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56.4 CLASSIFICATION OF PROTEINS

CLASSIFICATION BASED UPON MOLECULAR STRUCTURE

Proteins can be classified into two broad classes on the basis of their molecular structure:

Fibrous proteins and Globular proteins.

1. Fibrous Proteins

Such proteins have thread like molecules which tend to lie side by side to form fibres. The molecules are held together at many points by hydrogen and disulphide bonds: They are
generally insoluble in water.

2. Globular Proteins

Such proteins have molecules which are folded into compact units that often approach spheroidal shapes. The areas of contact between the molecules are small. Therefore, the intermolecular forces, here, are comparatively weak. These proteins are soluble in water or aqueous solutions of acids, bases or salts.

 CLASSIFICATION BASED UPON THE

CORRESPONDING HYDROLYSIS PRODUCTS

On the basis of chemical composition or the nature of products of hydrolysis, proteins are classified into three categories.

1. Simple Proteins

These proteins on hydrolysis give mixture of α-amino acids only. Some examples are albumins in white of egg, oxyzenin in rice, keratin in hair, nails, horns and glutenin in wheat.

2. Conjugated Proteins

These proteins on hydrolysis give a non-protein part in addition to the a-amino acids. This non-protein portion is called the prosthetic group. The, main function of the prosthetic group is to control the biological functions of the protein. Some examples of conjugate proteins along with their corresponding prosthetic groups are given below in tabular form:

Name of the Protein

Prosthetic Group

Nucleoproteins

Nucleic acids

Glycoproteins

Sugars

Lipoproteins

Lipids such as lecithin

Phosphoproteins

Phosphoric acid residues

Chromoproteins

Some colouring matter

Metalloproteins

Some metal.

3. Derived Proteins
These are the degradation products obtained by partial hydrolysis of simple or conjugated proteins with acids, alkalies or enzymes. Some examples are, proteoses, peptones and polypeptides.
Peptones → Polypeptides → Peptones → Polypeptides