Methionine
Essential amino acid, precursor to cysteine and the methyl donor rSAM.
Methionine is a proteinogenic amino acid with decisive biosynthetic roles, serving as the precursor to cysteine and the pervasive methylation agent S-adenosylmethionine (rSAM).
- codon
- AUG
- classification
- Essential amino acid, α-amino acid, nonpolar aliphatic
- key_roles
- Precursor to cysteine and rSAM; protein synthesis initiator via N-formylmethionine-sRNA
Lore & Background
In nuclear genes of eukaryotes and in Archaea, methionine is coded for by the start codon, meaning it indicates the start of the coding region and is the first amino acid produced in a nascent polypeptide during mRNA translation.
Reader's Guide
Methionine's significance lies in its essential role as a building block for proteins and as a precursor to critical molecules. It is required for protein synthesis, initiated by N-formylmethionine-sRNA, and serves as the precursor to the amino acid cysteine and the methylation agent rSAM. The methionine codon AUG is also the most common start codon, signaling the initiation of protein translation. In bacteria, the derivative N-formylmethionine is used as the initial amino acid. Methionine residues do not have a catalytic role, unlike cysteine residues, but the thioether within methionine has a minor structural role due to S/π interactions with aromatic amino acids. It has been conjectured that norleucine was present in early versions of the genetic code, but methionine intruded due to its role in rSAM. Methionine can be regenerated from homocysteine via methionine synthase, a reaction requiring vitamin B12, or via betaine-homocysteine methyltransferase.
Did You Know?
- It is one of only two amino acids encoded by a single codon (AUG) in the standard genetic code.
- Methionine is the precursor to the amino acid cysteine and the methylation agent rSAM.
- In bacteria, the derivative N-formylmethionine is used as the initial amino acid during translation.
Frequently Asked Questions
Who is Methionine?
Methionine (Met or M) is an essential, nonpolar aliphatic α-amino acid that humans cannot build internally and must pull from their diet. It occupies a singular position in biochemistry as both the universal translation-initiator and a key metabolic branch point.
What are Methionine's powers or roles?
It kicks off virtually every protein-translation event via the AUG start codon and, through N-formylmethionine-tRNA, caps the N-terminus of nascent chains. Beyond initiation, it is the direct precursor to cysteine and to S-adenosylmethionine (rSAM), the cell's dominant methyl-group donor.
How does Methionine's story end?
Once its methyl-donor duty is fulfilled, methionine is converted to homocysteine and then funneled either back into the methionine cycle or down the transsulfuration pathway into cysteine. Its carbon and sulfur skeletons are ultimately recycled into energy metabolism and other biosynthetic intermediates, so its 'ending' is really a handoff to downstream pathways.
Why is Methionine important?
Because rSAM-dependent methylation touches DNA, RNA, histones, and countless proteins, methionine availability acts as a global regulator of gene expression and signaling. Pair that with its non-negotiable role as the translation start signal, and you see why a single amino acid sits at the intersection of so many core cellular processes.
More in Cell And Molecular Biology 1-24
Elsewhere in the Cell And Molecular Biology universe
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record