Write classification of proteins with an example.

Answer: On the basis of molecular shape, proteins are classified into two types:

1. Fibrous proteins: polypeptide chains run parallel and are held together by hydrogen bonds and disulphide bonds, forming long thread-like fibres. They are generally insoluble in water. Examples: keratin (hair, wool, nails), myosin (muscles), fibroin (silk).
2. Globular proteins: polypeptide chains coil up into a spherical (globular) shape. They are usually soluble in water. Examples: insulin, egg albumin, haemoglobin.

(On the basis of composition, proteins are also classed as simple proteins, e.g. albumin, and conjugated proteins with a non-protein prosthetic group, e.g. haemoglobin.)