Ordinary and occasional amino acids

Ordinary amino acids and protein synthesis

The amino acids that appear most frequently in the protein structures of living organisms are called ordinary or proteinogenic amino acids. Although over 500 types of amino acids are known in nature, there are only twenty that we call ordinary. Along with them, there are some rarer ones, called occasional, which generally derive from the metabolism of ordinary ones after their incorporation into the protein chain. In collagen, for example, lysine and proline, two ordinary amino acids, are found in the hydroxylated form (hydroxyproline and hydroxylisine).

Most of the other non-ordinary amino acids mainly fall within the constitution of enzymes and hormones of a protein nature, or rather peptide (remember that to be considered such, a protein must be made up of more than 100 amino acids, while to form an oligo and a a polypeptide 2 to 9 and 10 to 100 are sufficient).


Among the 20 ordinary amino acids are remembered

Alanine, Arginine, Asparagine, Aspartic Acid, Cysteine, Glycine, Glutamic Acid, Glutamine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Proline, Serine, Threonine, Tryptophan, Tyrosine, Valine

Examples of non-ordinary amino acids are:

carnitine (involved in the transport of long-chain fatty acids into the mitochondrion, where they are used for energy);
ornithine, citrulline and homocysteine ​​(participate in the metabolism of amino acids - urea cycle);
hydroxyproline and hydroxylisine (they are part of the composition of collagen and some proteins);
sarcosine (or methylglycine).

Some amino acids may also be of commercial or pharmacological importance.

  • Sodium glutamate is used in the food industry as a flavor enhancer (see bouillon cube).
  • L-dihydroxyphenylalanine (L-DOPA) is a drug used for the treatment of Parkinson's disease.
  • 5-hydroxytryptophan (5-HTP) has been used to treat neurological symptoms associated with phenylketonuria (an inherited metabolic disease that prevents the use of phenylalanine, an essential amino acid, due to particular enzymatic deficiencies; the accumulation of phenylalanine in the tissues is responsible for severe skin and neurological damage).

Plants and bacteria are capable of producing particular amino acids, which can be found in peptide antibiotics, such as nisin and alamethicin.

Essential amino acids

Some of the 20 ordinary amino acids are called essential, as they cannot be synthesized by the body from other compounds, but must be taken with food.
For humans, these are phenylalanine, leucine, isoleucine, lysine, methionine, threonine, tryptophan, valine, and, in children, histidine and arginine.

Ordinary amino acids, chemical characteristics

Ordinary and occasional amino acids


Based on the chemical characteristics of the R radical (group of atoms forming a not very long chain), amino acids can be divided into various categories.


Amino acids with apolar R (not able to form hydrogen bonds):

with aliphatic chain: alanine, leucine, isoleucine, valine, proline
with aromatic chain: phenylalanine, tryptophan
with a chain containing a sulfur atom: methionine

Amino acids with polar R:

with OH group: serine, threonine, tyrosine
with SH group: cysteine
with CO-NH2 group: asparagine, glutamine
with H: glycine (the only non-chiral amino acid)

Amino acids with basic R:

lisina, arginina, istidina

Amino acids with acid R:

glutamic acid, aspartic acid

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