Amino Acids: Properties, Structure, Sources and Function

The organic compounds that make up a protein molecule are called amino acids. These compounds are present in the human body and are responsible for performing various functions including the growth and development of human beings. Protein and non-protein based amino acids are the two primary categories of amino acids. There are total 300 amino acids found in the nature, but only twenty amino acids are needed for the cellular functioning in the body.

• Except twenty two standard amino acids, rest are called non-protein amino acids. 
• Two newly discovered amino acids are selenocysteine and pyrrolysine. 
• All types of proteins required for cell growth and repair are coded by only twenty amino acids.

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What are Amino Acids?

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Amino acids are the organic compounds that form proteins and are generally referred to as the building blocks of a protein molecule. These organic compounds contain both the carboxyl-acid group (-COOH) and the amino Group (NH₂).

• Each group or atom of an amino acid is bonded to the central atom called the -R group.
• There are almost 300 amino acids that occur in nature, each performing a specific function.
• About 20 amino acids are present in both animal and plant proteins.

                                                                                                                             Amino Acids

The table below shows the list of all 20 amino acids: 

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Properties of Amino Acids

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The following are the physical and chemical properties of amino acids: 

Physical Properties of Amino Acids

The physical properties of amino acids include major parameters such as nature, solubility, and melting & boiling point of amino acids which are mentioned below in detail: 

• Nature: Amino acids are either bitter, tasteless, or sweet. These organic compounds are colorless. 
• Melting & Boiling Point: Amino acids have a very high boiling and melting point. They decompose upon heating at high temperatures. 
• Solubility: Amino acids are crystalline solid white substances. The majority of these compounds are insoluble in organic solvents and soluble in water.

Chemical Properties of Amino Acids

The chemical properties of amino acids are as follows:

• Zwitter Ion Property: Amino acids are the best example of molecules that shows zwitter ion property. The presence of positive and negative functional groups in a molecule is called zwitter ion. 
• Reaction with Sanger’s reagent: Under a mild alkaline medium, sanger’s reagent react with the amino group in the peptide chain. 
• Reaction with Acid & Base: These compounds react with both acid and base because of the presence of amine and carboxylic group.
• Amphoteric Property: The ability of amino acids to act as both acid as well as base is called its amphoteric property. 

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Structure of Amino Acids

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All amino acids have the same molecular structure and can be written as H₂NCH RCOOH. The 20 amino acids have this common structural feature – a central carbon atom (C) a carboxylate group (-COO^-), an amino group (NH₃⁺), and a hydrogen atom.

• They differ in their side chain which is the R group.
• The amino acids are joined together by peptide bonds. 
• Glycine does not have a side chain. Its α carbon consists of two hydrogen groups.

There are 4 groups of amino acids that are all attached to an alpha-carbon. They are:

• -COOH
• Hydrogen atom
• Amino group
• R- group (Sidechain)

Structure of amino acids are represented as:

Structure of Amino acids

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Classification of Amino Acids

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Our body requires all the 20 amino acids to function well. Out of all these 9 of them are the essential ones and the rest are non-essential. We will look into these types and their examples. Amino acids can be broadly classified into two categories which are briefly dicussed below: 

Non-Essential Amino Acids

These are the organic compounds that are synthesized by our body on their own. Asparagine, Aspartic acid, Glutamic acid, Proline, Alanine, Cysteine, Glycine, Serine, and Tyrosine are some of the examples of non-essential amino acids.

Essential Amino Acids

Amino acids that are required by our body but cannot be synthesized on their own are called essential amino acids. They are provided to the body through the intake of protein-rich food. Methionine, Threonine, Valine, Tryptophan, Phenylalanine, Leucine, Histidine, Lysine, and Isoleucine are the essential amino acids.

Essential and Non-Essential Amino Acids 

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Sources of Amino Acids

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Amino acids have the most important role in our body, they perform several chemical and biological functions in various body parts, the major functions they perform are repairing and building of tissues; formation and functioning of enzymes, digestion of food, transportation of molecules, etc.

The human body can synthesize only a few selected amino acids and the rest of them known as the essential amino acids are all provided to the body through food items rich in protein. Such food items in our diet include:

• Plant-based food: Beans, beetroots, broccoli, pumpkin, cucumber, carrots, onions, soybeans, lentils, whole grains, chia seeds, peanuts, green leafy vegetables, etc.
• Fruits: Bananas, berries, figs, apples, melons, oranges, papaya, pineapple, and pomegranates.
• Animal-based products: Eggs, dairy products, beef, chicken, meat, pork, seafood, etc.

Sources of Amino Acids 

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Functions of Amino Acids

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In the human body, both the essential and non-essential amino acids play different roles. 

The functions of essential amino acids are:

• Valine plays a role in promoting muscle growth.
• Threonine is responsible for improving the immune system.
• Phenylalanine boosts memory power and also maintains a healthy nervous system.
• Lysine is required to promote the formation of hormones, enzymes, and antibodies and to develop and fixate calcium within the bones.
• Leucine is responsible for promoting the synthesis of protein and the growth of hormones.
• Methionine helps in the treatment of kidney stones, maintaining healthy skin, and also invading pathogenic bacteria.
• Tryptophan works in the production of Vitamin B₃ and serotonin hormones.
• Isoleucine helps in the formation of hemoglobin, transporting oxygen from the lungs to various parts of the body.
• Histidine helps in various enzymatic processes and the synthesis of erythrocytes and leukocytes.

The functions of non-essential amino acids are:

• Cysteine works as an antioxidant to provide resistance to our body, inhibiting hair and nail growth.
• Alanine helps in the removal of toxins from the body and in the production of glucose & other amino acids.
• Glycine helps in maintaining proper cell growth and its function. It also helps in healing the wounds and acts as a neurotransmitter
• Glutamine is responsible for healthy brain function and for the synthesis of nucleic acids (DNA and RNA).
• Arginine works in promoting the synthesis of proteins and hormones, detoxification of kidneys, maintaining a healthy immune system, and healing wounds.
• Glutamic Acid helps in neurotransmission and is responsible for the development and functioning of the human brain.
• Serine helps in muscle growth and synthesis of immune system proteins
• Tyrosine helps in the production of thyroid hormones- T3 & T4, in melanin formation for the natural pigments found in hair, skin, and eyes. 
• Asparagine is involved in the formation of purines and pyrimidine for DNA synthesis, transportation of nitrogen in cells, and development of the nervous system. 
• Proline helps in repairing the tissues, in the formation of collagen, regeneration of new skin & in preventing the thickening and hardening of walls of arteries
• Aspartic acid is responsible for increasing metabolism and promoting the synthesis of other amino acids.

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Deficiency of Amino Acids 

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Amino acids play an important role in almost all life processes. The deficiency of amino acids in the body can lead to various disorders including pathological disorders like depression, anxiety, etc. The other disorders caused due to deficiency of amino acids are listed below:

• Anemia
• Depression
• Hypoglycemia
• Insomnia
• Edema
• Loss of Appetite
• Diarrhea
• Fat deposit in the liver
• Problems related to hair & skin
• Headaches
• Fatigue, weakness and irritability

To prevent the above disorders, one should maintain a healthy diet daily for the proper functioning of the body.

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Things to Remember 

• Amino acids are organic molecules that make up a protein.
• The shape, size, and functions of a protein is determined by the number and sequence of amino acids.
• Amino acids are also known as the monomers which make up proteins.
• Lysine, Arginine, and Histidine are basic amino acids.
• Based on the number of amino and carboxyl groups, they are divided into acidic, basic, and neutral. 
• These organic compounds can be polar and non-polar.