List of standard amino acids

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

Chemical properties
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.

Side chain properties
Note: The pKa values of amino acids are typically slightly different when the amino acid is inside a protein. Protein pKa calculations are sometimes used to calculate the change in the pKa value of an amino acid in this situation.

Gene expression and biochemistry
* UGA is normally a stop codon, but encodes selenocysteine if a SECIS element is present.

† The stop codon is not an amino acid, but is included for completeness.

‡ An essential amino acid cannot be synthesized in humans and must, therefore, be supplied in the diet. Conditionally essential amino acids are not normally required in the diet, but must be supplied exogenously to specific populations that do not synthesize it in adequate amounts.