Classification of Proteins:-

(A). On the basis of Molecular Structure – On the basis of their molecular structure proteins are of two types:

(A). On the basis of Molecular Structure – On the basis of their molecular structure proteins are of two types:

1. Fibrous Proteins:- These proteins consist of linear thread like molecules which tend to lie side by side to form fibers. The polypeptide chains are held together usually at many points by hydrogen bonds, hence intermolecular forces of attraction are very strong. These proteins are insoluble in water and stable towards moderate changes in temperature and pH e.g. Keratin (in skin, hair, nails, horn, wood, feather), collagen (in tendons), myosin (muscles), Fibroin (in silk) etc.

2. Globular Proteins:- In such proteins the molecules are folded which approach spheroidal shapes. This folding takes place in such a way that non polar groups are pushed inside while polar groups are pushed outwards. As a result water molecules interact strongly with polar groups and hence the proteins are soluble in water. These are sensitive to a small change in temperature and pH e.g. all enzymes, insulin, hemoglobin, fibrogen (clotting of blood), albumin (egg protin etc.).

(B) On the basis of Product they give on hydrolysis:- On the basis of products of hydrolysis these are of three types:-

1. Simple Proteins:- These proteins on hydrolysis give a mixture of α-amino acids only e.g. albumin in eggs, keratin etc.

2. Conjugated proteins:- These proteins on hydrolysis give a non protein part in addition to the α-amino acids. The non protein part is called prosthetic group. It controls the biological function of proteins. e.g. glycopr proteins have sugar as prosthetic group, Lipoproteins have Lipids as prosthetic group etc.

3. Derived Proteins:- These are the degradation products obtained by partial hydrolysis of simple or conjugated proteins with acids, alkalies or enzymes. e.g. Proteose, Peptones and polypeptides.

Proteins → Proteoses → Peptones → Polypeptides