In the previous article on the amino acid, we have studied what is an amino acid and its classification based on polarity. We have also studied what is a peptide bond? How a peptide bond is formed? We know that there are twenty amino acids that help in protein formation and the first discovered amino acid was asparagine (1806), and the last found amino acid was threonine (1938). The general structure of an amino acid contains a carboxyl group, amino group, and R side chain.


Amino acids that present in cells but don’t participate in peptide synthesis known as non-standard amino acids. Some of the non-standard amino acids are constituents of peptides, but they are formed by the modification of standard amino acid known as post-translational modification.

Examples of Non-standard amino acids:  

  • A derivative of proline (4-hydroxyproline, PubChem CID:825), and derivative of lysine (5-hydroxylysine, PubChem CID:550919). These are found in proteins of the plant cell wall and found in collagen, which is a fibrous protein of connective tissue.
  • Hydroxyproline was 1st isolated by Emil Fischer (1902), from hydrolyzed gelatin. Hydroxylysine was 1st discovered by Donald Van Slyke (1921).
  • A constituent of Myosin, 6-N-methyllysine which is found in contractile proteins of muscle.
  • Γ carboxyglutamate (PubChem CID: 40772), founded in Prothrombin which is a blood-clotting protein.
  • A derivative of four Lysine’s, Desmosine (PubChem CID: 25435) is a very complex nonstandard amino acid, and it is founded in elastin (fibrous protein).

There are 300 supplementary amino acids that have been observed in the cells, having a variety of roles but these are not the constituents of proteins.


  1. Kim, S., Thiessen, P. A., Bolton, E. E., Chen, J., Fu, G., Gindulyte, A., … & Wang, J. (2016). PubChem substance and compound databases. Nucleic acids research, 44(D1), D1202-D1213.
  2. Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2005). Lehninger principles of biochemistry. Macmillan.


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