Before a single protein can form, this molecule must arrive first β the universal start signal of all life on Earth.
Symbol
Met Β· M
Discovered
1922
Mol. Weight
149.21 g/mol
Essential
Yes
M
Discovery: The Sulfur Amino Acid
L-Methionine
In 1922, American biochemist John Howard Mueller was working at Columbia University in New York, studying what nutritional factors bacteria needed to grow. He was systematically testing different protein hydrolysates as growth media for Streptococcus hemolyticus. When he isolated a new sulfur-containing compound from casein (milk protein), he had found something genuinely new β but submitted an incorrect molecular formula. Three years later, Japanese researcher Shiro Odake corrected the formula and clarified its basic properties. The structural determination and the name came from George Barger and F. P. Coyne in 1928: working from Mueller's procedure, they confirmed the structure as Ξ³-methylthiol-Ξ±-aminobutyric acid and β after consulting Mueller β proposed the name methionine, in allusion to the compound's characteristic methylthiol group.
Methionine was one of the last standard amino acids to be discovered β amino acid chemistry had been going on for over a century by 1922, and the field thought most of the standard set had already been found. Methionine was a late and distinctly sulfurous surprise.
Every protein ever synthesized by a cell begins the same way: with a methionine residue, positioned by a single codon β AUG β that serves simultaneously as 'amino acid' and 'begin here.'
Of all the remarkable things about methionine, one stands above the rest: its codon, AUG, is the universal start signal for protein synthesis in virtually all life on Earth. When a ribosome β the molecular machine that builds proteins β encounters an mRNA molecule and begins scanning it, it looks for one thing above all: the sequence AUG. When it finds it, protein synthesis begins. The first amino acid to be incorporated is always methionine.
π¦ How the START Codon Works
Every messenger RNA has a 5' untranslated region before the protein-coding area begins. The ribosome scans along this region until it encounters the sequence AUG. This triggers assembly of the full ribosome and insertion of the first methionine. What follows β the rest of the protein β is entirely determined by what comes after that AUG. But without it, nothing begins.
This means that at the very beginning of its life, every protein has a methionine at its N-terminus (the starting end). In many cases, the cell removes this initial methionine afterwards through a process called N-terminal methionine excision β the protein doesn't need it for its function. But the methionine was still there, essential for getting things started.
Sulfur: The Element That Sets Methionine Apart
Methionine is one of only two amino acids in the standard set that contain sulfur (the other is cysteine). The sulfur atom in methionine is part of a thioether group β CHββSβCHββ β buried in its side chain. This gives methionine its characteristic chemistry: the compound dimethyl sulfide, which forms when methionine is metabolized, contributes to the aroma of cooked cabbage, seaweed, and some cheeses.
Sulfur chemistry is also the reason Brazil nuts have an unusual nutritional profile. Brazil nuts are notably rich in methionine among plant foods, and extraordinary in their selenium content β a single Brazil nut can provide more than the daily requirement for selenium, largely because of the sulfur-selenium chemistry in the proteins of this remarkable Amazonian tree.
Functions of L-Methionine in the Body
Methionine is an essential amino acid β humans cannot synthesize it and must obtain it from food. It participates in several critical metabolic processes beyond its universal role as the initiating residue in protein synthesis.
SAM: the methyl donor
When methionine reacts with ATP, it forms S-adenosylmethionine (SAM) β one of the most important methyl group donors in biochemistry. SAM transfers methyl groups (βCHβ) to DNA, RNA, proteins, neurotransmitters, and dozens of small molecules, participating in processes fundamental to gene regulation, neurotransmitter synthesis, and cellular metabolism. SAM-dependent methylation of DNA is a key mechanism in epigenetics. The liver produces SAM in large quantities and is the organ most dependent on adequate methionine supply; SAM supplementation has been studied in clinical settings for supporting liver function.
Cysteine synthesis
Methionine is the metabolic precursor of cysteine, via the transsulfuration pathway. The sulfur atom from methionine is transferred through a series of reactions to form cysteine β the other sulfur-containing amino acid, which is not essential in the diet precisely because the body can make it from methionine. This pathway also produces homocysteine as an intermediate, and impaired methionine metabolism leading to elevated homocysteine levels is associated with cardiovascular risk.
Antioxidant protection and detoxification
Methionine acts as an antioxidant by donating sulfur to quench reactive oxygen species and free radicals. It also maintains the pool of glutathione peroxidase β a key enzymatic antioxidant β by supporting the availability of cysteine for glutathione synthesis. Through this pathway, methionine contributes indirectly to the cell's broader detoxification capacity.
Did You Know?
When methionine reacts with ATP it forms S-adenosylmethionine (SAM) β one of the most widely used molecules in biochemistry. SAM donates methyl groups to DNA, RNA, neurotransmitters, and dozens of other molecules. Methionine's chemistry reaches far beyond protein building.
Interesting Facts
π
Every protein starts with Met. The AUG codon encodes methionine AND serves as the start signal for translation in all known organisms β from bacteria to blue whales. It is the most universal molecular instruction in biology.
βοΈ
Contains sulfur. Only methionine and cysteine among the 20 standard amino acids contain sulfur. This makes them chemically distinct β and makes methionine slightly larger and heavier than amino acids of similar carbon count.
πΏ
A methyl donor. In the form S-adenosylmethionine (SAM), methionine becomes one of the most important methyl group donors in biochemistry β transferring methyl groups (βCHβ) to DNA, RNA, proteins, and small molecules in a process fundamental to gene regulation.
π
Major industrial production. Methionine is one of the most commercially produced amino acids in the world, primarily used as an animal feed supplement. Poultry and pig feed is routinely supplemented with methionine because it's often the limiting amino acid in grain-based diets.
π₯
Brazil nuts: the extreme source. Brazil nuts have an unusually high methionine content β roughly 360 mg per 100g. Combined with their exceptional selenium content, they are in a nutritional category of their own among nuts and seeds.
Where to Find Methionine in Food
Methionine is an essential amino acid β the human body cannot synthesize it. It must come from food. It is particularly abundant in animal proteins and some seeds. Values below are approximate per 100g of food:
