Amino Acids: Definition, Structure, Classification

Amino Acids are chemical molecules that combine to produce proteins and are hence known as the building blocks of proteins. These biomolecules have a role in a variety of biological and chemical activities in the human body and are essential for human growth and development. There are around 300 amino acids found in nature.

Basic amino groups (-NH2) and carboxyl groups  (-COOH) are found in amino acids. Amino acids are the main components of proteins. Proteins and peptides are both made up of lengthy chains of amino acids. There are twenty amino acids in all that are used in the production of proteins. On this page, let us learn everything about Amino Acids in detaul. Read further to find more.

What is Amino Acids?

Proteins are the molecular mass complex biomolecules of amino acids present in all living cells. In other words, amino acids are the building blocks of proteins. There are about \(500\) naturally occurring amino acids known to us.

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Structure of Amino Acids

\(\alpha\)- amino acids are building blocks of proteins. Amino acids are organic compounds containing both amino groups and carboxylic groups.

General Formula: General formula:

The amino group \(\left( {{\rm{ – N}}{{\rm{H}}_{\rm{2}}}} \right)\) may be attached to any carbon atom other than that of the carboxyl \(\left( {{\rm{ – COOH}}} \right)\) group.

What are the Properties of Amino Acids?

1. They are colourless, crystalline substances. 
2. They have a high melting point.
3. The solubility in water varies with the nature of the side chain.  
4. They are amphoteric in nature, i.e., they react with both acids and bases.
5. They have asymmetric carbon except for glycine, hence rotate plane polarised light. This property is called optical activity.

Classification of Amino Acids

Classification Based on the Requirement of the Body:

(a) Non-essential amino acids: These amino acids are synthesized in the body and do not require them in our diet. There are ten non-essential amino acids out of \(20\). They are glycine, alanine, serine, cysteine, glutamine, tyrosine, proline, aspartic acid, asparagine, glutamic acid.

(b) Essential Amino Acids: These amino acids are not synthesised in the body, and we require them in our diet. There are ten non-essential amino acids out of \(20\). They are valine, leucine, isoleucine, arginine, lysine, threonine, phenylalanine, tryptophan, and histidine. These essential amino acids are required for our body’s growth, and lack of these in diet can cause diseases such as kwashiorkor.

Classification of Naturally Occurring Amino Acids

The naturally occurring amino acids can be classified into three groups: Aliphatic, Aromatic, and Heterocyclic amino acids.

Aliphatic Amino Acids: They are long-chain amino acids. They are further classified into the following types of amino acids:

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1. Neutral Amino Acid: It contains an equal number of carboxylic and amino groups in their molecules.
Example: Glycine, alanine, valine, etc. 

2. Acidic Amino Acid: Contains a greater number of the carboxylic group than amino groups in their molecules.
Examples: Aspartic acid, glutamic acid which contains two \({{\rm{ – COOH}}}\) groups and one \({\rm{ – N}}{{\rm{H}}_{\rm{2}}}\) group.

3. Basic Amino Acid: It contains a greater number of amino groups than carboxylic groups in their molecules.
Example: Lysine, arginine, and histidine, which contains two \({\rm{ – N}}{{\rm{H}}_{\rm{2}}}\) groups and \({{\rm{ – COOH}}}\) groups.

4. Sulphur Containing Amino Acid: It contains sulphahydryl \(\left( {{\rm{ – SH}}} \right)\) groups in their structure.
Example: Cysteine and Methionine

Aromatic Amino Acid: They contain a benzene ring in their structure.
Example: Phenylalanine and Tyrosine

Heterocyclic Amino Acids: They have a heterocyclic ring with at least one atom other than carbon in their side chain.
Example: Tryptophan, Proline, and Hydroxyproline

Some Common Amino Acids

How is Protein Formed from Amino Acids?

When amino acids unite to form proteins, the amino group of one amino acid molecule forms a chemical bond with the carboxylic group of the other. The resulting linkage is called a peptide bond. This linkage process continues until all amino acids necessary for the formation of a protein are joined together. This process involves the loss of one water molecule between the two amino acids. 

The joining of two amino acids in this manner forms a larger unit called a peptide. Several peptides joined together to form a polypeptide. In turn, polypeptides unite to form a complete protein.

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What is the Role of Amino Acid in Our Body?

1. Some amino acids are converted into biologically active compounds. Example: Tyrosine is converted into hormone thyroxine and adrenaline and also into a skin pigment, melanin.
2. Glycine is involved in the formation of heme (haemoglobin protein) and tryptophan from the vitamin nicotinamide and the plant hormone indole acetic acid.
3. Amino acids like citrulline and ornithine are actively involved in the urea cycle in the liver and thus helps in maintaining the ammonia concentration below toxics level.

Summary

The proteins we eat include around 20 amino acids. These amino acids produce a bigger protein molecule when they bind together. Because of the variable nature of carbon, amino acids, as organic compound molecules, may establish a variety of distinct linkages with one another. This allows for the enormous diversity of proteins observed in nature. Because of the roles they serve, they are considered a necessary component in our diet.

In this article, we acquired knowledge about the structure, classification, types, and uses of amino acids in detail. Let us use this knowledge for the consumption of food containing the right amount of amino acid.

FAQs on Amino Acids

Below are the frequently asked questions on Essential Amino Acids:

Q.1. Which foods contain amino acids?
Ans: Amino acids are obtained from both animal and plant sources. Grains, nuts, seeds, beans, legumes, fruits, vegetables, eggs, milk, cheese, fish, seafood, etc., are foods that contain amino acids.

Q.2. Is it safe to take amino acids every day?
Ans: Generally, it is safe to use a limited amount of amino acids every day. However, the large dosage of amino acids is harmful. They may lead to abdominal pain, diarrhoea, unhealthy drop in blood pressure, gastrointestinal distress, etc.

Q.3. What are the \(21\) amino acids?
Ans: The \(21\) amino acids are Glycine, Alanine, Valine, Leucine, Isoleucine, Phenylalanine, Methionine, Tryptophan, Proline, Serine, Cysteine, Asparagine, Glutamine, Threonine, Tyrosine, Aspartic acid, Glutamic acid, Lysine, Arginine, Histidine, and Selenocysteine.

Q.4. Do amino acids have side effects?
Ans: Excess intake of amino acids increases the risk of hypertension, heart disease, fatigue, and loss of contraction.

Q.5. What do amino acids do for your body?
Ans: Proteins are bodybuilding foods. Amino acids are fundamental building blocks of proteins. Hence, they help in the growth and repair of damaged cells and tissues of the body. Some amino acids are converted into biologically active compounds, e.g., tyrosine is converted into the hormone thyroxine and adrenaline, and also into a skin pigment, melanin.

Q.6. Do amino acids affect kidneys?
Ans: Amino acids like citrulline and ornithine are actively involved in the urea cycle in the liver. This helps in maintaining the ammonia concentration below toxic levels.

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