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Proline is the only proteinogenic amino acid whose side-group links to the α-amino group and, thus, is also the only proteinogenic amino acid containing a secondary amine at this position. The phrase " branched-chain amino acids" or BCAA refers to the amino acids having aliphatic side-chains that are non-linear these are leucine, isoleucine, and valine. The chemical structures of the 22 standard amino acids, along with their chemical properties, are described more fully in the article on these proteinogenic amino acids. The side-chain can make an amino acid a weak acid or a weak base, and a hydrophile if the side-chain is polar or a hydrophobe if it is nonpolar. In some amino acids, the amine group is attached to the β or γ-carbon, and these are therefore referred to as beta or gamma amino acids.Īmino acids are usually classified by the properties of their side-chain into four groups. In amino acids that have a carbon chain attached to the α–carbon (such as lysine, shown to the right) the carbons are labeled in order as α, β, γ, δ, and so on. In the alpha amino acids, the α–carbon is a chiral carbon atom, with the exception of glycine. These are the most common form found in nature. The carbon atom next to the carboxyl group is called the α–carbon and amino acids with a side-chain bonded to this carbon are referred to as alpha amino acids. In the structure shown at the top of the page, R represents a side-chain specific to each amino acid. Lysine with the carbon atoms in the side-chain labeled Usage of the term amino acid in the English language is from 1898. Glycine and leucine were also discovered around this time, in 1820. Another amino acid that was discovered in the early 19th century was cystine, in 1810, although its monomer, cysteine, was discovered much later, in 1884.

In 1806, the French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus that proved to be asparagine, the first amino acid to be discovered. The first few amino acids were discovered in the early 19th century.

6.1 Table of standard amino acid abbreviations and properties.

6 Physicochemical properties of amino acids.

3 Occurrence and functions in biochemistry.

In industry, applications include the production of biodegradable plastics, drugs, and chiral catalysts. Nine standard amino acids are called "essential" for humans because they cannot be created from other compounds by the human body, and so must be taken in as food.ĭue to their central role in biochemistry, amino acids are important in nutrition and are commonly used in nutrition supplements, fertilizers, food technology and industry. Of these, 20 are encoded by the universal genetic code. Twenty-two amino acids are naturally incorporated into polypeptides and are called proteinogenic or standard amino acids. Amino acids can be linked together in varying sequences to form a vast variety of proteins. One particularly important function is to serve as the building blocks of proteins, which are linear chains of amino acids. The various alpha-amino acids differ in which side-chain (R-group) is attached to their alpha carbon, and can vary in size from just one hydrogen atom in glycine to a large heterocyclic group in tryptophan.Īmino acids are critical to life, and have many functions in metabolism. Other types of amino acid exist when the amino group is attached to a different carbon atom for example, in gamma-amino acids (such as gamma-amino-butyric acid) the carbon atom to which the amino group attaches is separated from the carboxylate group by two other carbon atoms. They are particularly important in biochemistry, where the term usually refers to alpha-amino acids.Īn alpha-amino acid has the generic formula H 2NCHRCOOH, where R is an organic substituent the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (the α–carbon). The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen.

) are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The 21 amino acids found in eukaryotes, grouped according to their side-chains' pKas and charge at physiological pH 7.4Īmino acids ( pronounced /əˈmiːnoʊ.