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ICPP 2023 > Effector mechanisms > Flashcards

Effector mechanisms Flashcards

1
Q

A GPCR activates a G-protein which activates an …

A

Effector

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2
Q

An effector can be … (2)

A

an enzyme or ion channel

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3
Q

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

A

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

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4
Q

What does adenylyl cyclase do?

A

converts ATP to cyclic AMP (cAMP)

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5
Q

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

A
  • adenylyl cyclase produces cAMP
  • phospholipase C produces IP3 and DAG (via PIP2)
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6
Q

Example of ion channel effectors

A

Voltage-operated Ca2+ channels

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

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7
Q

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

A
  • 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
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8
Q

What is the most important effector of cyclic AMP?

A

Cyclic AMP-dependant protein kinase (PKA)

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9
Q

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

A
  • 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
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10
Q

What does Gs do?

A

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

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11
Q

Gs coupled receptors exampels

A

B-adrenoreceptors
D1-dopamine receptors
H2-histamine receptors

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12
Q

What do Gi do?

A

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

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13
Q

Gi coupled receptors examples

A

a2 adrenoreptors
D2 dopamine receptors
µ-opioid receptors

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14
Q

Cyclic AMP exerts the majority of its actions through …

A

cyclic AMP-dependent protein kinase A (PKA)

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15
Q

Describe the structure of PKA

A

Heterotetramer made up of 2 regulatory subunits and 2 catalytic subunits

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16
Q

2 subunits of PKA

A 
2x R (regulatory)
2x C (catalytic)
17
Q

What does R unit do?

A

Inhibits PKA by binding to C (catalytic) subunits

18
Q

What do C units do?

A

Covalently modify target proteins in a cell via phosphorylation

19
Q

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

A

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
20
Q

What happens when cAMP is low? (2)

A
  • The R subunits of PKA bind to the C subunits
  • To inhibit their action of phosphorylating target proteins in the cell
21
Q

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

A
  • IP3
  • DAG
22
Q

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

A
  • 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
23
Q

How do GaQ work?

A

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

24
Q

What do IP3 do? (3)

A

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.

25
Q

What does DAG do? (4)

A

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)

26
Q

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

A

Calcium

27
Q

What can IP3 receptor activation lead to?

A
  • Can lead to a large increase in cytoplasmic Ca2+ conc within a few seconds of agonist action
28
Q

Gaq coupled receptors examples (3)

A

a1 adrenoreceptors
M1 muscarinic receptors
H1 histamine receptors

29
Q

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

A

Signal amplification

30
Q

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

A
  • 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
31
Q

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

A
  • 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

ICPP 2023 (31 decks)

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