Types of 10 essential amino acids and classification

 • Definition of amino acid 

  • 10 essential amino acids

  • Properties of amino acids

  • Classification of amino acids

  

   Definition of amino acid 

Amino acids the simplest unit of proteins. The general formula of amino acid is as follows :        

     

             H

    R-------C-------COOH

              NH2

              

Amino acid consists of

• A free NH2 (amino group )

• A free COOH  (carboxyl group )

All amino acids of proteins have the NH2 group attached to the carbon atom which is next to the  COOH group            ( Alpha carbon atom ). So all amino acids of proteins are Alpha amino acid. Proteins are built up from 20 different Alpha amino acids.

10 Essential amino acids 

These are amino acids which are not synthesized in the body. So they must be supplied inadequate amounts through diet  .                                            

The following are the 10 essential amino acids :-

• Valine
• Isoleucine
• Phenylalanine
• Histidine
• Arginine
• Lycine
• Leucine
• Methionine
• Threonine
• Tryptophan 

All these amino acids are essential for normal growth and maintenance of Nitrogen equilibrium. The nutritional value of protein depends on the amount of essential amino acids present in it. Casein of milk contains all these essential amino acids. So it is a complete protein. Gliadin of Maize lacks in tryptophan. So it is an incomplete protein .

Physical properties of amino acid 

• Amino acids are white crystalline substances.

•  They are soluble in water and insoluble in organic solvents.

• They vary in taste. They may be tasteless sweet or bitter.

• They have a high melting point 200 to 300 degree Celsius. 

• All naturally occurring amino acids are Levorotatory except glycine which is optically inactive. 

• Amino acids have a free amino group and free carboxyl group .

Chemical properties of amino acids

• Reactions due to COOH group

• They from esters with alcohols and salts with bases. 

•With ammonia they form the corresponding amide that is glutamic acid forms glutamine aspartic acid forms asparagine .

• Decarboxylation of amino acids forms the the corresponding amides that is histidine to histamine.

Reactions due to NH2 group 

• they form salt with acids.

• The free amino group can be acylated with other Carboxylic acids 

• The amino group can be substituted by alkyl o r aryl groups.

• The amino group can react with nitrous acid liberating nitrogen quantitatively.

Classification of amino acids

 Amino acids are classified in different ways. The following classification based on the properties of side chains is convenient 

Neutral amino acids :

• Glycine
• Alanine
• Serine
• Threonine
• Valine
• Leucine
• Isoleucine                                                                                   

Acidic amino acids :

• Aspartic amino acid
• Glutamic acid                                                           

Basic amino acids :

• Arginine
• Lysine                                   

Sulphur containing amino acids :

• Cysteine
• Methionine 

Aromatic amino acids :

• Phenyalanine
• Tyrosine
• Tryptophan

Heterocyclic amino acids :

• Proline
• Hydroxyproline
• Histidine