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List of standard amino acids

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This list of standard proteinogenic amino acids details the chemical structures and properties of the twenty standard amino acids used in proteins by living cells.

Structures Edit

Structures and symbols of the 20 amino acids which are directly encoded for protein synthesis by the standard genetic code.

Chemical properties Edit

Following is a table listing the one letter symbols, the three-letter symbols, and the chemical properties of the side chains of the standard amino acids. The masses listed are based on weighted averages of the elemental isotopes at their natural abundances. Note that forming a peptide bond results in elimination of a molecule of water, so the mass of an amino acid unit within a protein chain is reduced by 18.01524 Da. The one-letter symbol for an undetermined amino acid is X. The three-letter symbol Asx or one-letter symbol B means the amino acid is either asparagine or aspartic acid; Glx or Z means either glutamic acid or glutamine; and Xle or J means either leucine or isoleucine. IUPAC/IUBMB now also recommends that Sec or U refers to selenocysteine, and Pyl or O refers to pyrrolysine.

General chemical propertiesEdit

Amino Acid Short Abbrev. Mass (Da) pI pK1
Alanine A Ala 89.09404 6.01 2.35 9.87
Cysteine C Cys 121.15404 5.05 1.92 10.70
Aspartic acid D Asp 133.10384 2.85 1.99 9.90
Glutamic acid E Glu 147.13074 3.15 2.10 9.47
Phenylalanine F Phe 165.19184 5.49 2.20 9.31
Glycine G Gly 75.06714 6.06 2.35 9.78
Histidine H His 155.15634 7.60 1.80 9.33
Isoleucine I Ile 131.17464 6.05 2.32 9.76
Lysine K Lys 146.18934 9.60 2.16 9.06
Leucine L Leu 131.17464 6.01 2.33 9.74
Methionine M Met 149.20784 5.74 2.13 9.28
Asparagine N Asn 132.11904 5.41 2.14 8.72
Proline P Pro 115.13194 6.30 1.95 10.64
Glutamine Q Gln 146.14594 5.65 2.17 9.13
Arginine R Arg 174.20274 10.76 1.82 8.99
Serine S Ser 105.09344 5.68 2.19 9.21
Threonine T Thr 119.12034 5.60 2.09 9.10
Selenocysteine U Sec 169.06
Valine V Val 117.14784 6.00 2.39 9.74
Tryptophan W Trp 204.22844 5.89 2.46 9.41
Tyrosine Y Tyr 181.19124 5.64 2.20 9.21

Side chain propertiesEdit

Amino Acid Short Abbrev. Side chain Hydro-
pKa Polar Charged Small Tiny Aromatic
or Aliphatic
van der Waals
Cysteine C Cys -CH2SH X 8.18 - acidic X - - 86
Glycine G Gly -H - - - - X X - 48
Glutamine Q Gln -CH2CH2CONH2 - - X - - - - 114
Arginine R Arg -(CH2)3NH-C(NH)NH2 - 12.48 X basic - - - 148
Serine S Ser -CH2OH - - X - X X - 73
Threonine T Thr -CH(OH)CH3 - - X weak acidic X - - 93
Selenocysteine U Sec -CH2SeH X 5.73 - - X - -
Tryptophan W Trp -CH2C8H6N X - - - - - Aromatic 163
Tyrosine Y Tyr -CH2-C6H4OH X 10.46 X - - - Aromatic 141


Gene expression and biochemistryEdit

Amino Acid Short Abbrev. Codon(s) Occurrence
in proteins
Essential in humans
Alanine A Ala GCU, GCC, GCA, GCG 7.8 -
Cysteine C Cys UGU, UGC 1.9 -
Aspartic acid D Asp GAU, GAC 5.3 -
Glutamate E Glu GAA, GAG 6.3 -
Phenylalanine F Phe UUU, UUC 3.9 X
Glycine G Gly GGU, GGC, GGA, GGG 7.2 -
Histidine H His CAU, CAC 2.3 -
Isoleucine I Ile AUU, AUC, AUA 5.3 X
Lysine K Lys AAA, AAG 5.9 X
Leucine L Leu UUA, UUG, CUU, CUC, CUA, CUG 9.1 X
Methionine M Met AUG 2.3 X
Asparagine N Asn AAU, AAC 4.3 -
Proline P Pro CCU, CCC, CCA, CCG 5.2 -
Glutamine Q Gln CAA, CAG 4.2 -
Arginine R Arg CGU, CGC, CGA, CGG, AGA, AGG 5.1 -
Serine S Ser UCU, UCC, UCA, UCG, AGU, AGC 6.8 -
Threonine T Thr ACU, ACC, ACA, ACG 5.9 X
Selenocysteine U Sec UGA -
Valine V Val GUU, GUC, GUA, GUG 6.6 X
Tryptophan W Trp UGG 1.4 X
Tyrosine Y Tyr UAU, UAC 3.2 -
Stop codon - Term UAA, UAG, UGA - -

Note: The Stop Codon is not an amino acid, but is included for completeness.


Amino Acid Abbrev. Remarks
Alanine A Ala Very abundant, very versatile. More stiff than glycine, but small enough to pose only small steric limits for the protein conformation. It behaves fairly neutrally, can be located in both hydrophilic regions on the protein outside and the hydrophobic areas inside.
Cysteine C Cys The sulfur atom binds readily to heavy metal ions. Under oxidizing conditions, two cysteines can join together in a disulfide bond to form the amino acid cystine. When cystines are part of a protein, insulin for example, this stabilises tertiary structure and makes the protein more resistant to denaturation; disulphide bridges are therefore common in proteins that have to function in harsh environments including digestive enzymes (e.g., pepsin and chymotrypsin) and structural proteins (e.g., keratin). Disulphides are also found in peptides too small to hold a stable shape on their own (eg. insulin).
Glutamate E Glu Behaves similar to aspartic acid. Has longer, slightly more flexible side chain.
Phenylalanine F Phe Essential for humans. Phenylalanine, tyrosine, and tryptophan contain large rigid aromatic group on the side chain. These are the biggest amino acids. Like isoleucine, leucine and valine, these are hydrophobic and tend to orient towards the interior of the folded protein molecule.
Arginine R Arg Functionally similar to lysine.
Threonine T Thr Essential for humans. Behaves similarly to serine.
Tyrosine Y Tyr Behaves similarly to phenylalanine and tryptophan (see phenylalanine). Precursor of melanin, epinephrine, and thyroid hormones.


  • Nelson, David L.; Cox, Michael M. (2000). Lehninger Principles of Biochemistry, 3rd ed, Worth Publishers.
  • Kyte, J., Doolittle, R. F. (1982). A simple method for displaying the hydropathic character of a protein. J. Mol. Biol. 157 (1): 105-132. PMID 7108955.

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