Humulin: why were the A and B chains made separately in E. coli and then joined, instead of one gene?
Nisha Verma12th Sample
4 Oct 2026 · 1 min read
Doing biotech applications for boards + NEET and I'm stuck on the insulin part. NCERT says insulin in the pancreas is made as a prohormone (proinsulin) that, like a proenzyme, needs processing before it becomes mature. The C peptide is removed during maturation, and what's left is the A chain (21 amino acids) and B chain (30 amino acids) held together by disulphide bridges. Then it says Eli Lilly prepared two DNA sequences corresponding to the A and B chains of human insulin and introduced them into plasmids of E. coli to produce the chains separately, which were then extracted and combined by creating disulphide bonds.
What I don't get:
- Why not just put the whole proinsulin gene into E. coli? Is it because bacteria can't remove the C peptide?
- In a PYQ-style question I saw the option "Humulin is produced from a single DNA sequence coding both chains". I marked it false but couldn't actually explain why.
- Humulin is "human insulin" but made by bacteria, so what makes it better than the old bovine/porcine insulin? I wrote "fewer allergies" in my notes but I'm not sure that's the real reason.
I made a table for the whole chapter and this is the only row still blank 😭 please explain like I haven't slept (true).
7 answers & replies
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[+][–]Tanvi Deshmukh12thSample· 15d ago
Okay so YES, bacteria don't have the machinery to chop the C peptide out of proinsulin!! That's the whole reason for the NCERT method. Proinsulin has A + C + B in one chain, and a cell has to cleave the C peptide to get the mature two-chain insulin. Eukaryotic pancreatic cells do that, E. coli doesn't. So the workaround was: make the A chain DNA and the B chain DNA separately, put each in its own plasmid, grow bacteria that make only that chain, purify both, and then join them with disulphide bonds in the lab.
Small extra for exams: mature insulin = 21 + 30 = 51 amino acids. Later methods did use proinsulin and cut it afterwards in the lab, but for NEET stick to the NCERT two-chain story.
[+][–]Nisha Verma12thSample· 15d agoAuthor
ok the "bacteria can't cut the C peptide" line finally made it click. adding it to the table now
[+][–]Siddharth RaoDropperSample· 15d ago
Your PYQ option is false for exactly the reason Tanvi said, plus one more thing worth writing in your notes. NCERT says "two DNA sequences", so two separate genes (A and B), and the chains are made separately and combined afterwards. If an option says "single DNA sequence coding both chains" or "chains are produced as one polypeptide", it's wrong.
I lost a mark on this last year because I read "combined by creating disulphide bonds" and my brain went "peptide bonds". Disulphide bridges between cysteine residues. Peptide bonds are inside each chain already. Examiners love that swap.
[+][–]Pooja Iyer12thSample· 15d ago
For your third doubt, "fewer allergies" is actually the real idea. 🙂 Insulin from cattle and pigs differs from human insulin by a few amino acids, and some people's immune system treats the foreign version as a threat and builds a reaction. Humulin has the human sequence, so there's much less chance of that. Also supply: you don't need to depend on slaughterhouse pancreas, bacteria can be grown in fermenters in bulk.
I remember it as "Humulin = HUMan insuLIN", so it's the same as ours. Dramatic but it works.
[+][–]Aarav Kulkarni11thSample· 15d ago
I'm in 11th so not sure, but isn't the C peptide still present in the mature insulin and that's why the A and B chains stay joined? Like it's the bridge between them? So Humulin = A chain + C peptide + B chain? Please correct me if I'm wrong, I made a flowchart on this and now I'm scared 😅
[+][–]Rahul DubeyDropperSample· 15d ago
Aarav, small fix: the C peptide is the part that gets removed. In proinsulin the order is B–C–A (C connects them in the single chain) and after cleavage the A and B chains are held together only by the two disulphide bridges between them (plus one inside A). So mature insulin has no C peptide at all. Redo the flowchart, you'll be fine.
On the topic heading itself, vaccines go the same direction as insulin: instead of growing a dangerous virus, you clone only the gene for one protein. The recombinant hepatitis B vaccine is the standard example: the gene for the surface antigen is put into yeast, yeast makes the protein in bulk, and that protein alone is the vaccine. No whole virus, so nothing to infect with.
[+][–]Aarav Kulkarni11thSample· 15d ago
Ohh that makes sense, I mixed up "connects" with "stays". Thanks Rahul bhaiya, flowchart redone 🙏
