How are amino acids classified?
Amino acid classifications are numerous and varied depending on the target audience. Knowing how amino acids are classified can be important for your overall health and vitality.
You will learn
Standard and non-standard amino acid classification
Most of the amino acids we are familiar with are known as standard amino acids. These are amino acids that are formed from universal genetic coding. Standard amino acids (also known as canonical amino acids) are protein ‘building-blocks’. These are the amino acids that together with DNA help organism to form and function.
Non-standard amino acids do not help to build proteins. There are only three exceptions to this rule. Selenocysteine, pyrrolysine, and N-formylmethionine are non-standard amino acids that can still be naturally incorporated into proteins.
They do this by exploiting genetic code that has not been used or in a certain position within an amino acid chain. Non-standard amino acids can also be modified forms of standard amino acids. Many non-standard amino acids are used in genetic engineering.
Proteinogenic and non-proteinogenic amino acid classification
Proteinogenic amino acids are broadly defined as the amino acids that are used to form proteins. Many of these are naturally produced by the body. However, there are proteinogenic amino acids that must be sourced from diet. In all cases, proteinogenic amino acids are necessary for healthy body function. You can read about these in greater detail in our article – The 20 Proteinogenic Amino Acids.
Non-proteinogenic amino acids are not required to build proteins. However, this doesn’t mean that they are not important. These amino acids have a vital role as metabolic intermediates. There are approximately 700 known non-protein amino acids, of which around 300 are plant derived. Their routes of origin include modification of an existing amino acid or pathway, or their own novel pathways.
Amino acid classification based on human nutrition
One of the most common ways to classify amino acids is based on human health requirements. Some amino acids can be synthesised by the body. These are classified as non-essential amino acids. There are eleven non-essential amino acids. Conversely, essential amino acids must be acquired from food sources. A balanced diet usually ensures that the body acquires enough essential amino acids
There is also a third category of amino acids classified based on nutrition. These are called semi-essential amino acids. These are amino acids that are normally considered as non-essential although during periods of stress they may become semi-essential. For example, if the immune system is compromised the body may struggle to produce certain amino acids. Consequently diet becomes a necessary source of these amino acids.
Amino acid classification based on chemical properties
There are many different chemical properties of amino acid side chains that can be classified. These variations can help to order and classify amino acids into groups with common characteristics.
Some of the common chemical properties used to classify amino acids include the charge of the side chain, whether it is acid or basic, and whether it has hydrogen bonding ability. Broadly, the twenty common amino acids can be split into non-polar amino acids (hydrophobic) and polar amino acids (hydrophilic).
Non-polar amino acids can be further split into alkyl or aromatic, while the polar amino acids can be split into neutral, acid, or basic. The following video goes into more detail about the various chemical properties that help to classify different amino acid groups:
There are many different ways to classify amino acids. Unless you are a chemist, amino acid classification based on nutrition is most widely utilised. This information can help us to understand what our body needs to improve certain functions. This classification can be used to determine various nutrient deficiencies. It may also help us to make better food choices based on the various proteins available.