Question

Describe briefly the structure of insulin. How is gentically engineered insulin synthesized ?

Answer 1

Insulin consists of two short polypeptide chains: chain A and chain B, that are linked together by disulphide bridges. In mammals, including humans, insulin is synthesised as a pro-hormone (like a pro-enzyme, the pro-hormone also needs to be processed before it becomes a fully mature and functional hormone) which contains an extra stretch called the C peptide. This C peptide is not present in the mature insulin and is removed during maturation into insulin.

For synthesis of insulin, human insulin is extracted from pancreas cells and an insulin-producing gene is isolated. A plasmid DNA is extracted from a bacterium and cut with restriction enzyme, forming plasmid vector. Human insulin-producing gene is inserted into the bacterial plasmid vector to form the recombinant DNA of human insulin-producing gene. This recombinant DNA is introduced into a bacterial cell to form the recombinant bacterium. The recombinant bacteria multiply in a fermentation tank and produce human insulin. Insulin is extracted, purified and bottled. It is then ready to be injected into diabetic patients.

The main challenge for production of insulin using rDNA techniques was getting insulin assembled into a mature form. In 1983, Eli Lilly an American company prepared two DNA sequences corresponding to A and B, chains of human insulin and introduced them in plasmids of E. coli to produce insulin chains. Chains A and B were produced separately, extracted and combined by creating disulfide bonds to form human insulin.