• This amino acid is coded for by the codon AUG, which is the start codon for translation. In bacteria the derivative of Methionine, N-formylmethionine (fMet) is used to initiate translation. The formation of fMet is catalyzed by the enzyme methionyl-tRNA formyltransferase.
• fMet is highly important not only for the bacterial cell itself but also for the mitochondria and the chloroplasts.
• Interestingly there has been some research looking into the possibility of methionine restriction can extend life by alternative lengthening of telomeres genes. http://www.nature.com/onc/journal/v26/n32/full/1210260a.html 
• Methionine is an intermediate in the biosynthesis of cysteine, caritine, lecithin, phosphatidylcholine, taurine, and other phospholipids.

Synthesis:

Methionine biosynthesis

The enzymes involved in this synthesis are:

1. aspartokinase
2. β-aspartate semialdehyde dehydrogenase
3. homoserine dehydrogenase
4. homoserine O-transsuccinylase
5. cystathionine-γ-sythase
6. cystathionine-β-lyase
7. Methionine sythase

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 produce homocysteine. Subsequent methylation of the thiol group by folates affords methionine. Both cystathionine-γ-sythase and cystathionine-β-lyase require pyridoxyl-5'-phosphate as a cofactor, whereas homocysteine methyltransferase  requires vitamin B12 as a cofactor. (Lehninger, Albert L.; Nelson, David L.; Cox, Michael M. (2000), Principles of Biochemistry (3rd ed.), New York: W. H. Freeman, ISBN 1-57259-153-6).

To learn more about the initiation of protein synthesis with fMet in bacteria read the article, Initiation of Protein Synthesis in Bacteria LINK: http://mmbr.asm.org/content/69/1/101.short