9 min readPathway sheetOften a 3-mark answer on how human insulin was made in E. coli; 2-mark questions on the C peptide; also a NEET favourite

Human insulin by recombinant DNA technology

Insulin has chains A and B joined by disulphide bridges. DNA for each chain was expressed separately in E. coli, and the chains were joined to make human insulin.

StructureTwo polypeptide chains, A (21 amino acids) and B (30 amino acids), linked by disulphide bridges

Pro-hormoneMade as proinsulin, with an extra stretch called the C peptide that is removed during maturation

Recombinant human insulin1983, by Eli Lilly, an American company, using E. coli

Older sourcePancreas of slaughtered cattle and pigs; it caused allergy in some patients

Why make insulin in bacteria?

People with diabetes may need insulin at regular intervals. Earlier, insulin was extracted from the pancreas of slaughtered cattle and pigs. In some patients this animal insulin caused allergy or other reactions to the foreign protein. Animal sources also could not meet the large demand. Making human insulin in bacteria solves both problems.

Proinsulin vs mature insulin

ProinsulinThe pro-hormone made in mammals. It has chains A and B plus an extra stretch, the C peptide. Like a pro-enzyme, it must be processed before it works.

Mature insulinOnly chains A and B, joined by disulphide bridges. The C peptide has been removed. This is the active hormone.

The main challenge for recombinant insulin was getting it assembled into the mature form.

How human insulin was made (Eli Lilly, 1983)

Making the two chains separately avoided the problem of removing the C peptide.

1
Two DNA sequences were prepared: one for chain A and one for chain B of human insulin.
2
Each was introduced into a plasmid of E. coli.
3
The bacteria produced chain A and chain B separately.
Grown on a large scale in bioreactors
4
The chains were extracted and purified.
5★
They were combined by creating disulphide bonds to form human insulin.

★ key step

How to draw the maturation of proinsulin

Draw one long folded chain and label it proinsulin. Mark its two parts A peptide (chain A) and B peptide (chain B), and the loop joining them C peptide.
Draw short bars between chains A and B and label them S-S (disulphide bridges).
Draw an arrow downwards to show maturation.
Below it, draw chains A and B side by side, still joined by the S-S bridges, and label them insulin. Draw the loop on its own and label it free C peptide.
Title: Maturation of proinsulin into insulin (simplified).

Animal insulin vs recombinant human insulin

Animal insulinFrom cattle or pig pancreas; differs from human insulin in a few amino acids; may cause allergy; supply limited.

Recombinant human insulinSame as human insulin; avoids reactions to a foreign protein; can be mass-produced in E. coli.

In real life

India has a very large number of people with diabetes. The insulin they inject today is recombinant human insulin or a modified form of it. Diet, exercise and regular blood sugar checks remain part of care.

Exam angle

3 marks: “How was human insulin produced by recombinant DNA technology? Why is it better than insulin from animals?” Eli Lilly (1983) prepared two DNA sequences for chains A and B, put them into E. coli plasmids, produced the chains separately, extracted them and joined them by disulphide bonds. It avoids allergy to a foreign protein and can be mass-produced.

2 marks: “What is the C peptide? Why was it a challenge for producing insulin by rDNA technology?” An extra stretch in proinsulin that is removed during maturation. Getting insulin assembled into its mature form was hard, so chains A and B were made separately and joined.

1 marks: “Name the bacterium used to make the chains of human insulin.” Escherichia coli.

Remember it

A plus B, no C: mature insulin keeps chains A and B, joined by S-S bridges, and drops the C peptide.