Week 4

Question 1

What is the function of insulin?

Answer 1

• Stimulation of glucose uptake by cells from systemic circulation
• Suppression of hepatic gluconeogenesis
  -(generation of glucose from non-
  carbohydrate sources)
• Suppression of glycogenolysis
  - breakdown of glycogen to
  glucose in muscle and liver

REGULATE GLUCOSE HOMEOSTASIS

Question 2

What two polypeptide chains does the insulin molecule have?

Answer 2

• A chain (21 aa) and B chain (30aa)

- Linked by two disulphide bridges

Question 3

What chromosome is insulin gene located?

Answer 3

Chromosome 11

Question 4

Breakdown the insulin biosynthesis

Answer 4

1. mRNA transcription
   
   • Excision of intervening sequence
2. Translation of mRNA (L,B,C,A) = pre proinsulin
   -transport of pre-proinsulin to lumen
   of ER
   
   • (L signal peptide)

3. Signaal peptide (L) removed
     = proinsulin 
    Contains three chains A B C
    Disulfide bonds and amino acids links 
    hold chains together

4. Pro insulin goes through golgi compartments
   Endopeptidases cleave Lys‐Arg and Arg‐Arg
   
   • Carboxypeptidases cleave off terminal Lys
     and Arg amino acids
     - Liberate C‐peptide
     = final insulin consisting of A and B chains held
     together by disulfide bond

Question 5

How is insulin secrected?

Answer 5

1. Co‐precipitates with zinc ions as microcrystals in secretory granules
2. Microtubules transport granules to plasma membrane
3. Granules are released by exocytosis

Question 6

What happens when glucose is increase (e.g. you just ate a meal)

Answer 6

1. Glucose binds to GLUT-2 receptor on plasma membrane and enters the cell
2. glucose is converted to glucose-6-phosphate via glycolysis
3. glucose will be metabolised to generate a lot of ATP
4. ATP triggers ATP-sensitive K+ channel to block channel, so K+ in cell goes up and results in depolarisation
5. voltaged gated calcium channel is triggered to open and allow calcium into the cell. Triggers vesicles that contain insulin to be transported to plasma membrane and released into blood stream.

Question 7

What does an insulin receptor contain?

Answer 7

4 subunits (two alpha and two beta)
Tyrosine kinase

Question 8

What happens when insulin binds?

Answer 8

When insulin binds we get:

• Autphosphorylation of the tyosine residues of the receptor (aka becomes phosphorylated the tyrosine kinase)
• once phophorylated binds to protein IRS
• the IRS becomes phosphorylated

Question 9

Explain what happens to P13K and the cascade of events

Answer 9

• PI3K converts PIP2 to PIP3
  
  • Phosphatidylinositol 4,5‐bisphosphate
  • Phosphatidylinositol (3,4,5)‐trisphosphate
• PIP3 activates protein kinase B (PKB/Akt) via its PH domain
• PIP3 activates phosphatidylinositol dependant protein kinase (PDK1) via PH domain
• PDK1 phosphorylates PKB/Akt, releases it to the cytosol and nucleus for signaling

Question 10

Name the three events that PKB/Akt activates

Answer 10

• • Gluocse uptake by glucose transporters ( GLUT4)
• • Protein synthesis
    -Stimulation of glycogen synthase
    > conversion of glucose to glycogen (storage)

Question 11

Summary of insulin receptor action

Answer 11

1. Phosphorylation of insulin receptor (its a tyrosine kinase)
2. IRS-1 is phosphorylated
3. IRS-1 binds P13K: phosphorylates phospholipids to PIP3
4. PIP3 recruits PDK1 and PKB/Akt to the membrane
5. PKB/Akt releases to activate signalling pathways