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Amino acid

Amino acids are organic compounds that combine to form proteins; they have an amino and a carboxyl group plus a variable side chain and serve structural, metabolic, and signaling roles.

Author: Leandro Alegsa Creation date: November 13, 2022 Last updated: May 20, 2026

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Amino acids are organic molecules that serve as the monomer units of proteins and perform many other roles in cells. In living organisms, most proteins are assembled from a set of twenty standard amino acids. The sequence and chemical properties of these amino acids determine a protein's shape and function. For a concise introduction to their role in proteins see proteins.

Basic structure and chemical properties

In biochemistry the term usually refers to alpha-amino acids, molecules with a central (alpha) carbon bonded to four groups: an amino group, a carboxyl group, a hydrogen, and a distinctive side chain commonly called the R group. This generic description appears in many texts on biochemistry. The amino group itself consists of a nitrogen and hydrogen atoms; the chemical nature of that group is often noted simply as the amine group, while the carboxyl contributes acidic behavior. At physiological pH most amino acids exist as zwitterions, carrying both positive and negative charges. Each amino acid's R group gives it characteristic polarity, size and reactivity.

Peptide bonds and protein formation

Amino acids link together through peptide bonds to form polypeptides and proteins. A peptide bond forms when the carboxyl group of one amino acid reacts with the amino group of another, releasing water. The backbone of any protein is therefore a repeating pattern derived from these molecule units, while side chains project outward and interact to fold the chain into a functional three-dimensional structure.

Classification, special cases and biosynthesis

Amino acids are commonly classified by the chemical properties of their side chains: nonpolar, polar uncharged, acidic or basic. Most organisms use the same set of twenty standard amino acids, but two additional amino acids—selenocysteine and pyrrolysine—are incorporated into proteins in certain contexts. Some amino acids cannot be synthesized by a species and must be obtained from the diet; in adult humans a subset—often described as nine essential amino acids—must be supplied externally.

Functions and biological importance

Beyond building proteins, amino acids serve as metabolic intermediates, precursors for neurotransmitters and hormones, and as nitrogen donors in biosynthetic reactions. For example, glutamate functions as a key neurotransmitter and as an amino donor in transamination reactions. Amino acids also influence acid–base balance and can be oxidized for energy when needed.

History, study and resources

Recognition of amino acids emerged in the 19th century through protein hydrolysis and chemical characterization; their central role in genetics became clear with the deciphering of the genetic code, which assigns sets of three nucleotides (codons) to specific amino acids. For more technical treatment of functional groups and related terminology, see entries on functional groups and introductory pages about proteins. Further reading on molecular structure and biochemical pathways is available through general biochemistry resources amine group descriptions and pathway summaries at textbooks and review sites biochemistry.

Key features: amine and carboxyl groups, variable R group
Essential vs nonessential amino acids
Forms proteins via peptide bonds and participates in metabolism

Basic structure of α-amino acids (residue R is a H atom in the case of glycine).

History

The first amino acid was isolated in 1805 in the Paris laboratory of Louis-Nicolas Vauquelin and his student Pierre Jean Robiquet from the juice of asparagus (Asparagus officinalis) and was subsequently named asparagine. The last of the common protein-building amino acids, threonine was discovered in fibrin in 1931 and its structure clarified by William Rose in 1935. Rose had found out by experiments with different feeds that the 19 amino acids discovered so far were not sufficient as an additive. He also established the essentiality of other amino acids and determined the minimum daily dose required for optimal growth.

In the period between 1805 and 1935, many of the chemists and pharmacists known at the time were involved in isolating amino acids for the first time and clarifying their structure. Emil Fischer, for example, to whom the Fischer projection also goes back, succeeded in finally elucidating the structure of serine (1901), lysine (1902), valine (1906) and cysteine (1908). Albrecht Kossel (1896 histidine from sturgeon sperm), Richard Willstätter (1900 proline via synthesis) and Frederick Hopkins (1901 tryptophan from casein) also later became Nobel Prize winners. The German chemist Ernst Schulze isolated three amino acids for the first time - glutamine from beets in 1877, phenylalanine in 1881 and arginine from lupins in 1886 - and was involved in the structural elucidation of other amino acids. Heinrich Ritthausen had previously obtained glutamic acid from cereal protein, gluten, in crystalline form in 1866. In 1872, Wilhelm Dittmar clarified the structure of glutamine and glutamic acid, whose salts are glutamates.

As early as 1810, William Hyde Wollaston discovered the sulfur-containing cystine as "cystic oxide" in bladder stones, but it was not until 1884 that Eugen Baumann discovered the monomeric cysteine. In 1819 Henri Braconnot separated glycine from glue and Joseph Louis Proust separated leucine from cereals. Eugen von Gorup-Besánez isolated valine from pancreatic juice in 1856. As early as 1846, Justus von Liebig was able to separate tyrosine from casein for the first time, the structure of which was clarified by Ludwig von Barth in 1869. In the hydrolysate of casein, Edmund Drechsel also discovered lysine in 1889 and later John Howard Mueller in 1922 discovered the sulphur-containing methionine as the 19th amino acid, whose structural formula was given by George Barger and Philip Coine in 1928. In molasses, Felix Ehrlich had already found isoleucine, a structural isomer of leucine, as the 18th amino acid in 1903.

