Amino acid

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The generic structure of an alpha amino acid in its un-ionized form
Table of Amino Acids.
The 21 amino acids found in eukaryotes, grouped according to their side-chains' pKa values and charges carried at physiological pH 7.4

Amino acids (/əˈmn/, /əˈmn/, or /ˈæmɪn/) are biologically important organic compounds composed of amine (-NH2) and carboxylic acid (-COOH) functional groups, along with a side-chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen, though other elements are found in the side-chains of certain amino acids. About 500 amino acids are known[1] and can be classified in many ways. They can be classified according to the core structural functional groups' locations as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, pH level, and side-chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.). In the form of proteins, amino acids comprise the second-largest component (water is the largest) of human muscles, cells and other tissues.[2] Outside proteins, amino acids perform critical roles in processes such as neurotransmitter transport and biosynthesis.

Amino acids having both the amine and the carboxylic acid groups attached to the first (alpha-) carbon atom have particular importance in biochemistry. They are known as 2-, alpha-, or α-amino acids (generic formula H2NCHRCOOH in most cases[3] where R is an organic substituent known as a "side-chain");[4] often the term "amino acid" is used to refer specifically to these. They include the 23 proteinogenic ("protein-building") amino acids,[5][6][7] which combine into peptide chains ("polypeptides") to form the building-blocks of a vast array of proteins.[8] These are all L-stereoisomers ("left-handed" isomers), although a few D-amino acids ("right-handed") occur in bacterial envelopes and some antibiotics.[9] Twenty of the proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as "standard" amino acids. The other two ("non-standard" or "non-canonical") are pyrrolysine (found in methanogenic organisms and other eukaryotes) and selenocysteine (present in many noneukaryotes as well as most eukaryotes). For example, 25 human proteins include selenocysteine (Sec) in their primary structure,[10] and the structurally characterized enzymes (selenoenzymes) employ Sec as the catalytic moiety in their active sites.[11] Pyrrolysine and selenocysteine are encoded via variant codons; for example, selenocysteine is encoded by stop codon and SECIS element.[12][13][14] Codon–tRNA combinations not found in nature can also be used to "expand" the genetic code and create novel proteins known as alloproteins incorporating non-proteinogenic amino acids.[15][16][17]

Many important proteinogenic and non-proteinogenic amino acids also play critical non-protein roles within the body. For example, in the human brain, glutamate (standard glutamic acid) and gamma-amino-butyric acid ("GABA", non-standard gamma-amino acid) are, respectively, the main excitatory and inhibitory neurotransmitters;[18] hydroxyproline (a major component of the connective tissue collagen) is synthesised from proline; the standard amino acid glycine is used to synthesise porphyrins used in red blood cells; and the non-standard carnitine is used in lipid transport.

Nine proteinogenic 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. Others may be conditionally essential for certain ages or medical conditions. Essential amino acids may also differ between species.[19]



The first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus that was subsequently named asparagine, the first amino acid to be discovered.[20][21] Cystine was discovered in 1810,[22] although its monomer, cysteine, remained undiscovered until 1884.[21][23] Glycine and leucine were discovered in 1820.[24] Usage of the term amino acid in the English language is from 1898.[25] Proteins were found to yield amino acids after enzymatic digestion or acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister proposed that proteins are the result of the formation of bonds between the amino group of one amino acid with the carboxyl group of another, in a linear structure that Fischer termed peptide.[26]

General structure[edit]

Further information: Proteinogenic amino acid
Lysine with the carbon atoms in the side-chain labeled



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. 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.[27] In chemical terms, proline is, therefore, an imino acid, since it lacks a primary amino group,[29] although it is still classed as an amino acid in the current biochemical nomenclature,[30] and may also be called an "N-alkylated alpha-amino acid".[31]

Animation of two mirror image molecules rotating around a central axis.
The two enantiomers of alanine, D-Alanine and L-Alanine


