Insulin Structure: A and B Chains and Disulphide Bonds
Biology · Chemical Coordination and Integration · NEET
Mature insulin is a peptide hormone made of two polypeptide chains, called the A chain and the B chain. These two chains are joined by covalent disulphide bridges (-S-S-), not by hydrogen bonds. Memory hook: think "A-B tied by S-S" - two chains, sulphur-to-sulphur bridges.
Mature insulin has two chains, A and B, held together by covalent disulphide (-S-S-) bridges. Proinsulin additionally carries a C-peptide, which is removed during maturation.
Your doubts, answered
Are the A and B chains of insulin held by hydrogen bonds or disulphide bonds?
They are held by covalent disulphide bridges (-S-S-), formed between cysteine amino acids. This is the single most tested point. Hydrogen bonds only help fold a chain weakly; they do NOT link the two chains. If a NEET option says 'A and B chains are linked by hydrogen bonds', it is wrong.
How many chains does mature (functional) insulin have?
Only TWO chains: A and B. A common trap says insulin has three chains 'A, B and C'. That is false. There is no C chain in mature insulin. The C-peptide is present only in the earlier form (proinsulin) and is cut out before insulin becomes active.
What is proinsulin and what is C-peptide?
In humans, insulin is first made as a single long chain called proinsulin. Proinsulin has an extra stretch called the C-peptide. During maturation this C-peptide is removed (cleaved out), leaving the A chain and B chain joined by disulphide bridges. So C-peptide is present in proinsulin but absent in mature insulin.
Is insulin a peptide hormone or a steroid hormone?
Insulin is a peptide (protein) hormone. It is made of amino acids linked by peptide bonds. Because it is a protein, it can be digested by protease enzymes in the stomach - this is why it cannot be taken as a tablet and must be injected.
Where is insulin made and by which cells?
Insulin is secreted by the beta-cells of the Islets of Langerhans in the pancreas. (The alpha-cells secrete glucagon.) Genetically engineered human insulin used for treatment is produced in the bacterium E. coli - another NEET fact worth remembering.
⚠️ The NEET trap ✗ Mature insulin has three polypeptide chains - A, B and C - joined together. ✓ Mature insulin has only TWO chains, A and B, joined by disulphide bridges. The C-peptide belongs to proinsulin and is removed during maturation, so it is absent in mature insulin. 🧠 C-peptide is CUT out. If you see 'C chain' or 'three chains' in mature insulin, mark it wrong.
Real NEET questions
2016
The two polypeptides of human insulin are linked together by:
A · Hydrogen bonds
B · Phosphodiester bond
C · Covalent bond
D · Disulphide bridges ✓
Solution: The A and B chains of mature insulin are held together by covalent disulphide (-S-S-) bridges between cysteine residues. Hydrogen bonds only weakly stabilise folding; phosphodiester bonds link nucleotides (not proteins); 'covalent bond' is too vague. The specific answer is disulphide bridges.
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 a prohormone which contains an extra stretch of C-peptide
Solution: Mature insulin has only TWO chains (A and B), not three. The C-peptide is part of proinsulin and is cleaved off during maturation, so it is absent in mature insulin. The other three statements are correct, making option C the incorrect one asked for.
NEET
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: In functional insulin the A and B chains are joined by covalent disulphide bridges, NOT hydrogen bonds - so statement A is not correct. The other three are true: recombinant human insulin is made in E. coli, insulin is first synthesised as proinsulin, and proinsulin carries an extra C-peptide.
Solved Chemical Coordination and Integration NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Insulin has three disulphide bonds in total. Two are inter-chain bonds joining the A and B chains, and one is an intra-chain bond within the A chain itself. For NEET, the key point is that the A and B chains are linked by disulphide bridges.
Why does NEET keep asking about insulin structure?
Because it packs several traps into one line: chain number (two, not three), bond type (disulphide, not hydrogen), and the proinsulin/C-peptide story. NCERT states these plainly, so examiners test whether you read carefully. It has appeared in 2016, 2023 and other years.
What is the difference between proinsulin and mature insulin?
Proinsulin is a single continuous chain that includes the A chain, B chain and the connecting C-peptide. Mature insulin is the active form after the C-peptide is removed, leaving only the A and B chains held by disulphide bridges.
Does insulin contain a C chain?
No. There is no 'C chain' in mature insulin. There is a C-peptide, but it exists only in proinsulin and is cut out during maturation. Any statement giving mature insulin a C chain is wrong.
Is insulin secreted by alpha or beta cells?
Insulin is secreted by the beta-cells of the Islets of Langerhans. The alpha-cells secrete glucagon, which raises blood glucose - the opposite action to insulin.
