Amino acids


Amino acids are the monomeric constituents of proteins. The precise sequence of amino acids in a protein (i.e. the primary structure of a protein) is specified by the genetic code.

This article provides a list of the 22 amino acids found in biological proteins (i.e. the proteinogenic amino acids) as well as their one and three letter codes, their chemical formulae, their chemical properties, the mRNA codons which specify them in protein translation, and whether they are essential (i.e. required from the diet) or non-essential (i.e. synthesised by the organism de novo).

Alanine (ala, A): alanine has the chemical formula C3H7NO2 and a non-polar (hydrophobic), alipathic side chain. In the genetic code, alanine is specified by the following codons: GCU GCC GCA GCG. Alanine is a non-essential amino acid.

Arginine (arg, R): arginine has the chemical formula C6H14N4O2 and a basic (positively charged) side chain. In the genetic code, arginine is specified by the following codons: CGU CGC CGA CGG AGA AGG. Arginine is an essential amino acid.

Asparagine (asn, N): asparagine has the chemical formula C4H8N2O3 and a polar but uncharged side chain. In the genetic code, asparagine is specified by the following codons: AAU and AAC. Asparagine is a non-essential amino acid.

Aspartic acid (asp, D, also aspartate): aspartic acid has the chemical formula C4H7NO4 and an acidic (negatively charged) side chain. In the genetic code, aspartic acid is specified by the following codons: GAU GAC. Aspartic acid is a non-essential amino acid.

Cysteine (cys, C): cysteine has the chemical formula C3H7NO2S and a non-polar (hydrophobic), aliphatic side chain. In the genetic code, cysteine is specified by the following codons: UGU and UGC. Cysteine is one of two sulphur-containing amino acids, methionine being the other, however, only cysteine can form disulphide bridges. Cysteine is a non-essential amino acid.

Glutamic acid (glu, E, also glutamate): glutamic acid has the chemical formula C5H9NO4 and an acidic (negatively charged) side chain. In the genetic code, glutamic acid is specified by the following codons: GAA and GAG. Glutamic acid is a non-essential amino acid.

Glutamine (gln, Q): glutamine has the chemical formula C5H10N2O3 and a polar, uncharged side chain. In the genetic code, glutamine is specified by the following codons: CAA and CAG. Glutamine is a non-essential amino acid.

Glycine (gly, G): glycine has the chemical formula C2H5NO2 and a single H atom as its side chain, making it either hydrophilic or hydrophobic depending on its environment. In the genetic code, glycine is specified by the following codons: GGU, GGC, GGA, GGG. Glycine is a non-essential amino acid.

Histidine (his, H): histidine has the chemical formula C6H9N3O2 and a basic (positively charged) side chain. In the genetic code, histidine is specified by the following codons: CAU and CAC. Histidine is an essential amino acid in infants; after several years of development it becomes non-essential.

Isoleucine (ile, I): isoleucine has the chemical formula C6H13NO2 and a non-polar, alipathic side chain. In the genetic code, isoleucine is specified by the following codons: AUU, AUC and AUA. Isoleucine is an essential amino acid.

Leucine (leu, L): leucine has the chemical formula C6H13NO2 and a non-polar (hydrophobic), aliphatic side chain. In the genetic code, leucine is specified by the following codons: UUA, UUG, CUU, CUC, CUA, and CUG. Leucine is an essential amino acid.

Lysine (lys, K): lysine has the chemical formula C6H14N2O2and a basic (positively charged) side chain. In the genetic code, lysine is specified by the following codons: AAA, AAG. Lysine is an essential amino acid.

Methionine (met, M): methionine has the chemical formula C5H11NO2S and a non-polar (hydrophobic), alipathic side chain. In the genetic code, methionine is specified only by the start codon, AUG. This makes methionine the first amino acid found at the N-terminal end of all eukaryotic and archaean proteins (although it is often removed post-translationally). Bacterial proteins have a similar amino acid, formylmethionine (fmet), encoded by the start codon. Methionine is one of two sulphur-containing amino acids, cysteine being the other. Methionine is an essential amino acid.

Phenylalanine (phe, F): phenylalanine has the chemical formula C9H11NO2 and a non-polar (hydrophobic), aromatic side chain. In the genetic code, phenylalanine is specified by the following codons: UUU UUC. Phenylalanine is an essential amino acid.

Proline (pro, P): proline has the chemical formula C9H11NO2 and a non-polar (hydrophobic), aromatic side chain. In the genetic code, proline is specified by the following codons: CCU CCC CCA CCG. Proline is a non-essential amino acid. It is unique among the twenty proteinogenic amino acids, in that its a-amino group is secondary.

Pyrrolysine (pyl, O): pyrrolysine has the chemical formula C12H21N3O3 and is similar in structure to lysine, although a pyrroline ring is linked to the end of the lysine side chain. In the genetic code, pyrrolysine is specified by the amber stop codon, UAG.

Selenocysteine (sec, U): selenocysteine has the chemical formula C3H7NO2Se and is similar in structure to cysteine, although a selenium atom replaces the usual sulphur. In the genetic code, selenocysteine is specified by the opal stop codon, UGA.

Serine (ser, S): serine has the chemical formula C3H7NO3 and a polar but uncharged side chain. In the genetic code, serine is specified by the following codons: UCU, UCC, UCA, UCG, AGU and AGC. Serine is a non-essential amino acid.

Threonine (thr, T): threonine has the chemical formula C4H9NO3 and a polar but uncharged side chain. In the genetic code, threonine is specified by the following codons: ACU, ACA, ACC, and ACG. Threonine is an essential amino acid.

Tryptophan (trp, W): tryptophan has the chemical formula C11H12N2O2 and a non-polar (hydrophobic), aromatic side chain. In the genetic code, tryptophan is specified only by the UGG codon. Tryptophan is an essential amino acid

Tyrosine (tyr, Y): tyrosine has the chemical formula C9H11NO3 and a polar but uncharged side chain. In the genetic code, tyrosine is specified by the following codons: UAU and UAC. Tyrosine is generally a non-essential amino acid; however, it can only be synthesised from the essential amino acid, phenylalanine, and thus is essential in a person who has a genetic impairment that prevents them from metabolising phenylalanine.

Valine (val, V): valine has the chemical formula C5H11NO2 and a non-polar, alipathic side chain. In the genetic code, valine is specified by the following codons: GUU, GUC, GUA, and GUG. Valine is an essential amino acid