Of the standard α-amino acids, all but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other (see also Chirality). While L-amino acids represent all of the amino acids found in proteins during translation in the ribosome, D-amino acids are found in some proteins produced by enzyme posttranslational modifications after translation and translocation to the endoplasmic reticulum, as in exotic sea-dwelling organisms such as cone snails.[32] They are also abundant components of the peptidoglycan cell walls of bacteria,[33] and D-serine may act as a neurotransmitter in the brain.[34] D-amino acids are used in racemic crystallography to create centrosymmetric crystals, which (depending on the protein) may allow for easier and more robust protein structure determination.[35] The L and D convention for amino acid configuration refers not to the optical activity of the amino acid itself but rather to the optical activity of the isomer of glyceraldehyde from which that amino acid can, in theory, be synthesized (D-glyceraldehyde is dextrorotatory; L-glyceraldehyde is levorotatory). In alternative fashion, the (S) and (R) designators are used to indicate the absolute stereochemistry. Almost all of the amino acids in proteins are (S) at the α carbon, with cysteine being (R) and glycine non-chiral.[36] Cysteine is unusual since it has a sulfur atom at the second position in its side-chain, which has a larger atomic mass than the groups attached to the first carbon, which is attached to the α-carbon in the other standard amino acids, thus the (R) instead of (S).




Isoelectric point[edit]


Composite of Titration Curves Grouped by Side Chain Category using applet


Occurrence and functions in biochemistry[edit]

A polypeptide is an unbranched chain of amino acids.

Essential amino acids[edit]

Main article: Essential amino acids


Twenty-two amino acids are naturally incorporated into polypeptides and are called proteinogenic or natural amino acids.[27] Of these, 20 are encoded by the universal genetic code. The remaining 2, selenocysteine and pyrrolysine, are incorporated into proteins by unique synthetic mechanisms. Selenocysteine is incorporated when the mRNA being translated includes a SECIS element, which causes the UGA codon to encode selenocysteine instead of a stop codon.[40] Pyrrolysine is used by some methanogenic archaea in enzymes that they use to produce methane. It is coded for with the codon UAG, which is normally a stop codon in other organisms.[41] This UAG codon is followed by a PYLIS downstream sequence.[42]

The amino acid selenocysteine

Non-standard amino acids[edit]

Aside from the 23 proteinogenic amino acids, there are many other amino acids that are called non-proteinogenic or non-standard (although three of the proteinogenic amino acids are also called non-standard, not being encoded simply by the universal genetic code but requiring additional contextual information). Those either are not found in proteins (for example carnitine, GABA) or are not produced directly and in isolation by standard cellular machinery (for example, hydroxyproline and selenomethionine).

Non-proteinogenic amino acids that are found in proteins are formed by post-translational modification, which is modification after translation during protein synthesis. These modifications are often essential for the function or regulation of a protein; for example, the carboxylation of glutamate allows for better binding of calcium cations,[43] and the hydroxylation of proline is critical for maintaining connective tissues.[44] Another example is the formation of hypusine in the translation initiation factor EIF5A, through modification of a lysine residue.[45] Such modifications can also determine the localization of the protein, e.g., the addition of long hydrophobic groups can cause a protein to bind to a phospholipid membrane.[46]

β-alanine and its α-alanine isomer


In human nutrition[edit]

When taken up into the human body from the diet, the 22 standard amino acids either are used to synthesize proteins and other biomolecules or are oxidized to urea and carbon dioxide as a source of energy.[49] The oxidation pathway starts with the removal of the amino group by a transaminase; the amino group is then fed into the urea cycle. The other product of transamidation is a keto acid that enters the citric acid cycle.[50] Glucogenic amino acids can also be converted into glucose, through gluconeogenesis.[51]

Pyrrolysine trait is restricted to several microbes, and only one organism has both Pyl and Sec. Of the 22 standard amino acids, 9 are called essential amino acids because the human body cannot synthesize them from other compounds at the level needed for normal growth, so they must be obtained from food.[52] In addition, cysteine, taurine, tyrosine, and arginine are considered semiessential amino-acids in children (though taurine is not technically an amino acid), because the metabolic pathways that synthesize these amino acids are not fully developed.[53][54] The amounts required also depend on the age and health of the individual, so it is hard to make general statements about the dietary requirement for some amino acids.

