Proteins:-

The term 'protein' was coined by berzelius(1837) and Mulder(1838). Though approximately 300 amino acids occurs in nature but only 20 make the composition of proteins.

Proteins are polymers of amino acids.

 All amino  acids have a common structure.

 The only difference between the different amino acids lies With R-Group in general formula.

 The R-Group have quite diverse chemical  properties.

All amino acids, apart from the simplest one (glycine) show optical isomerism. 

This can result in two different arrangements as shown in the diagram. With a very few

minor exceptions (e.g., bacterial cell wall contains D-amino acids) only the L-forms are found in living organisms.

Gamma Amino Butyric Acid (GABA), histamine serotonin, ornithine, citruline and B-alanine are the amino acids which are not found in proteins.

Glycine is the simplest amino acid with lowest molecular weight and absence of asymmetrical carbon atom. 

It is involved in the formation of heme.

Tryptophan is the most complex amino acid containing indole ring.

Methionine and cysteine are sulphur containing amino  acids.

In proline and hydroxyproline, insteadof NH2 (amino)group, NH (imino) group is present.

These amino acids are called imino acids.

Proteins contain L-(laevorotatory) isomers of amino acids.

D-(Dextrorotatory) isomers of amino acids, are found only in bacterial cell walls.

Lysine and arginine are basic amino acids, which contain more than one amino groups.          Glutamic acid and asparticacid are acidic amino acids, which contain more than one

acidic groups.

Tryptophan amino acid forms the vitamin nicotinamide and a plant hormone Indole Acetic Acid (IAA). 

Amino acids that cannot be synthesized in the body are called essential amino acids, while those which can be synthesized in the body and need not be supplied in the diet are called non-essential or

dispensable amino acids. 

For human beings eight amino acids are essential. Infants require arginine and histidine in

addition.