Amino Acid Definition and Examples

How to recognize an amino acid

Arginine, like other amino acids, is characterized by having an amino end and a carboxyl end.
Arginine, like other amino acids, is characterized by having an amino end and a carboxyl end. Martin McCarthy / Getty Images

Amino acids are important in biology, biochemistry, and medicine. Learn about the chemical composition of the amino acids, their functions, abbreviations, and properties:

Key Takeaways: Amino Acids

  • An amino acid is an organic compound characterized by having a carboxyl group, amino group, and side chain attached to a central carbon atom.
  • Amino acids are used as precursors for other molecules in the body. Linking amino acids forms polypeptides. Polypeptides may be modified and combined to form proteins.
  • The genetic code is basically a code for proteins made within cells. DNA is translated into RNA. Three bases (combinations of adenine, uracil, guanine, and cytosine) code for an amino acid. There is more than one code for most amino acids.
  • Amino acids are made in the ribosomes of eukaryotic cells.
  • Some amino acids may not be made by an organism. These "essential" amino acids must be present in the organism's diet.
  • In addition to making amino acids from the genetic code and obtaining them from the diet, other metabolic processes convert molecules into amino acids.

Amino Acid Definition

An amino acid is a type of organic acid that contains a carboxyl functional group (-COOH) and an amine functional group (-NH2) as well as a side chain (designated as R) that is specific to the individual amino acid. Amino acids are considered to be the building blocks of polypeptides and proteins. The elements found in all amino acids are carbon, hydrogen, oxygen, and nitrogen. Amino acids may contain other elements on their side chains.

Shorthand notation for amino acids may be either a three-letter abbreviation or a single letter. For example, valine may be indicated by V or val; histidine is H or his.

Amino acids may function on their own, but more commonly act as monomers to form larger molecules. Linking a few amino acids forms peptides. A chain of many amino acids is called a polypeptide. Polypeptides may become proteins.

The process of producing proteins based on an RNA template is called translation. Translation occurs in ribosomes of cells. There are 22 amino acids involved in protein production. These amino acids are considered to be proteinogenic. In addition to the proteinogenic amino acids, there are some amino acids that are not found in any protein. An example is the neurotransmitter gamma-aminobutyric acid. Typically, nonproteinogenic amino acids function in amino acid metabolism.

The translation of the genetic code involves 20 amino acids, which are called canonical amino acids or standard amino acids. For each amino acid, a series of three mRNA residues acts as a codon during translation (the genetic code). The other two amino acids found in proteins are pyrrolysine and selenocysteine. These two amino acids are specially coded, usually by an mRNA codon that otherwise functions as a stop codon.

Common Misspellings: ammino acid

Examples: lysine, glycine, tryptophan

Functions of Amino Acids

Because they are used to build proteins, most of the human body consists of amino acids. Their abundance is second only to water. Amino acids are used to build a variety of molecules and are used in neurotransmitter and lipid transport.

Amino Acid Chirality

Amino acids are capable of chirality, where the functional groups may be on either side of a C-C bond. In the natural world, most amino acids are the L-isomers. There are a few instances of D-isomers. An example is the polypeptide gramicidin, which consists of a mixture of D- and L-isomers.

One and Three Letter Abbreviations

The amino acids most commonly memorized and encountered in biochemistry are:

  • Glycine, Gly, G
  • Valine, Val, V
  • Leucine, Leu, L
  • Isoeucine, Leu, L
  • Proline, Pro, P
  • Threonine, Thr, T
  • Cysteine, Cys, C 
  • Methionine, Met, M
  • Phenylalanine, Phe, F
  • Tyrosine, Tyr, Y 
  • Tryptophan, Trp, W 
  • Arginine, Arg, R
  • Aspartate, Asp, D
  • Glutamate, Glu, E
  • Aparagine, Asn, N
  • Glutamine, Gln, Q
  • Aparagine, Asn, N

Properties of the Amino Acids

The characteristics of the amino acids depend on the composition of their R side chain. Using the single-letter abbreviations:

  • Polar or Hydrophilic: N, Q, S, T, K, R, H, D, E
  • Non-Polar or Hydrophobic: A, V, L, I, P, Y, F, M, C
  • Contain Sulfur: C, M
  • Hydrogen Bonding: C, W, N, Q, S, T, Y, K, R, H, D, E
  • Ionizable: D, E, H, C, Y, K, R
  • Cyclic: P
  • Aromatic: F, W, Y (H also, but doesn't display much UV absorption)
  • Aliphatic: G, A, V, L, I, P
  • Forms a Disulfide Bond: C
  • Acidic (Positively Charged at Neutral pH): D, E
  • Basic (Negatively Charged at Neutral pH): K, R