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

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.