Question

Total Amino acids in insulin(bovine) are in 51 polypeptide chains A and B. How many amino acids are there in chain A?  

A. 21

B. 20

C. 30

D. 31

Answer 1

Hint: Insulin is a hormone produced by the pancreatic gland. It is secreted exclusively by the beta cells of the pancreas. The blood glucose level of the blood is maintained by insulin.

Complete answer:
Insulin consists of two peptide chains referred to as chain A and chain B. A and B chains are bound together by two disulfide bonds, and an additional disulfide is formed within the A chain. 

In most organisms, the A chain consists of 21 amino acids and the B chain consists of 30 amino acids. While the sequence of insulin amino acids varies from one individual to another, the positions of the three disulfide bonds, all ends of the A chain and the C-terminal residues of the B chain remains the same in all. 

Insulin is synthesised in the beta cells of the Langerhans islets. First, the Insulin mRNA is converted into a single chain precursor called preproinsulin. The signal peptide is then withdrawn at the N-terminus after injection at the endoplasmic reticulum. This is producing proinsulin. 

In the endoplasmic reticulum, the endopeptidases decompose the binding peptide (c-peptide) between the chains A and B. This splits a single chain into two chains (A and B) which are bound together by disulfide bridges. This gives way to the mature form of insulin.

Hence, the correct answer is (A).

Note: Bovine Insulin, or beef insulin, is a two-chain polypeptide hormone that has been produced in pancreatic beta cells. Bovine insulin varies from human insulin in just three residues of amino acids, similar enough to be clinically active in humans.