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 amino acids or proteinogens. Although more than 500 types of amino acids are known in nature, there are only twenty that we call ordinary. Along with these, there are some more rare, called occasional ones, 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 fall mainly into the constitution of enzymes and hormones of protein nature, or rather peptide (remember that to be considered such, a protein must consist of more than 100 amino acids, while to form an oligo and one polypeptide is sufficient from 2 to 9 and from 10 to 100).

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 inside the mitochondria, where they are used to produce energy);

ornithine, citrulline and homocysteine (participating in the metabolism of amino acids - urea cycle);

hydroxyproline and hydroxylisine (part of the composition of collagen and some proteins);

sarcosine (or methylglycine).

Some amino acids may also have commercial or pharmacological importance.

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

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

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, isucine, lysine, methionine, threonine, tryptophan, valine, and, in children, histidine and arginine.

Ordinary amino acids, chemical characteristics

On the basis of the chemical characteristics of the radical R (group of atoms that forms a not very long chain), the 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 chain containing a sulfur atom: methionine

Amino acids with polar R:

with OH group: serine, threonine, tyrosine

with SH group: cysteine

with CO-NH _{2 group} : asparagine, glutamine

with H: glycine (the only non-chiral amino acid)

Amino acids with basic R:

lysine, arginine, histidine

Amino acids with R acid: