Question

Which of the following statements regarding the structure of proinsulin and mature insulin are not correct?
(i) Proinsulin is made up of three polypeptide chains- A, B and C.
(ii) C - polypepfidd chain with 33 amino acids is removed prior to insulin formation.
(iii) Mature insulin is made up of 51 amino acids arranged in two polypeptide chains- A and B.
(iv) Polypeptide chain A has 30 amino acids and polypeptide chain B has 21 amino acids.
(v) Polypeptide chains A and B are interconnected by only one S - S linkage.

• A. (i) and (ii)
• B. (iii) and(iv)
• C. (iv) and (v)
• D. (iii), (iv) and (v)

Answer 1

Answer: (C)

Mature insulin consists of two polypeptide chains (51 amino acids), A and B consisting of 21 and 30 amino acids respectively. These A and B chains are linked together by disulfide bonds. In mammals, insulin is synthesised from a preprohormone called preproinsulin. This inactive form has 110 amino acids with the signal peptide or C peptide. Proinsulin is formed by removal of the signal peptide and mature insulin is formed by removal of C peptide. This is followed by the formation of disulfide bonds between A and B chains leading to the formation of active insulin consisting of 51 amino acids.

Eli Lilly, an American company, first prepared DNA sequences corresponding to A and B chains of human insulin and introduced them in E. coli to produce insulin chains through a transformation with the pBR 322 vector.
Hence, in the question the product A is polypeptide chain C.