L14: Insulin Signalling

Question 1

Descibe the structure of the final insulin molecule

Answer 1

Consists of 51 AAs in 2 chains joined by disulfide bonds, ~6kDa

Question 2

Outline insulin storage and release

Answer 2

• Released by pancreatic beta cells in response to elevated glucose levels
• Stored in a hexatrimeric form complexed with Zn

Question 3

What are insulin’s 3 forms?

Answer 3

1. Prepoinsulin: in ER, 24 AAs cleaved as signal peptide is forming
2. Proinsulin: further processed in golgi, where terminal C peptide is removed
3. Activate insulin: stored in secretory granules

Question 4

What happens to blood glucose after eating?

Answer 4

It increases so insulin increases

Question 5

What is the function of glucokinase?

Answer 5

Is a glucose sensor for insulin secretion

Question 6

How does glucose generate energy?

Answer 6

Glucose is phosphorylated to glucose 6-P and metabolised by glycolysis and mitochondrial oxidation

Question 7

What is the functino of insulin receptor substrates (IRSs)?

Answer 7

Post receptor they become a docking protein for proteins that contain SH2 domains
- this contains PTB (phosphotyrosine binding) and PH domains

Question 8

What happens to SH2 domains in proteins?

Answer 8

• phosphorylated on tyrosines by activated receptor

- phosphorylation on serines may inhibit action

Question 9

What is the function of Src Homology 2 domains (SH3)?

Answer 9

Bind phosphotyrosine residues surrounded by unique protein sequences, i.e. on SH2 domain in P13k will not bind to phosphotyrosine recognised by Grb2

Question 10

What do SH3s specifically bind to?

Answer 10

Proline regions i.e. SH3 domain in Grb2 will only bind Sos (son of sevenless)

Question 11

Explain the ras-dependent pathway

Answer 11

• Grb2 SH2/SH3 domain contains proteins that interacts with Sos
• Sos acts as GDP/GTP exchange factor for Ras (small GTP binding protein) and is activated
• Ras activates Raf which activates MAPKK and MAPK, driving differentiation, survival and growth

Question 12

Explain the ras-independent pathway

Answer 12

IRS acts on P13k causing phosphorylation then activates Akt (inv. in GLUT4 movement) and acts on GSK3 to inhibit glycogen synthase (its action) so glycogen synthesis increases

Question 13

What is a further function of Akt?

Answer 13

It activates mTor which regulates protein synthesis, PDE3B which acts on hormone sensitive lipase (HPL) which controls fatty acid so stops fatty acids being released into blood

Question 14

How can glucose be stored?

Answer 14

• can be converted and stored as glycogen
• converted to pyruvate and stored in protein or fat
• ised in the tricarboxylic acid (TCA) cycle

Question 15

How can diabetes/ starvation affect insulin?

Answer 15

There is increased

• free fatty acids levels
• beta oxidation, breakdown of FA to acetyl CoA for TCA cycle (PDH inhbited by acetyl CoA so no no glucose produces pyruvate)
• fatty acids converted to muscle
• citrate (as TCA cycle active), which leaves mitochondria and inhibits PFK hence reducing glucose utilisation

Question 16

What is the overall effect of insulin on glucose?

Answer 16

Insulin shuts down glucose production from liver and stimulates glucose uptake in tissue

Question 17

What is the overall effect of glucagon on glucose

Answer 17

Glucagon increase breakdown of glycogen to give glucose & stimulates release of FFa from adipocytes

Question 18

What are consequences of failure to lower blood glucose levles?

Answer 18

• stops liver producing glucose
• stimulates muscles and adipocytes to take up glucose
• blood glucose levels elevated leading to end-organ damage