Essential Nonessential
Histidine Alanine
Isoleucine Arginine*
Leucine Asparagine
Lysine Aspartic acid
Methionine Cysteine*
Phenylalanine Glutamic acid
Threonine Glutamine*
Tryptophan Glycine
Valine Ornithine*

(*) Essential only in certain cases.[55][56]


Although there are many ways to classify amino acids, these molecules can be assorted into six main groups, on the basis of their structure and the general chemical characteristics of their R groups.

Class Name of the amino acids
Aliphatic Glycine, Alanine, Valine, Leucine, Isoleucine
Hydroxyl or Sulfur/Selenium-containing Serine, Cysteine, Selenocysteine, Threonine, Methionine
Cyclic Proline
Aromatic Phenylalanine, Tyrosine, Tryptophan
Basic Histidine, Lysine, Arginine
Acidic and their Amide Aspartate, Glutamate, Asparagine, Glutamine

Non-protein functions[edit]

Further information: Amino acid neurotransmitter



Some non-standard amino acids are used as defenses against herbivores in plants.[62] For example, canavanine is an analogue of arginine that is found in many legumes,[63] and in particularly large amounts in Canavalia gladiata (sword bean).[64] This amino acid protects the plants from predators such as insects and can cause illness in people if some types of legumes are eaten without processing.[65] The non-protein amino acid mimosine is found in other species of legume, in particular Leucaena leucocephala.[66] This compound is an analogue of tyrosine and can poison animals that graze on these plants.

Uses in industry[edit]


The food industry is also a major consumer of amino acids, in particular, glutamic acid, which is used as a flavor enhancer,[69] and Aspartame (aspartyl-phenylalanine-1-methyl ester) as a low-calorie artificial sweetener.[70] Similar technology to that used for animal nutrition is employed in the human nutrition industry to alleviate symptoms of mineral deficiencies, such as anemia, by improving mineral absorption and reducing negative side effects from inorganic mineral supplementation.[71]

The chelating ability of amino acids has been used in fertilizers for agriculture to facilitate the delivery of minerals to plants in order to correct mineral deficiencies, such as iron chlorosis. These fertilizers are also used to prevent deficiencies from occurring and improving the overall health of the plants.[72] The remaining production of amino acids is used in the synthesis of drugs and cosmetics.[67]

Amino acid derivative Pharmaceutical application
5-HTP (5-hydroxytryptophan) Experimental treatment for depression.[73]
L-DOPA (L-dihydroxyphenylalanine) Treatment for Parkinsonism.[74]
Eflornithine Drug that inhibits ornithine decarboxylase and is used in the treatment of sleeping sickness.[75]

Expanded genetic code[edit]

Main article: Expanded genetic code



Main article: Nullomers

Nullomers are codons that in theory code for an amino acid, however in nature there is a selective bias against using this codon in favor of another, for example bacteria prefer to use CGA instead of AGA to code for arginine.[76] This creates some sequences that do not appear in the genome. This characteristic can be taken advantage of and used to create new selective cancer-fighting drugs[77] and to prevent cross-contamination of DNA samples from crime-scene investigations.[78]

Chemical building blocks[edit]

Further information: Asymmetric synthesis



Biodegradable plastics[edit]

Further information: Biodegradable plastic and Biopolymer

Amino acids are under development as components of a range of biodegradable polymers. These materials have applications as environmentally friendly packaging and in medicine in drug delivery and the construction of prosthetic implants. These polymers include polypeptides, polyamides, polyesters, polysulfides, and polyurethanes with amino acids either forming part of their main chains or bonded as side-chains. These modifications alter the physical properties and reactivities of the polymers.[81] An interesting example of such materials is polyaspartate, a water-soluble biodegradable polymer that may have applications in disposable diapers and agriculture.[82] Due to its solubility and ability to chelate metal ions, polyaspartate is also being used as a biodegradeable anti-scaling agent and a corrosion inhibitor.[83][84] In addition, the aromatic amino acid tyrosine is being developed as a possible replacement for toxic phenols such as bisphenol A in the manufacture of polycarbonates.[85]


As amino acids have both a primary amine group and a primary carboxyl group, these chemicals can undergo most of the reactions associated with these functional groups. These include nucleophilic addition, amide bond formation, and imine formation for the amine group, and esterification, amide bond formation, and decarboxylation for the carboxylic acid group.[86] The combination of these functional groups allow amino acids to be effective polydentate ligands for metal-amino acid chelates.[87] The multiple side-chains of amino acids can also undergo chemical reactions.[88] The types of these reactions are determined by the groups on these side-chains and are, therefore, different between the various types of amino acid.

