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 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