Question

During the processing of proinsulin into mature insulin.
A. C –peptide is added to proinsulin
B. C –peptide is removed from proinsulin
C. B –peptide is added to proinsulin
D. B –peptide is removed from proinsulin.

Answer 1

Proinsulin is a single polypeptide chain which is mainly composed of the B and A subunits of insulin joined by the peptide region. C –peptide and mature insulin are active biologically. an extra stretch called as C peptide is present in the proinsulin.

Complete step-by-step answer:
Insulin consists of two short polypeptide chains: chain A and chain B, which are linked together by disulphide bridges. C peptide is not present in the mature insulin and it is removed during the maturation process in which proinsulin is converted into insulin. This is the major difference between insulin and proinsulin. Human insulin is made up of 51 amino acids and they are arranged in two polypeptide chains: A chain having 21 amino acids and B chain with 30 amino acids.
A, B and C are the three chains which are present in proinsulin. In which C chain has 33 amino acids which are removed prior to insulin formation. Proinsulin is closely similar to insulin in many properties, such as solubility, isoelectric point. Proinsulin is converted to insulin during the maturation of secretory vesicles by the action of two proteases and conversion is inhibited by ionophores which is disrupted by the intracellular ${H^+}$ gradients. Proinsulin is the prohormone before the insulin which is developed in the beta cells of the islets of Langerhans, which are specialized regions of the pancreas. In humans, proinsulin is encoded by the gene called INS gene. The functional insulin produced is considered better than the ones earlier used by the diabetic patients because they are less expensive and cause fewer allergic reactions.

The correct answer is option B.

Note: Insulin is used for diabetes in which it is synthesised by a pro–hormone present in humans. The prohormones need to be processed to become a fully mature and functional hormone as they contain an extra stretch called the C peptide. In the mature insulin the C peptide is not present.