Select the correct set of the names labelled A, B, C and D in the given diagram.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A–Proinsulin, B–A-peptide, C–B-peptide, D–Free C-peptide
Insulin is made in the body as a pro-hormone, proinsulin. Proinsulin has chain A, chain B and an extra stretch called the C peptide. During maturation the C peptide is removed. Chains A and B stay joined by disulphide bridges (S–S) and form mature insulin.
A — this is Proinsulin. It is the whole loop before the arrow: chain A, chain B and the C peptide still joined as one molecule.
B — this is the A-peptide. It is the upper chain after processing. It carries an S–S bridge within itself and is linked to the lower chain by two more.
C — this is the B-peptide. It is the lower chain, joined to chain A by the disulphide bridges.
D — this is the Free C-peptide. It is the curved strand set apart below. It has been cut out and is not part of mature insulin.
NCERT Extract:
Genetically Engineered Insulin – “… Insulin consists of two short polypeptide chains: chain A and chain B, that are linked together by disulphide bridges (Figure 10.3). 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.”
Figure 10.3 Maturation of pro-insulin into insulin (simplified)