Biotechnological Applications In Medicine

Insulin is initially synthesised as a single-chain precursor called proinsulin, which contains the A-chain and B-chain joined together by an extra stretch known as the C-peptide, as outlined in NCERT Class 12, Chapter 11 (Biotechnology and its Applications). For the hormone to become biologically active, this connecting C-peptide is enzymatically excised, leaving the A-chain and B-chain linked only by disulphide bridges. This processing requirement was a key challenge in producing recombinant insulin, which scientists overcame by synthesising the two chains separately in bacteria and then joining them. The A-chain cannot be the answer because it is retained as part of mature insulin, not removed. Likewise, the B-chain remains in the functional hormone, so eliminating it would destroy the molecule. Removing only the signal peptide is incomplete, because while a signal sequence is cleaved during secretion, the defining maturation step that converts proinsulin to insulin is the removal of the C-peptide. The C-peptide also plays a role in ensuring the correct folding of the precursor before the disulphide bridges form, after which it is no longer needed. A biological consistency check confirms this: mature insulin must retain both functional chains, so only the non-essential connecting peptide can be the segment that is cut out.