Effector mechanisms

Question 1

A GPCR activates a G-protein which activates an …

Answer 1

Effector

Question 2

An effector can be … (2)

Answer 2

an enzyme or ion channel

Question 3

Effectors can be enzymes. Give 4 examples of effectors that are enzymes

Answer 3

Adenylyl cyclase
Phospholipase C
Phosphoinositide 3-kinase (PI3K)
cGMP Phosphodiesterase

Question 4

What does adenylyl cyclase do?

Answer 4

converts ATP to cyclic AMP (cAMP)

Question 5

Give 4 examples of effectors that are enzymes, and name their second messengers

Answer 5

• adenylyl cyclase produces cAMP
• phospholipase C produces IP3 and DAG (via PIP2)

Question 6

Example of ion channel effectors

Answer 6

Voltage-operated Ca2+ channels

GIRKS (G protein-regulated inwardly-rectifying K+ channels)

Question 7

Describe how adenylyl cyclase is stimulated to produce cAMP (7)

Answer 7

• Agonist binds to the GPCR, activating it
• The GPCR interacts with the G protein (Gs protein with an alpha s subunit)
• This Gs-protein exchanges GDP for GTP
• The active component of the G Protein (αlpha s with GTP bound to it) is released
• That moves along the plane of the membrane, interacts with adenylyl cyclase
  and activates it.
• Adenylyl cylase then converts ATP to cyclic AMP (the pathway second messanger)
• cAMP can interact with a number of cellular targets to regulate cellular activity e.g. PKA

Question 8

What is the most important effector of cyclic AMP?

Answer 8

Cyclic AMP-dependant protein kinase (PKA)

Question 9

Describe how adenylyl cyclase is inhibited from producing cAMP (6)

Answer 9

• Agonist binds to the GPCR, activating it
• The GPCR interacts with the G protein (Gi protein with an alpha i subunit)
• This Gi-protein exchanges GDP for GTP
• The active component of the G Protein (αlpha i with GTP bound to it) is released
• That moves along the plane of the membrane, interacts with adenylyl cyclase
  and inhibits the activity of it.
• cAMP is thus not produced from ATP so other molecules, eg. PKA, aren’t activated

Question 10

What does Gs do?

Answer 10

GDP –> GTP on alpha s unit
STIMULATES adenylyl cyclase
ATP –> cAMP
cAMP activates PKA

Question 11

Gs coupled receptors exampels

Answer 11

B-adrenoreceptors
D1-dopamine receptors
H2-histamine receptors

Question 12

What do Gi do?

Answer 12

GDP –> GTP alpha i unit
INHIBITS adenylyl cyclase
ATP not converted to cAMP
PKA not activated

Question 13

Gi coupled receptors examples

Answer 13

a2 adrenoreptors
D2 dopamine receptors
µ-opioid receptors

Question 14

Cyclic AMP exerts the majority of its actions through …

Answer 14

cyclic AMP-dependent protein kinase A (PKA)

Question 15

Describe the structure of PKA

Answer 15

Heterotetramer made up of 2 regulatory subunits and 2 catalytic subunits

Question 16

2 subunits of PKA

Answer 16

2x R (regulatory)
2x C (catalytic)

Question 17

What does R unit do?

Answer 17

Inhibits PKA by binding to C (catalytic) subunits

Question 18

What do C units do?

Answer 18

Covalently modify target proteins in a cell via phosphorylation

Question 19

1) When does cAMP activate PKA?
2) How does cAMP activate PKA?

Answer 19

1) When levels of cAMP are high

2) - When cAMP is high, it binds to R (regulatory) subunits on PKA

• the interactions between the R and C subunits are weakened
• C (catalytic) units are released and now free to phosphorylate target proteins in the cell

Question 20

What happens when cAMP is low? (2)

Answer 20

• The R subunits of PKA bind to the C subunits
• To inhibit their action of phosphorylating target proteins in the cell

Question 21

What are the second messengers that are produced from phospholipase C?

Answer 21

• IP3
• DAG

Question 22

Describe how phospholipase C (PLC) is regulated to produce IP3 and DAG, the 2 second messengers (7)

Answer 22

• Agonist binds to the GPCR, activating it
• The GPCR interacts with the G protein (Gq protein with an alpha q subunit)
• This Gq-protein exchanges GDP for GTP
• The active component of the G Protein (αlpha q with GTP bound to it) is released
• That moves along the plane of the membrane, interacts with phospholipase C and activates it.
• Phospholipase C then interacts with a membrane phospholipid, PIP2
• PLC cleaves PIP2 into IP3 and DAG, its 2 second messengers

Question 23

How do GaQ work?

Answer 23

GDP –> GTP on alpha q unit
Activates Phospholipase C
PIP2 is cleaved into IP3 and DAG

Question 24

What do IP3 do? (3)

Answer 24

IP3:
- Moves freely through cytoplasm
- Binds to IP3 receptors on ER
- Opens Ca2+ channels causing Ca2+ to be released into the cytoplasm down its conc. grad.

Question 25

What does DAG do? (4)

Answer 25

DAG: - It remains in the membrane - Binds to and activates PKC (protein kinase C) - PKC phosphorylates key substrate proteins (- Sometimes calcium helps to activate PKC activity)

Question 26

What can sometimes help DAG to activate protein kinase C activity?

Answer 26

Calcium

Question 27

What can IP3 receptor activation lead to?

Answer 27

- Can lead to a large increase in cytoplasmic Ca2+ conc within a few seconds of agonist action

Question 28

Gaq coupled receptors examples (3)

Answer 28

a1 adrenoreceptors M1 muscarinic receptors H1 histamine receptors

Question 29

Adenylyl cyclase activating many cAMP molecules which activate many PKA molecules which activate many target proteins is an example of ...

Answer 29

Signal amplification

Question 30

What is the key feature of signalling transduction pathways? Describe this (4)

Answer 30

- Signal amplification - It means you require a very small change in the initial stimulus to cause a physiologically relevant change in the cell - Few ligands = massive cellular response - Efficient use of body's molecules

Question 31

Give an example of signal amplification in the body (4)

Answer 31

- For example, a few molecules of adrenaline binding to cell surface β-adrenoceptors may cause a relatively massive cellular response - The β-adrenoceptor → Gs protein → adenylyl cyclase part of the cascade causes relatively little amplification. - The activation of adenylyl cyclase generates many molecules of cyclic AMP which then activate the enzyme PKA. - PKA is also an enzyme, the it can efficiently phosphorylate lots of substrate proteins