Uncommon Amino Acids & Their Functions

Uncommon amino acids are those that are not commonly found in the standard set of 20 amino acids used in protein synthesis.

While the vast majority of proteins are composed of the 20 standard amino acids, there are several uncommon amino acids that occur naturally in certain organisms or are post-translationally modified.

Here are a few examples of uncommon amino acids:

Selenocysteine (Sec)

Selenocysteine is an amino acid that contains selenium instead of sulfur in its side chain. It is incorporated into proteins via a special recording mechanism called “selenocysteine insertion sequence” (SECIS).

Selenocysteine is found in a small number of proteins and has important roles in antioxidant defense and thyroid hormone metabolism.

Pyrrolysine (Pyl)

Pyrrolysine is a rare amino acid that contains a pyrroline ring. It is found in certain archaea and bacteria and is incorporated into proteins through a specific genetic codon and a unique tRNA molecule. Pyrrolysine is involved in the catalytic activity of enzymes called methyltransferases.


It is a modified form of the amino acid lysine. Hydroxylysine contains a hydroxyl group (-OH) attached to the side chain of lysine.

Hydroxylysine plays a crucial role in the post-translational modification of collagen, a major structural protein in the body. It contributes to the stability and strength of collagen fibers.

4-Hydroxyproline (Hyp)

Hydroxyproline is a modified form of the amino acid proline. It is produced by post-translational hydroxylation of proline residues in collagen proteins.

Hydroxyproline contributes to the stability and structure of collagen and is essential for connective tissue function.

Gamma-aminobutyric acid (GABA)

While GABA is primarily known as a neurotransmitter, it can also act as an amino acid. It is synthesized from glutamate through decarboxylation and plays a role in the regulation of neuronal excitability.

Source- Lehninger Biochemistry


Desmosine is formed through a series of enzymatic reactions involving lysine residues in the elastin protein. It derives from four lysine residues.

It is a unique component of elastin, a protein found in connective tissues that provides elasticity and resilience.


Ornithine is an amino acid involved in the urea cycle, which is responsible for the detoxification of ammonia in the body. Ornithine acts as an intermediate molecule, linking the reactions that occur in the mitochondria and cytoplasm.

During the urea cycle, ornithine combines with carbamoyl phosphate to form citrulline. Citrulline is then transported out of the mitochondria into the cytoplasm, where it continues to participate in the urea cycle.

Source- Lehninger Biochemistry


It is classified as a non-proteinogenic amino acid, meaning it is not directly incorporated into proteins during translation.

Citrulline gets its name from watermelon (Citrullus vulgaris), as it was first isolated from this fruit.

Citrulline involves in the urea cycle in the liver. Citrulline is produced in the urea cycle when ornithine reacts with carbamoyl phosphate. It is subsequently converted to arginine in the urea cycle, which plays a critical role in the detoxification of ammonia.

They are ornithine, citrulline (both are involved in the ornithine cycle to synthesize urea), and diaminopimelic acid.

γ-Carboxyglutamate (Gla or γ-Glu)

It is a modified form of the amino acid glutamate. It is produced through a post-translational modification known as carboxylation

γ-Carboxyglutamate plays a vital role in the structure and function of blood-clotting proteins, such as prothrombin, factors VII, IX, X, and proteins C, S, and Z.


Methyllysine is a modified form of the amino acid lysine. It is a constituent of the muscle protein Myosine.

About Uncommon amino acids

Some 300 additional uncommon amino acids have been found in cells.

These are just a few examples of uncommon amino acids. There are other rare amino acids that can be found in specific organisms or are chemically modified during or after protein synthesis.

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