Determining the number of amino acids in nature is complex, as the answer depends on classification criteria. These molecules serve different functions; some are components of proteins, while hundreds of others perform a vast array of physiological roles. Understanding these categories reveals the full scope of their presence in nature.
The 22 Proteinogenic Amino Acids
The most frequently discussed amino acids are the proteinogenic, or protein-building, types. For decades, the standard number taught was 20, as these are the ones specified by the universal genetic code. During a process called translation, cellular machinery reads a messenger RNA (mRNA) template in three-letter units called codons. Each codon instructs the ribosome to add a specific amino acid to a growing polypeptide chain, which then folds into a functional protein.
This set of 20 standard amino acids includes well-known examples like leucine, glycine, and tryptophan, each with a unique side chain that gives it specific chemical properties. These properties dictate how the protein chain folds and interacts with its environment, ultimately determining its biological function. The sequence of these amino acids is dictated by an organism’s genes, making them the direct products of genetic expression.
The family of protein-building amino acids has expanded to include two more, bringing the total to 22. Selenocysteine, the 21st amino acid, is found in proteins across all domains of life, including humans. It is incorporated when a specific RNA sequence, known as a SECIS element, signals the ribosome to interpret a UGA codon, which normally means “stop,” as a signal to insert selenocysteine instead. This process allows for the creation of specialized enzymes involved in antioxidant defense and thyroid hormone metabolism.
A 22nd amino acid, pyrrolysine, is used by some methane-producing microbes (methanogenic archaea) and at least one bacterium. Similar to selenocysteine, it is encoded by a stop codon, UAG, but its insertion requires a downstream sequence called PYLIS. Pyrrolysine is part of enzymes these organisms use to generate energy from methylamines. The discoveries of selenocysteine and pyrrolysine demonstrate that the protein-building toolkit of nature is slightly larger and more flexible than once thought.
Essential vs. Non-Essential Amino Acids
Amino acids can also be classified by dietary needs, a system relevant to human nutrition. This classification divides the standard 20 amino acids into two main groups: essential and non-essential. The distinction is not based on an amino acid’s importance, as all 20 are required for health, but on whether the body can produce them on its own.
Nine amino acids are essential for humans because our bodies cannot synthesize them. They must be obtained from food sources and include:
- Histidine
- Isoleucine
- Leucine
- Lysine
- Methionine
- Phenylalanine
- Threonine
- Tryptophan
- Valine
A diet lacking in any of these nine can lead to the breakdown of the body’s own proteins, particularly muscle, to supply the missing component for new protein synthesis.
The remaining 11 amino acids are non-essential, meaning the human body can synthesize them from glucose or other amino acids. This group includes alanine, asparagine, aspartic acid, and glutamic acid. A third category is conditionally essential amino acids. Under normal circumstances, amino acids like arginine, cysteine, and glutamine are non-essential, but during periods of significant physical stress, illness, or growth, the body’s ability to produce them may not meet its demands, making them temporarily essential in the diet.
Hundreds of Non-Standard Amino Acids
Beyond the 22 amino acids used to construct proteins, nature is home to a vast number of non-proteinogenic amino acids. Over 500 have been identified, and these molecules are not encoded in an organism’s DNA for protein assembly. Instead, they perform a wide variety of independent biological functions, acting as metabolic intermediates, signaling molecules, or components of specialized peptides. Their roles are distinct but important to an organism’s survival.
One prominent example is gamma-aminobutyric acid (GABA), a non-proteinogenic amino acid that functions as the primary inhibitory neurotransmitter in the mammalian central nervous system. It is synthesized in the brain from glutamate, one of the standard amino acids. GABA’s role is to reduce neuronal excitability, which is important for managing anxiety, stress, and promoting sleep.
Other non-standard amino acids serve as intermediates in metabolic pathways. Ornithine and citrulline are two such examples found in the urea cycle, the process by which the body disposes of excess nitrogen as urea. Another is carnitine, which is synthesized from the essential amino acids lysine and methionine and is responsible for transporting long-chain fatty acids into the mitochondria, where they can be oxidized to produce energy.
The Universal Chemistry of Amino Acids
All amino acids, whether protein-building or not, are unified by a common chemical architecture. Each molecule contains a central carbon atom bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain known as the R-group. It is the unique chemical structure of the R-group that distinguishes one amino acid from another and determines its specific properties, such as size, charge, and polarity.
This structure is not unique to life on Earth. Scientists have discovered a diverse array of amino acids in meteorites, such as the Murchison meteorite, which fell in Australia in 1969. Analyses have identified over 70 distinct amino acids within these extraterrestrial rocks, including some of the proteinogenic types used by terrestrial life and many that are not. Their presence in these ancient materials suggests that amino acids can form abiotically through natural chemical processes in space.
The discovery of amino acids in meteorites provides compelling evidence that the building blocks of proteins, and life itself, may be widespread throughout the universe. These molecules likely seeded the early Earth, providing a ready-made toolkit for the origin of life. The shared chemical foundation of all amino acids connects terrestrial biology to the broader chemistry of the cosmos, highlighting a universal principle of molecular construction.