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# Essential Amino Acids

After carbohydrates, proteins are essential biomolecules for living beings. The main sources of protein are milk, cheese, pulses, fish etc. They are vital chemical substances which are essential for the growth and maintenance of the life. Proteins are high molecular mass complexes, biopolymers of amino acids. They are formed by the condensation polymerization of monomeric units; $\alpha$-amino acids which bonded by peptide linkage.

There are total twenty α-amino acids which involve in the formation of proteins in living systems. All proteins are essentially made up of carbon, hydrogen, oxygen with nitrogen and sulfur. Some of the proteins contain non-metal like phosphorus, iodine as well as metals like iron, zinc, cobalt etc.

## What are Essential Amino Acids?

All proteins are made up of α-amino acids in which both functional groups; amino group and carboxyl group bonded on same carbon atom. Hence α-amino acids are building blocks of all proteins.

• In each $\alpha$-amino acid there are two functional group; amino group and carboxyl group bonded on same carbon atom.
• Excluding one amino acid, glycine, all other $\alpha$-amino acids are optically active as the α-carbon atom bonded with two different functional groups , one hydrogen atom and one alkyl group (-R).
• In optically inactive $\alpha$-amino acids; glycine, there are two hydrogen atoms on $\alpha$-carbon atom.
• In amino acids, both functional groups are of opposite nature.
• Amino group is basic in nature whereas carboxyl group is acidic in nature.
• Hence presence of both opposite functional groups makes amino acids natural in nature.
• If out of these functional groups, any one functional group is more than another one, molecules becomes acidic or basic.
• More than one amino group make amino acid basic in nature and called as basic amino acid. Like, Arginine contains total four amino groups with one carboxyl group.

If more than one carboxyl groups in amino acid make molecule acidic like aspartic acid (HOOCCH(NH2)CH2COOH) contains two carboxyl groups with one amino acid.

Both functional groups present in amino acids can interact with each other and affect the properties of each other. Because of opposite nature of functional groups; both neutralize each other by involving transfer of a proton from the carboxyl groups to amino group within a molecule. As a results amino acid exists as dipolar ion also called as zwitterion or internal salt.

The pH of solution would be the deciding factor for the existence of Zwitterion of any amino acid. For example, at low pH i.e. in acidic medium the proton will accept by amino group to form a cation with NH3+ group which move towards cathode.

While in basic medium, the carboxyl group releases proton and forms carboxylate ion which move towards anode. At a certain pH both forms are in equilibrium, hence show no overall movement towards electrodes in electrolysis. That pH value is known as isoelectric point. It is a characteristic value for each amino acid.

On the basis of the need in living system, α-amino acids can be classified in two types.
• Essential α-amino acid
• Non-essential α-amino acid

Essential α-amino acids

Out of the twenty amino acids, about ten amino acids cannot be synthesized by human body and must be supplied by food to body and called as essential amino acid or indispensable amino acids. These amino acids are required for the growth of the body and their deficiency causes diseases like kwashiorkor in which the water balance of living body get disturbed and some of the organs of the body become watery and bloated. Therefore essential amino acids must be supplied by human diet.

Other bio molecules like fat and carbohydrate can store in human body, while amino acid cannot store, hence continues supply of amino acids are necessary for growth and development and they must be present in the food of every day. Some of the essential amino acids are Isoleucine, Leucine, Trypophan, and Valine. Some essential amino acids synthesized in body but with a very low rate which is not sufficient for the necessary amount of needed amino acids.

For example, Arginine can be synthesized by mammalian cells but with very slow speed, hence insufficient for all requirements. Some amino acids are complementary to each other, hence one required for the growth of another one. Like Methionine is required to produce cysteine and phenyalanine is needed to form tyrosine.