About Non-Standard Amino Acids
Non- amino acids are organic molecules that are not composed of amino acids. There are many different types of non-amino acids, and they can be found in both natural and synthetic forms. Some of the most common non-amino acids include sugars, lipids, and nucleic acids. Non-amino acids are used in various areas of the plant and even humans for different purposes. For example, when the non-amino acid ribonucleic acid (RNA) is combined with DNA, it becomes messenger RNA (mRNA), which serves as a transcriptional control protein that works in the nucleus of a cell. The cell can then use this messenger RNA to produce proteins and the cell is thus able to function. The same concept applies to amino acids in which some of the non-amino acids can be utilized as a building block and others will be degraded. The role of the non-amino acids can be diverse. Some of them will be used in metabolic processes. However, some of them may not be used for a long time and then will become no longer usable. In addition to the need for amino acids in biological processes, there are also non-amino acids that must be present in a certain amount.
These non-amino acids are known as essential amino acids. The different types of non-amino acids are not the same, and they are not all manufactured the same way. For example, there are certain metabolic processes that do not require the presence of certain non-amino acids. Examples of non-essential amino acids include hydroxyproline, glycine, serine, threonine, lysine, arginine, and aspartic acid. In cases of long-term starvation, the body may use its protein stores and depleted non-essential amino acids will be used instead of the essential ones. To maintain a healthy body and to prevent many health problems, it is very important that we consume certain essential amino acids on a regular basis. Supplements With Essential Amino Acids for Health The body needs the essential amino acids in different quantities and it is not possible to provide them through supplements. When the body is not supplied with the required amount of essential amino acids, these amino acids will be converted into other compounds, which are called by-products. Some of these by-products are toxic. For example, the body makes some proteins that are harmful to the liver. The main amino acids that the body needs are listed below. We should remember, however, that the recommended daily amount of each of these amino acids depends on an individual's gender, age, and weight.
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.
(Image will be Uploaded Soon)
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 as 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 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 food additives, 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. They're known to be phospholipids and also to be found in intracellular membranes.
Inositol: This type of amino acid is found in many kinds of viruses.
FAQs on Non-Standard Amino Acids
1. What are non-standard amino acids?
Non-standard amino acids are amino acids that are not among the 20 common amino acids directly encoded by the universal genetic code. They are typically formed by the chemical modification of a standard amino acid after it has been incorporated into a protein chain. This process is known as post-translational modification.
2. How do non-standard amino acids differ from the 20 standard amino acids?
The primary difference lies in their origin. Standard amino acids are directly specified by codons in an organism's DNA and are used as building blocks during protein synthesis. In contrast, most non-standard amino acids are not directly coded for. Instead, they are created by enzymatic modifications to a standard amino acid residue already present in a polypeptide, giving them unique structures and functions.
3. What are some important examples of non-standard amino acids and where are they found?
Several non-standard amino acids play critical roles in biological systems. Key examples include:
- Hydroxyproline and Hydroxylysine: Essential for the structural integrity of collagen, the most abundant protein in mammals, found in skin, bones, and cartilage.
- Gamma-carboxyglutamate: Found in proteins involved in blood coagulation, such as prothrombin, where it helps in binding calcium ions.
- Phosphoserine: A modified form of serine that plays a crucial role in cell signalling and regulating protein activity through phosphorylation.
- Cystine: Formed by the oxidation of two cysteine residues to create a disulfide bridge, which is vital for stabilising the three-dimensional structure of many proteins.
4. How are non-standard amino acids incorporated into proteins if they are not coded by DNA?
Most non-standard amino acids are introduced into proteins through post-translational modification (PTM). This is a multi-step process: First, a standard amino acid is incorporated into the growing polypeptide chain during normal translation. Afterwards, specific enzymes act on this amino acid residue, chemically altering it to form the non-standard version. For example, the enzyme prolyl hydroxylase converts proline residues into hydroxyproline within a newly formed collagen chain.
5. Why are non-standard amino acids biologically important?
Non-standard amino acids provide specialised chemical properties that the 20 standard amino acids cannot offer. Their importance lies in their ability to expand the functional capacity of proteins. They are crucial for:
- Structural Reinforcement: Creating strong cross-links that stabilise proteins like collagen and elastin.
- Cellular Regulation: Acting as molecular switches, such as in phosphorylation, to turn protein activity on or off.
- Metabolic Processes: Serving as intermediates in critical metabolic pathways, like the urea cycle.
6. Do non-standard amino acids exist outside of proteins? If so, what roles do they play?
Yes, many non-standard amino acids function as independent molecules and are not part of proteins. For instance, ornithine and citrulline are key intermediates in the urea cycle, the metabolic pathway for disposing of nitrogenous waste. Another vital example is gamma-aminobutyric acid (GABA), derived from the standard amino acid glutamate, which acts as a primary inhibitory neurotransmitter in the central nervous system.
7. Are selenocysteine and pyrrolysine considered standard or non-standard amino acids?
Selenocysteine and pyrrolysine represent a special case. While they are often called the '21st and 22nd' amino acids and are not part of the original 20, they are unique because they are genetically encoded. Unlike other non-standard amino acids, they have their own tRNA molecules and are incorporated into proteins during translation in response to specific codons (UGA and UAG, respectively), which typically function as stop signals. This makes them an exception to the rule that non-standard amino acids are formed only by post-translational modification.
