Proinsulin, C-Peptide and How Mature Insulin Forms
Biology · Biotechnology and Its Applications · NEET
In humans, insulin is first made as a pro-hormone called proinsulin. Proinsulin has an extra stretch called the C-peptide. During maturation this C-peptide is cut out (removed), leaving only two chains, A and B, joined by disulphide bridges — this is mature, functional insulin. Memory hook: "Pro has C, Mature is free" — proinsulin carries C-peptide, mature insulin does not.
Insulin is first made as proinsulin (A + B + C-peptide). During maturation the C-peptide is removed, leaving only the A and B chains held together by disulphide (S–S) bridges — the mature, functional insulin.
Your doubts, answered
Does mature insulin contain the C-peptide?
No. The C-peptide is present only in proinsulin (the pro-hormone). It is cut off during maturation, so mature insulin does NOT have the C-peptide. Mature insulin has just two chains, A and B.
What exactly is proinsulin?
Proinsulin is the pro-hormone (immature form) of insulin. Like a pro-enzyme, it must be processed before it works. It is a single stretch that contains the A chain, the B chain, and an extra C-peptide connecting them. NCERT calls insulin's early form a pro-hormone.
Why does the C-peptide have to be removed?
The C-peptide holds the chains in a foldable form while insulin is being made, but it is not part of the working hormone. It is removed during maturation so that only chains A and B remain, joined by disulphide bridges. Only this two-chain form is fully mature and functional.
How many polypeptide chains does mature insulin have?
Two — chain A and chain B. A common NEET trap says 'three chains A, B and C'. That is wrong: the C-peptide is gone in mature insulin, so only A and B remain, linked by disulphide bridges.
Does insulin made by rDNA technology (in E. coli) have a C-peptide?
No. In the recombinant (Eli Lilly) method, chains A and B are produced separately in E. coli, extracted, and joined by creating disulphide bonds. There is no proinsulin stage and no C-peptide in this rDNA product.
What links the A and B chains together?
Disulphide bridges (–S–S– bonds), which are covalent. A frequent trap option says 'hydrogen bonds' — that is incorrect for the inter-chain link in insulin.
⚠️ The NEET trap ✗ Mature insulin has three chains — A, B and C-peptide — held together by hydrogen bonds. ✓ Mature insulin has only two chains, A and B, linked by disulphide bridges. The C-peptide belongs to proinsulin and is removed during maturation. 🧠 NTA loves two traps in one line: (1) counting three chains, and (2) saying hydrogen bonds. Remember 'two chains, disulphide, no C'.
Real NEET questions
2021
With regard to insulin choose the correct options. (a) C-peptide is not present in mature insulin. (b) The insulin produced by rDNA technology has C-peptide. (c) The pro-insulin has C-peptide. (d) A-peptide and B-peptide of insulin are interconnected by disulphide bridges.
A · (a), (c) and (d) only ✓
B · (a) and (d) only
C · (b) and (d) only
D · (b) and (c) only
Solution: Proinsulin carries an extra C-peptide (c is true). This C-peptide is removed during maturation, so it is absent in mature insulin (a is true). Chains A and B are joined by disulphide bridges (d is true). Statement (b) is false — rDNA insulin is assembled from separate A and B chains and has no C-peptide. So (a), (c) and (d).
2023
Select the incorrect statement regarding the chemical structure of insulin.
A · C-peptide is not present in mature insulin molecule.
B · Polypeptide chains A and B are linked by disulphide bridges.
C · Mature insulin molecule consists of three polypeptide chains — A, B and C. ✓
D · Insulin is synthesized as prohormone which contains extra stretch of C-peptide.
Solution: Mature insulin has only two chains, A and B, joined by disulphide bridges. The C-peptide is removed during maturation and is absent in mature insulin, so 'three chains A, B and C' is incorrect. The other three statements are all correct per NCERT.
2016
Which of the following statements is not correct?
A · The functional insulin has A and B chains linked together by hydrogen bonds ✓
B · Genetically engineered insulin is produced in E. coli
C · In man insulin is synthesised as a proinsulin
D · The proinsulin has an extra peptide called C-peptide
Solution: The A and B chains are linked by disulphide bridges, not hydrogen bonds — so the first statement is NOT correct. The other three are true: rDNA insulin is made in E. coli, insulin is first synthesised as proinsulin, and proinsulin carries an extra C-peptide.
Solved Biotechnology and Its Applications NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT does not require this, but clinically the C-peptide is a marker: because natural proinsulin releases C-peptide while rDNA insulin does not, doctors can measure how much insulin a person's own body is making. For NEET, just remember: C-peptide is in proinsulin, removed during maturation, absent in mature and in rDNA insulin.
Is proinsulin the same as pro-enzyme?
They are analogous. NCERT says the pro-hormone (proinsulin) is 'like a pro-enzyme' — both are inactive precursors that must be processed before they become active. Proinsulin is processed by removing the C-peptide.
Why can't animal insulin simply replace human insulin?
Insulin from cattle and pigs is slightly foreign to the human body and caused allergic reactions in some patients. This is why human insulin made by rDNA technology (Humulin) was developed — the C-peptide/maturation topic connects directly to this rDNA production story.
In which cells and organ is insulin made?
Insulin is made by the beta-cells of the Islets of Langerhans in the pancreas. It is first synthesised there as proinsulin, then matured by removing the C-peptide.
