Are there 20 or 25 amino acids? Unpacking the Proteinogenic Question

December 6, 2025 •

3 min read

Over 500 amino acids have been identified in nature, yet the number central to genetics is often debated. The simple answer to "are there 20 or 25 amino acids?" is that neither number tells the whole story, as it depends on whether one is referring to the standard building blocks of proteins or the total number of genetically encoded amino acids. This nuance is crucial for understanding the diversity of life on a molecular level.

Quick Summary

The exact number of amino acids depends on the context: 20 are standard in the universal genetic code, while the total of genetically encoded protein-building amino acids is 22, including selenocysteine and pyrrolysine, with other modified amino acids existing as well.

Key Points

Standard Set is 20: The universal genetic code for protein synthesis includes 20 standard, or canonical, amino acids used by most life forms.
Expanded Code Includes 22: In some organisms, the genetic code is expanded to include two additional amino acids, selenocysteine (the 21st) and pyrrolysine (the 22nd).
Humans Use 21: Humans and other eukaryotes utilize 21 genetically encoded amino acids, incorporating selenocysteine but not pyrrolysine.
Hundreds Exist in Nature: Over 500 amino acids occur in nature, many of which are non-standard and not genetically encoded for protein synthesis.
Post-Translational Modification: Some non-standard amino acids are created by modifying standard amino acids after they have been incorporated into a protein chain.
Depends on the Context: The seemingly simple question has a complex answer, with the correct number depending on whether one refers to the standard, genetically encoded, or total naturally occurring set.

The 20 Standard Amino Acids: The Universal Foundation

The standard genetic code utilizes 20 common, or 'canonical', amino acids to build proteins in most organisms. These are categorized based on their side chain properties and include nine essential amino acids that humans must obtain through diet: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Complete proteins, like meat and eggs, provide all nine essential amino acids.

The Expansion: Introducing the 21st and 22nd Amino Acids

The traditional set of 20 proteinogenic amino acids is expanded by two additional genetically encoded amino acids in some organisms. Selenocysteine, the 21st, is found in all domains of life, including humans, and is incorporated via a special mechanism that reinterprets a UGA stop codon. Pyrrolysine, the 22nd, is found in certain archaea and bacteria, also incorporated via a recoded stop codon (UAG), and is not present in humans.

More Than Just 20 or 22: The Non-Standard Amino Acids

Nature contains over 500 amino acids beyond the genetically encoded ones. These non-proteinogenic amino acids are not directly coded by DNA. They can arise from post-translational modification of existing amino acids in a protein, such as hydroxyproline and hydroxylysine crucial for collagen. Other non-standard amino acids function as metabolic intermediates or neurotransmitters like ornithine, citrulline, and GABA.

Comparison: Standard vs. Expanded Amino Acid Sets

The table below highlights the differences between the standard and expanded sets of protein-building amino acids.


Feature	20 Standard Amino Acids	22 Genetically Encoded Amino Acids
Definition	Universally encoded by the standard genetic code in most organisms.	Includes the 20 standard plus selenocysteine and pyrrolysine, which require special decoding mechanisms.
Inclusion	The 20 standard amino acids are listed on {Link: The Twenty Amino Acids https://www.cryst.bbk.ac.uk/education/AminoAcid/the_twenty.html}.	The 20 standard, plus Selenocysteine and Pyrrolysine.
Genetic Coding	Each has a dedicated set of codons in the standard genetic code.	Selenocysteine is encoded by a UGA (stop) codon with a special sequence, and Pyrrolysine by a UAG (stop) codon via a different mechanism.
Human Proteins	All 20 are used in human protein synthesis.	Humans use 21 (the 20 standard plus selenocysteine), but not pyrrolysine.
Prevalence	Present in virtually all life forms.	Present only in specific organisms, often dependent on unique genetic and environmental conditions.

Conclusion: More Than Meets the Eye

The number of amino acids depends on the definition used. While 20 are standard in the universal genetic code, the inclusion of selenocysteine and pyrrolysine brings the total of genetically encoded amino acids to 22 in some organisms. Humans utilize 21 of these. Additionally, hundreds of other non-standard amino acids exist naturally, serving various functions. Thus, the answer depends on the context of the question.

List of 20 Standard Proteinogenic Amino Acids

A list of the 20 standard proteinogenic amino acids is available from {Link: The Twenty Amino Acids https://www.cryst.bbk.ac.uk/education/AminoAcid/the_twenty.html}.

A Final Thought on the Count

The number of amino acids is context-dependent. Introductory biology typically focuses on the 20 standard amino acids. In advanced contexts, the count for proteinogenic amino acids across life is 22, and for humans, it's 21. Considering all naturally occurring amino acids, the number is in the hundreds.

Frequently Asked Questions

The confusion stems from a difference in definition. The common set of protein-building amino acids is 20, as taught in introductory biology. However, advanced studies reveal two more, selenocysteine and pyrrolysine, that are genetically encoded in some organisms, expanding the proteinogenic set to 22. This, along with hundreds of other non-proteinogenic amino acids, complicates the simple count.

The terms are often used interchangeably, but proteinogenic means 'protein-creating' and encompasses all genetically encoded amino acids. While 20 are the standard, the inclusion of selenocysteine and pyrrolysine brings the total proteinogenic count to 22 across different forms of life.

No, humans use 21 genetically encoded amino acids for protein synthesis, not 22. The human body does incorporate selenocysteine, but it does not have the genetic machinery to incorporate pyrrolysine.

Non-proteinogenic amino acids are those not directly coded for by DNA and incorporated into proteins during translation. They may be created by post-translational modification of a standard amino acid or serve other biological functions, such as neurotransmitters or metabolic intermediates.

They are special because they are genetically encoded using codons that normally function as stop signals (UGA and UAG, respectively), requiring unique decoding mechanisms. This expands the repertoire of amino acids available for protein synthesis beyond the standard 20.

The 20 standard amino acids are the essential building blocks for all proteins in the human body. These proteins are vital for virtually every biological process, including immune function, structural support, enzymatic reactions, and hormone production.

The nine essential amino acids for humans are a subset of the 20 standard amino acids. They are 'essential' not because they are special protein-builders, but because the human body cannot synthesize them and must obtain them from diet.