Friedrich Wöhler, whose syntheses in the 1820s opened up the field of biochemistry, did not discover any amino acid, but three of his students were involved, in addition to the aforementioned Gorup-Besánez and Schulze, also Georg Städeler (1863 serine from raw silk). 18 of the 20 amino acids discovered were isolated from plant or animal material, only the two amino acids alanine (1850 Adolph Strecker) and proline (Willstätter) were obtained by organic synthesis. While the analysis of the material composition up to the sum formula could be well accomplished with the methods of the time, the structural formula of many amino acids could often only be finally elucidated by partial steps of the synthesis, which was sometimes only achieved years later. The structure of asparagine and that of aspartic acid was not clarified by Hermann Kolbe until 1862, 57 years after the first description.

Amino acids owe their generic names to two functional groups, their individual names sometimes to a bright appearance (e.g. arginine, leucine), a sweet taste (e.g. glycine) or the material in which they were found (e.g. asparagine, cysteine, serine, tyrosine), features of the chemical structure (e.g. proline, valine, isoleucine) or both (e.g. glucine). e.g. asparagine, cysteine, serine, tyrosine), features of the chemical structure (e.g. proline, valine, isoleucine) or both (e.g. glutamine, glutamic acid) and sometimes also the reactants of their synthesis (e.g. alanine).

The fact that proteins are built up as chains of amino acids linked by peptide bonds was first proposed simultaneously and independently of each other by both Emil Fischer and Franz Hofmeister at the Assembly of German Natural Scientists and Physicians in Karlsbad in 1902 (Hofmeister-Fischer theory).

Simplified structural formulas of 20 natural α-amino acids, without configuration information.

Structure

Carbamidsäure

Carbamic acid

Amino acids consist of at least two carbon atoms. The unstable carbamic acid has only one carbon atom and is therefore not an amino acid, but a carbonic acid amide. Amino acids can be divided into classes according to the carbon atom at which the amino group is located relative to the carboxy group. If more than one amino group is present in the molecule, the carbon atom whose amino group is closest to the carboxy carbon determines which class of amino acid it is.

General structure of amino acids
(R: side chain)

α-amino acid

β-amino acid

γ-amino acid

α-Amino acids: The amino group of α-amino acids is located at the second carbon atom, including the carboxy carbon atom. The counting always starts with the carboxy-carbon. Therefore, the IUPAC designation is 2-aminocarboxylic acids. The simplest representative of the α-amino acids is the proteinogenic amino acid glycine. All proteinogenic amino acids are α-amino acids.

The term amino acids often refers to a specific group of α-amino acids consisting mainly of L-α-amino acids: the proteinogenic amino acids. These are the building blocks of all proteins of all life on earth and, along with nucleic acids, the basic building blocks of life.

β-Amino acids: The amino group of β-amino acids is located on the third carbon atom (counting the carboxy carbon atom). The IUPAC designation is 3-aminocarboxylic acids. The simplest representative is β-alanine.
γ-Amino acids: The amino group of γ-amino acids is located at the fourth carbon atom (counting the carboxy carbon atom). The IUPAC designation is 4-aminocarboxylic acids. The simplest representative is γ-aminobutyric acid (GABA).

The designation of other classes of amino acids follows the same scheme.

The amino acids of a class are distinguished by their side chain R. If the side chain R is different from the other substituents located on the carbon with the amino group, then a stereocenter is located here and two enantiomers exist of the corresponding amino acid. If the side chain R itself contains further stereocentres, diastereomers also result and the number of possible stereoisomers increases in proportion to the number of further stereocentres. There are four stereoisomers of amino acids with two differently substituted stereocentres.

Questions and Answers

Q: What are amino acids?

A: Amino acids are molecules that have both amine (NH2+R) and carboxyl (C=O) functional groups, and they are the building blocks of proteins.

Q: How many "standard" amino acids exist in eukaryotes?

A: In eukaryotes, there are 20 "standard" amino acids out of which almost all proteins are made.

Q: What is the general formula for alpha-amino acids?

A: The general formula for alpha-amino acids is H2NCHRCOOH, where R is one of many side groups.

Q: What does biochemistry refer to when it mentions amino acids?

A: In biochemistry, the term 'amino acid' refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is one of many side groups.

Q: How do proteins get their structure?

A: Proteins get their structure from the combination of different types of amino acids.

Q: What role do amine and carboxyl functional groups play in an amino acid molecule?

A: Amine and carboxyl functional groups make up an amino acid molecule; they provide a nitrogen atom as well as a carbon atom that can form bonds with other molecules.