Question

Insulin (bovine) has 51 amino acids in A and B polypeptides. The polypeptide A possesses amino acids
A. 31
B. 21
C. 20
D. 30

Answer 1

Hint: Insulin is a polypeptide hormone secreted by the beta- cells of Islets of Langerhans, which plays a major role in glucose homeostasis. Pro-insulin is made up of three chains, chain A, chain B, chain and C; chain A and C are connected by the chain B.

Complete answer:
Pancreas gland produces Insulin hormone. Insulin is specifically secreted from the beta-cells of pancreas.
Insulin is a protein formed of two polypeptide chains. The two polypeptide chains consist of 51 amino acids and designated as A (21 amino acid residues) and B (30 amino acid residues).
Insulin is first synthesized as preproinsulin made up of 110 amino acid residues. Preproinsulin is a biologically inactive precursor.
Functions:
> It lowers the blood glucose level by accelerating transport of glucose into the cell.
> Insulin stimulates glycogenesis.
> Insulin inhibits glycogenolysis.
> Insulin inhibits gluconeogenesis in the liver.
> It stimulates protein synthesis and inhibits protein degradation.
> The polypeptide A is made up of 21 amino acids.

Hence the correct answer is option B.

Note:
Points to remember:
> Preproinsulin→ Proinsulin→ Insulin.
> Finally formed insulin contains only A and C chains joined together by two disulphide bonds.
> It is also called hypoglycemic hormone because it lowers the blood sugar (glucose) level.
> Beta or B-cells constitute about 70 percent of pancreatic islet cells and secrete insulin.
> Glucagon is the antagonistic hormone to insulin secreted by alpha cells of Islets of Langerhans and contributes about 17 percent of islet cells of pancreas.
> Glucagon is made up of 29 amino acid residues.