Structure of Insulin

Ans. B i.e. 10 minutes

Degradation of insulin

• Once an insulin molecule has docked onto the receptor and effected its action, it may be released back into the extracellular environment, or it may be degraded by the cell.
• The two primary sites for insulin clearance are the liver and the kidney.
• The liver clears most insulin during first-pass transit, whereas the kidney clears most of the insulin in systemic circulation.
• Degradation normally involves endocytosis of the insulin-receptor complex, followed by the action of insulin-degrading enzyme.
• An insulin molecule produced endogenously by the pancreatic beta cells is estimated to be degraded within about one hour after its initial release into circulation (insulin half-life – 4-6 minutes)

Ans. is `b’ i.e., Zn⁺⁺

Zinc ions are essential for the formation of hexameric insulin and hormone crystallization.

Following the synthesis of proinsulin, zinc promotes the formation of proinsulin hexamers and increases its solubility before conversion to insulin.

Proinsulin binds 30 zinc ions, of which 2 to 4 are coordinated within the molecule. These zinc ions appear to be important for the solubility of proinsulin hexamers.

With the removal of C-peptide from each proinsulin monomers, the resulting insulin increases its coordination of zinc to up to six ions per hexamer, which decreases its solubility and increases its crystallization within the secretory vesicle.

The insulin, stored as crystalline hexamer, is resistant from proteolytic attack within the vesicle.

Upon release of insulin into the bloodsteam, zinc is also released and the insulin becomes soluble in the blood.

• Insulin is two-chain polypeptide hormone, with 51 amino acids.
• Structure: 
  • 2 chains – A & B chain.
  • A – chain – 21 amino acids; B – chain – 30 amino acids.
• Half-life-: About 5 mins