Select the correct set of the names labelled A, B, C and D in the given diagram.

A–Proinsulin, B–A-peptide, C–B-peptide, D–Free C-peptide

Earlier, insulin used for diabetes treatment was extracted from the pancreas of slaughtered cattle and pigs. However, this animal-derived insulin could cause allergic reactions or other adverse responses in some patients due to the foreign proteins present. Insulin is composed of two short polypeptide chains, chain A and chain B, connected by disulfide bridges. In mammals, including humans, insulin is initially synthesized as a pro-hormone, which includes an additional segment known as the C peptide. During maturation into insulin, this C peptide is removed, and the insulin becomes fully mature and functional.

The main challenge faced in producing insulin using recombinant DNA (rDNA) techniques was achieving the proper assembly of insulin into its mature form. This process involved ensuring that the two polypeptide chains (A and B) correctly combined and formed the necessary disulfide bridges to create functional insulin. Overcoming this challenge was critical in the successful production of human insulin using rDNA technology, which has become the standard insulin treatment for diabetic patients due to its higher purity and reduced risk of complications compared to animal-derived insulin.