Non-Standard Amino Acids

Non-Standard Amino Acids Definition

Non-standard amino acids are amino acids that are considered to be chemically modified after they are incorporated into a protein; this is known as post-translational modification. This type of amino acid is known to be found in various living organisms but does not occur in all proteins. They are not found in the genetic code of any organism. This type of amino acid can get created naturally as well as artificially in the laboratory. They are known to be present in meteorites and are capable of getting used in prebiotic experiments.

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Standard and Non-Standard Amino Acids

Amino acids are considered to be molecules that are characterized by the presence of an amino group and a carboxylic group in the molecular structure of the compounds. Hence, these molecules are termed amino acids. The main function of an amino acid is to act like a manometer for the purpose of the synthesis of proteins and peptides. From all the amino acids that exist naturally, there is a set of 20 amino acids which are termed standard amino acids. These standard amino acids serve as building blocks for the proteins that are produced in a living organism. These acids are studied widely all over the world.  The standard amino acids are:

  • Glycine

  • Alanine

  • Proline

  • Valine

  • Leucine

  • Isoleucine

  • Methionine

  • Phenylalanine

  • Tyrosine

  • Tryptophan

  • Serine

  • Threonine

  • Cysteine

  • Asparagine

  • Glutamine

  • Lysine

  • Arginine

  • Histidine

  • Aspartate

  • Glutamate

Besides standard amino acids, there are some acids that are found in some proteins and are known as non-standard amino acids. The acids have a different composition that doesn't match with the composition of standard amino acids. They are known for presenting different physicochemical properties. Therefore, when it becomes necessary to introduce a local in a particular protein with specific properties, and it can't get incorporated by the standard amino acids, then the non-standard amino acids come into the scene, and they are used for incorporation. So it is clear that the only difference between standard and non-standard amino acids is their composition. Some examples of non-standard amino acids are:

  • Cystine, desmosine, and isodesmosine

  • Hydroxyproline and hydroxylysine

  • Gamma-carboxyglutamate

  • Phosphoserine, phosphothreonine, and phosphotyrosine

  • N-acetyl lysine

  • Methyllysine

Non-Standard Amino Acids and Their Functions

Non-standard amino acids being an essential molecule in biochemistry, have some key functions that help humans to produce different things which act as useful products in their daily lives. Some of the important functions of non-standard amino acids are as follow:

  • The most important posttranslational modification of amino acids occurring in eukaryotic organisms is considered to be the reversible addition of a phosphate molecule to the hydroxyl portion of the R groups of serine, threonine, and tyrosine. The event is referred to as phosphorylation and gets used for regulating the activity of proteins that seem to be functioning in cells continuously. This is considered to be an important function of non-standard amino acids.

  • Non-standard amino acids that are produced in industries are considered to have some important functions that can be very useful. The first commercial production of amino acids was in the year 1908. At that time, a flavouring agent, monosodium glutamate, was prepared using a type of large seaweed. Glycine and cysteine can also do the work as a food additive, and some mixtures of amino acids are also used as flavour enhancers in some food industries. This is considered to be another important function of amino acids that is useful to humans.

List of Non-Standard Amino Acids

Non-standard amino acids which one cannot find in proteins are lanthionine, 2-aminoisobutyric acid, dehydroalanine, and the neurotransmitter gamma-aminobutyric acid. Non-standard amino acids are considered to be intermediaries that are present in the metabolic pathway of standard amino acids. In order to have a clear understanding, let's discuss an example: ornithine and citrulline seem to occur in the urea cycle, which is considered to be a part of amino acid catabolism. Some other examples of non-standard amino acids are as follows:

  • Cystine, desmosine, and isodesmosine: This type of amino acid is known to be found in extracellular matrix proteins such as elastin.

  • Hydroxyproline and hydroxylysine: This type of amino acid is known to be found in one type of extracellular matrix, which is known as collagen.

  • Gamma-carboxyglutamate: These are found in osteocalcin, which is considered to be a  type of extracellular matrix that is found in bones but also in the prothrombin. This is considered to be important for the coagulation cascade.

  • Phosphoserine, phosphothreonine, and phosphotyrosine: These are found in many different proteins.

FAQs (Frequently Asked Questions)

1. State the Classification of Standard Amino Acids.

Ans: One of the best ways to classify standard amino acids is based on the polarity of the R group. Polarity is considered to be the distribution of electric charge. The two types based on polarity are as follows:


Non-Polar Amino Acids: Non-polar amino acids are glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, and tryptophan. The R groups of this type of amino acids are considered to have either aliphatic or aromatic groups.


Polar Amino Acids: Polar amino acids are serine, cysteine, threonine, tyrosine, asparagine, and glutamine. The side chains in these groups of amino acids possess a spectrum of functional groups.

2. What are the Basic Amino Acids?

Ans: The three basic types of amino acids that are known to humans and are included in the biochemistry course are arginine, lysine, and histidine. These three amino acids have basic side chains at neutral pH. Their side chains are known to have side chains that contain nitrogen in them and are also capable of resembling ammonia which is considered to be a base. Their pKa's are considered to be high enough that they have the ability to bind protons; thus; as a result, they gain a positive charge during the process. This is what basic amino acids mean; in order to get more knowledge, one has to choose biochemistry as his/her career.