Methionine

Lore & Background

It is encoded by the single codon AUG, which also serves as the most common start codon in eukaryotes and Archaea, initiating protein translation. In bacteria, the derivative N-formylmethionine is used as the initial amino acid. Methionine is one of only two amino acids encoded by a single codon in the standard genetic code, the other being tryptophan (UGG).

Reader's Guide

Methionine is an essential amino acid that humans must obtain from diet, as it is not synthesized de novo. Its biosynthetic roles are decisive: it is the precursor to cysteine and to S-adenosylmethionine (rSAM), a cofactor that serves mainly as a methyl donor. Methionine is required for protein synthesis, initiated by N-formylmethionine-sRNA. In proteins, methionine residues generally lack catalytic roles, unlike cysteine, but the thioether side chain contributes structural stability through S/π interactions with aromatic amino acids. The methionine codon AUG is the most common start codon, signaling the initiation of translation. In eukaryotes and Archaea, methionine is often the first amino acid in nascent polypeptides, though it may be removed post-translationally. The biosynthesis of methionine in plants and microorganisms belongs to the aspartate family, with pathways involving transsulfurylation or direct sulfurylation. Methionine can be regenerated from homocysteine via methionine synthase, requiring vitamin B12, or via betaine-homocysteine methyltransferase. Its significance extends to its role in methylation reactions and as a key component of the genetic code.

Did You Know?

• Methionine is one of only two amino acids encoded by a single codon (AUG) in the standard genetic code; the other is tryptophan (UGG).
• The methionine derivative S-adenosylmethionine (rSAM) is a cofactor that serves mainly as a methyl donor.
• In bacteria, the derivative N-formylmethionine is used as the initial amino acid during translation.

Discovery & Nomenclature

The abbreviation Met, or simply M, has since become standard in biochemistry. What makes methionine's identity particularly striking is its genetic address: it is specified by the codon AUG, a triplet that carries extraordinary significance in the language of the cell. Unlike most amino acids, which are recognized by several synonymous codons, methionine is one of only two in the standard genetic code assigned a single codon—the other being tryptophan, which uses UGG. This uniqueness hints at a deeper evolutionary story, one in which methionine's role as the universal kickoff signal for protein translation may have shaped the very architecture of the code itself.

The Start of Every Protein

In the nuclear genomes of eukaryotes and in Archaea, the AUG codon serves as the universal start signal, telling the ribosome where to begin reading a messenger RNA strand. Because methionine is the amino acid tied to that codon, it is the first residue laid down in every newly forming polypeptide chain. In bacteria, a modified form called N-formylmethionine takes this initial position. In eukaryotes and archaea, the methionine at the N-terminus can later be cleaved away through post-translational modification, but its presence at the very start is a near-universal feature of protein synthesis. The neighboring AUN codons, where N is any nucleotide, encode isoleucine, another hydrophobic residue, suggesting a shared evolutionary lineage. In the mitochondrial genomes of animals and yeast, the codon AUA—which normally specifies isoleucine in the standard code—has been reassigned to methionine as well. The tRNA machinery in bacteria and archaea employs unusual modified bases, lysidine and agmatidine respectively, to ensure faithful discrimination between AUG and AUA.

Structure, Sulfur, and Subtle Structural Roles

Methionine is one of just two sulfur-bearing amino acids that are incorporated into proteins, the other being cysteine. Its side chain is an S-methyl thioether, which classifies it as a nonpolar, aliphatic residue. Despite containing sulfur, methionine does not play a catalytic role in the way cysteine's reactive thiol group does across countless enzymes. A rare exception exists in the form of methionine sulfoxide, which can function as a redox sensor in certain contexts. The thioether does, however, contribute a subtle structural influence: in roughly one-third of all characterized protein structures, the sulfur atom engages in stabilizing S/π interactions with nearby aromatic rings. This effect is modest, as demonstrated by experiments in which methionine is swapped for norleucine, a straight hydrocarbon chain lacking the thioether, with surprisingly little disruption to protein function. Some researchers have speculated that norleucine may have occupied this position in an ancestral genetic code before methionine displaced it, driven by the latter's indispensable role in generating the cofactor S-adenosylmethionine.

An Essential Nutrient and a Biosynthetic Hub

Because humans and other animals cannot manufacture methionine from scratch, it is classified as an essential amino acid; it must be obtained through diet, either as free methionine or within protein-rich foods. In plants and microorganisms, however, a full biosynthetic route exists, placing methionine in the aspartate family alongside threonine and lysine. The carbon backbone is drawn from aspartic acid, while the sulfur atom can be sourced from cysteine, methanethiol, or hydrogen sulfide. The pathway proceeds through homoserine, a key intermediate that also branches into threonine synthesis, and through a series of activation steps involving phosphate, acetyl, or succinyl groups depending on the organism. The final sulfur-incorporation step is catalyzed by PLP-dependent enzymes that stabilize carbanion intermediates. Beyond protein synthesis, methionine serves as the direct precursor to cysteine and to S-adenosylmethionine, a sulfonium cation that acts as the cell's principal methyl-group donor, making methionine a linchpin of methylation chemistry throughout biology.