Amino acids (Building blocks of proteins) : General Structure and Function

Amino acids are the building units of proteins. They are small molecules made up of carbon, hydrogen, oxygen and nitrogen. A few amino acids contain sulphur too.

There are 20 different amino acids isolated from plant and animal proteins. They differ according to the nature of the amino acid side chain R. Glycine is the simplest amino acid. Here the side chain R is represented by a hydrogen atom. The side chain can be short hydrocarbon chain in alanine, a hydrocarbon ring as in tyrosine or sulphur containing as in cysteine. R groups of same amino acids contain carboxyl group (glutamic acid) whereas others contain an amino group

With the exception of glycine the alpha carbon atom of all amino acids is symmetrical. So D and L stereoisomer of most of the amino acids exist. All the amino acids found in proteins are levorotatory.

Generally amino acids have a common basic structure. There is a central four valence carbon atom called the α carbon. Four groups are attached to this carbon atom ( central carbon C^α) attached to:hydrogen (H), amino group (-NH₂), carboxyl group (-COOH) and side chain (R); it may be polar or non polar. The name α amino acid is given due to the attachment of amino group to the α carbon atom.

Many amino acids can be synthesized by animals from the existing precursors. For man 7 amino acids can not be synthesized in his body. So they should be taken through food. They are called essential amino acids. They are isoleucine, leucine, lysine, methionine, phenylalanine, trytophan and valine.

The amino group of one amino acid joins to the carboxyl group of another in a condensation reaction. It results in the formation of a peptide linkage. Two amino acids joined by a peptide linkage is called a dipeptide. Long chain of amino acids residues formed by peptide bond is called a polypeptide. A protein molecule is formed of one or many polypeptides.

• Formation of proteins: Amino acids act as building blocks of proteins.
• Biosynthesis of other compounds: Various amino acids give rise to different biologically important compounds.

For example,Tyrosine is converted into thyroxine and adrenaline. The skin pigment melanin is synthesised fro thyrosine

Glycine is involved in the synthesis of haem.

Tryptophan involved in the synthesis of vitamin nicotinamide and the plant hormone indole 3 acetic acid.

• Formation of glucose: The carbon chain of many amino acids are converted into glucose after the removal of the amino group.
• Formation of amines: Biologically active amines such as histamine are formed from amino acids by losing the carboxyl group.