How many amino acids are there?

Proteins are built from 20 standard amino acids. Two more, selenocysteine and pyrrolysine, are also genetically encoded, which brings the protein alphabet across known life to 22. Humans use 21 of them, because human cells insert selenocysteine but not pyrrolysine. Nine of the standard 20 are essential in the adult diet, and about 500 amino acids were already known in nature by 1983, most of which never enter a protein.

Any protein sequence shows the 20 at work, each type repeated many times along the chain. In ProteinIQ's count of reviewed UniProt sequences, leucine is the most common amino acid, at 9.65% of standard residues. Amino acid composition gives the same per-type breakdown for any sequence you paste in.

Are there 20, 21, or 22 amino acids?

The conventional protein alphabet contains 20 amino acids. This is the set meant by the familiar textbook answer and by the NHGRI glossary.^[1] Humans also incorporate selenocysteine through specialized translation machinery, bringing the human encoded set to 21.^[2][3] Pyrrolysine, found in some archaea and bacteria, extends the naturally genetically encoded set across life to 22.^[2][4]

These are nested sets rather than competing estimates. The count of 22 spans organisms with different translation machinery; it does not mean that every organism uses all 22. Likewise, nine essential amino acids is a dietary count within the standard 20, rather than an alternative estimate of the entire protein alphabet.^[4][5]

Some sources mention a 23rd amino acid, but none is known to be encoded the way selenocysteine and pyrrolysine are. Lobanov and colleagues scanned 16 archaeal and 130 bacterial genomes for tRNAs that could read a stop codon as a further amino acid, and concluded that another widely distributed encoded amino acid is unlikely.^[6]

The usual candidate, N-formylmethionine, is the methionine that starts proteins in bacteria and mitochondria, delivered on a formylated initiator tRNA. It is a modified methionine, not a new entry in the code.^[7]

The distinction between types and residues also matters. A residue is an amino acid unit within a peptide or protein chain, where the same type can occur many times. A protein can therefore contain hundreds of residues while using only the standard alphabet. The average protein size guide covers chain lengths rather than the number of available building blocks.

What are the 22 genetically encoded amino acids?

The list consists of the 20 standard amino acids plus selenocysteine and pyrrolysine. The table gives their sequence codes and separates the human dietary classification from their inclusion in the genetic alphabet.

Amino acid	Three-letter code	One-letter code	Healthy-adult dietary classification
Alanine	Ala	A	Nonessential
Arginine	Arg	R	Nonessential
Asparagine	Asn	N	Nonessential
Aspartic acid	Asp	D	Nonessential
Cysteine	Cys	C	Nonessential
Glutamic acid	Glu	E	Nonessential
Glutamine	Gln	Q	Nonessential
Glycine	Gly	G	Nonessential
Histidine	His	H	Essential
Isoleucine	Ile	I	Essential
Leucine	Leu	L	Essential
Lysine	Lys	K	Essential
Methionine	Met	M	Essential
Phenylalanine	Phe	F	Essential
Proline	Pro	P	Nonessential
Serine	Ser	S	Nonessential
Threonine	Thr	T	Essential
Tryptophan	Trp	W	Essential
Tyrosine	Tyr	Y	Nonessential
Valine	Val	V	Essential
Selenocysteine	Sec	U	Outside the standard 9/11 classification
Pyrrolysine	Pyl	O	Not encoded by humans

Sequence notation follows UniProt; the dietary labels for the standard 20 follow MedlinePlus. Selenocysteine and pyrrolysine are listed separately because the 9-essential/11-nonessential division applies to the standard 20.^[8][5] The nonessential category includes amino acids that can become conditionally essential, as explained below.

Aspartic acid and glutamic acid are often called aspartate and glutamate when discussing their ionized forms. These names do not add two more amino acid types. Their charge, like that of other ionizable groups, depends on pH and the surrounding chemical environment.

The largest amino acid by mass also depends on the set being compared. Among the standard 20, tryptophan has the highest average residue mass, about 186.21 Da, while glycine has the lowest, about 57.05 Da. Including all 22 genetically encoded types makes pyrrolysine the heaviest, at about 237.30 Da per residue. These are masses of residues within a chain, not free amino acids; the latter include the equivalent of an additional water molecule.^[9]

Residue mass also distinguishes peptides with the same length. Among unmodified linear dipeptides made from the standard 20 amino acids, Trp-Trp is heavier than Tyr-Tyr because tryptophan residues are heavier than tyrosine residues. Both peptides have the same terminal groups, so Tyr-Tyr is not the highest-mass dipeptide in that set.^[9] Peptide-bond count alone cannot determine molecular mass.

How many amino acids are in the human body?

Human protein synthesis uses 21 genetically encoded amino acids, but the body contains amino acids beyond that set. The count of 21 includes the standard 20 and selenocysteine; it is not a complete inventory of human amino acid chemistry.^[1][3]

Selenocysteine is inserted at selected UGA codons. UGA is one of the three stop codons that ordinarily end translation, and specialized machinery lets those positions specify an amino acid instead.^[2]

The human genome contains 25 selenoprotein genes. That is a count of genes whose products contain selenocysteine, not 25 additional amino acid types.^[3] Their products participate in processes including thyroid hormone metabolism and protection against oxidative damage.^[10]

The body synthesizes the selenocysteine needed for these proteins using dietary selenium. Food can supply selenium in several forms, including selenocysteine and selenomethionine. Selenium remains an essential mineral even though the body makes its own selenocysteine for protein synthesis.^[10]

Other amino acids have metabolic roles outside the encoded protein alphabet. Ornithine and citrulline, for example, participate in the urea cycle, which clears waste nitrogen.^[11] Including such metabolites changes the question from which amino acids are genetically encoded to which amino acids are present in the body. The latter requires a defined tissue, sample, and chemical scope before a count can be interpreted.

How many amino acids are essential for humans?

Healthy adults need nine essential amino acids from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The body normally makes the other 11 standard amino acids.^[5] Essential describes dietary supply rather than biological importance.

Some become conditionally essential when synthesis cannot meet demand. MedlinePlus lists arginine, cysteine, glutamine, tyrosine, glycine, proline, and serine, particularly in illness or stress.^[5] The classification depends on physiological context, so conditional essentiality does not define a fixed third group to add to the 20.

This also explains the term complete protein: nutritional completeness concerns the supply of the nine essential amino acids. It does not require a food to contain every amino acid found across nature.

How many amino acids exist in nature?

About 500 naturally occurring amino acids were known when Wagner and Musso reviewed the field in 1983. Most had been discovered in the previous 30 years, in microbial culture media and antibiotics, fungi, seeds, plants, fruits, and animal body fluids. Some 240 of them occur free in nature, some only as intermediates in metabolism.^[12]

That figure counts a chemical category with very different inclusion rules from the 20-, 21-, and 22-member sets. It was also a snapshot of a list that had been growing quickly, so "about 500" is best read as an order of magnitude rather than a census, and it cannot be compared directly with the number of amino acids specified during translation.