For the steps in the reaction, see the text.
The Strecker amino acid synthesis

Chemical synthesis[edit]

Main article: Peptide synthesis

Several methods exist to synthesize amino acids. One of the oldest methods begins with the bromination at the α-carbon of a carboxylic acid. Nucleophilic substitution with ammonia then converts the alkyl bromide to the amino acid.[89] In alternative fashion, the Strecker amino acid synthesis involves the treatment of an aldehyde with potassium cyanide and ammonia, this produces an α-amino nitrile as an intermediate. Hydrolysis of the nitrile in acid then yields a α-amino acid.[90] Using ammonia or ammonium salts in this reaction gives unsubstituted amino acids, whereas substituting primary and secondary amines will yield substituted amino acids.[91] Likewise, using ketones, instead of aldehydes, gives α,α-disubstituted amino acids.[92] The classical synthesis gives racemic mixtures of α-amino acids as products, but several alternative procedures using asymmetric auxiliaries[93] or asymmetric catalysts[94][95] have been developed.[96]


Peptide bond formation[edit]

For more details on this topic, see Peptide bond.






Nonstandard amino acids are usually formed through modifications to standard amino acids. For example, homocysteine is formed through the transsulfuration pathway or by the demethylation of methionine via the intermediate metabolite S-adenosyl methionine,[104] while hydroxyproline is made by a posttranslational modification of proline.[105]

Microorganisms and plants can synthesize many uncommon amino acids. For example, some microbes make 2-aminoisobutyric acid and lanthionine, which is a sulfide-bridged derivative of alanine. Both of these amino acids are found in peptidic lantibiotics such as alamethicin.[106] However, in plants, 1-aminocyclopropane-1-carboxylic acid is a small disubstituted cyclic amino acid that is a key intermediate in the production of the plant hormone ethylene.[107]


* Amino acids catabolized into both glucogenic and ketogenic products.


Physicochemical properties of amino acids[edit]



Many proteins undergo a range of posttranslational modifications, when additional chemical groups are attached to the amino acids in proteins. Some modifications can produce hydrophobic lipoproteins,[111] or hydrophilic glycoproteins.[112] These type of modification allow the reversible targeting of a protein to a membrane. For example, the addition and removal of the fatty acid palmitic acid to cysteine residues in some signaling proteins causes the proteins to attach and then detach from cell membranes.[113]

Table of standard amino acid abbreviations and properties[edit]

Amino Acid 3-Letter[114] 1-Letter[114] Side-chain polarity[114] Side-chain charge (pH 7.4)[114] Hydropathy index[115] Absorbance λmax(nm)[116] ε at λmax (x10−3 M−1 cm−1)[116]
Alanine Ala A nonpolar neutral 1.8
Arginine Arg R Basic polar positive −4.5
Asparagine Asn N polar neutral −3.5
Aspartic acid Asp D acidic polar negative −3.5
Cysteine Cys C nonpolar neutral 2.5 250 0.3
Glutamic acid Glu E acidic polar negative −3.5
Glutamine Gln Q polar neutral −3.5
Glycine Gly G nonpolar neutral −0.4
Histidine His H Basic polar positive(10%)


−3.2 211 5.9
Isoleucine Ile I nonpolar neutral 4.5
Leucine Leu L nonpolar neutral 3.8
Lysine Lys K Basic polar positive −3.9
Methionine Met M nonpolar neutral 1.9
Proline Pro P nonpolar neutral −1.6
Serine Ser S polar neutral −0.8
Threonine Thr T polar neutral −0.7
Valine Val V nonpolar neutral 4.2


21st and 22nd amino acids 3-Letter 1-Letter
Selenocysteine Sec U
Pyrrolysine Pyl O


Ambiguous Amino Acids 3-Letter 1-Letter
Asparagine or aspartic acid Asx B
Glutamine or glutamic acid Glx Z
Leucine or Isoleucine Xle J
Unspecified or unknown amino acid Xaa X



See also[edit]

References and notes[edit]

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Further reading[edit]

External links[edit]