Read the paragraph given below and answer the question.
Insulin used for diabetes was earlier extracted from slaughtered cattle and pigs. Insulin from an animal source, though caused some patients to develop allergy or other type of reactions. Insulin consists of two short polypeptide chains: chain A and chain B, that are linked together by disulphide bridges. In mammals, including human, insulin is synthesised as a pro-hormone.
Proinsulin is made up:
(A) Peptide A
(B) Free peptide
(C) Peptide B
(D) Peptide C
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (C), (D) only
Insulin is first made as a single chain called pro-insulin (a pro-hormone). As the figure in the passage shows, pro-insulin has three parts joined in one chain: peptide A, peptide B and, linking them, an extra stretch called peptide C.
To become mature insulin, pro-insulin is processed: peptide C is cut out, and peptides A and B stay joined by disulphide bridges. The cut-out peptide C is then the free C peptide. So the free peptide is a product of processing, not a part of pro-insulin.
(A) Peptide A — True. Chain A of insulin is already present in pro-insulin.
(B) Free peptide — False. The C peptide becomes free only after it is removed during maturation; inside pro-insulin it is still part of the chain.
(C) Peptide B — True. Chain B of insulin is already present in pro-insulin.
(D) Peptide C — True. The extra stretch that links A and B in pro-insulin and is removed when insulin matures.
NCERT Extract:
Genetically Engineered Insulin – “… 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.”