Explain the classification of proteins based on their chemical composition.

On the basis of structure, proteins are classified into three categories: 

1. Simple proteins: 

a. Simple proteins on hydrolysis yield only amino acids. 

b. These are soluble in one or more solvents.

c. Simple proteins may be soluble in water. 

d. Histones of nucleoproteins are soluble in water. 

e. Globular molecules of histones are not coagulated by heat. 

f. Albumins are also soluble in water but they get coagulated on heating. 

g. Albumins are widely distributed e.g. egg albumin, serum albumin and legumelin of pulses are albumins. 

Importance: They are involved in structural components; they also act as a storage kind of protein. 

Some are associated with nucleic acids in nucleoproteins of cell.

2. Conjugated proteins: 

a. Conjugated proteins consist of a simple protein united with some non-protein substance. 

b. The non-protein group is called prosthetic group e.g. haemoglobin.

c. Globin is the protein and the iron containing pigment haem is the prosthetic group. 

d. Similarly, nucleoproteins have nucleic acids. 

e. Proteins are classified as glycoproteins and mucoproteins. 

f. Mucoproteins are carbohydrate-protein complexes e.g. mucin of saliva and heparin of blood. 

g. Lipoproteins are lipid-protein complexes e.g. conjugate protein found in brain, plasma membrane, milk etc. 

Importance: They are involved in structural components of cell membranes and organelles. 

They also act as a transporter. 

Some conjugated proteins are important in electron transport chain in respiration. 

3. Derived proteins: 

a. These proteins are not found in nature as such. 

b. These proteins are derived from native protein molecules on hydrolysis. 

c. Metaproteins, peptones are derived proteins. 

Importance: They act as a precursor for many molecules which are essential for life.