Effector mechanisms Flashcards
A GPCR activates a G-protein which activates an …
Effector
An effector can be … (2)
an enzyme or ion channel
Effectors can be enzymes. Give 4 examples of effectors that are enzymes
Adenylyl cyclase
Phospholipase C
Phosphoinositide 3-kinase (PI3K)
cGMP Phosphodiesterase
What does adenylyl cyclase do?
converts ATP to cyclic AMP (cAMP)
Give 4 examples of effectors that are enzymes, and name their second messengers
- adenylyl cyclase produces cAMP
- phospholipase C produces IP3 and DAG (via PIP2)
Example of ion channel effectors
Voltage-operated Ca2+ channels
GIRKS (G protein-regulated inwardly-rectifying K+ channels)
Describe how adenylyl cyclase is stimulated to produce cAMP (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
What is the most important effector of cyclic AMP?
Cyclic AMP-dependant protein kinase (PKA)
Describe how adenylyl cyclase is inhibited from producing cAMP (6)
- 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
What does Gs do?
GDP –> GTP on alpha s unit
STIMULATES adenylyl cyclase
ATP –> cAMP
cAMP activates PKA
Gs coupled receptors exampels
B-adrenoreceptors
D1-dopamine receptors
H2-histamine receptors
What do Gi do?
GDP –> GTP alpha i unit
INHIBITS adenylyl cyclase
ATP not converted to cAMP
PKA not activated
Gi coupled receptors examples
a2 adrenoreptors
D2 dopamine receptors
µ-opioid receptors
Cyclic AMP exerts the majority of its actions through …
cyclic AMP-dependent protein kinase A (PKA)
Describe the structure of PKA
Heterotetramer made up of 2 regulatory subunits and 2 catalytic subunits
2 subunits of PKA
2x R (regulatory) 2x C (catalytic)
What does R unit do?
Inhibits PKA by binding to C (catalytic) subunits
What do C units do?
Covalently modify target proteins in a cell via phosphorylation
1) When does cAMP activate PKA?
2) How does cAMP activate PKA?
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
What happens when cAMP is low? (2)
- The R subunits of PKA bind to the C subunits
- To inhibit their action of phosphorylating target proteins in the cell
What are the second messengers that are produced from phospholipase C?
- IP3
- DAG
Describe how phospholipase C (PLC) is regulated to produce IP3 and DAG, the 2 second messengers (7)
- 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
How do GaQ work?
GDP –> GTP on alpha q unit
Activates Phospholipase C
PIP2 is cleaved into IP3 and DAG
What do IP3 do? (3)
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.
What does DAG do? (4)
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)
What can sometimes help DAG to activate protein kinase C activity?
Calcium
What can IP3 receptor activation lead to?
- Can lead to a large increase in cytoplasmic Ca2+ conc within a few seconds of agonist action
Gaq coupled receptors examples (3)
a1 adrenoreceptors
M1 muscarinic receptors
H1 histamine receptors
Adenylyl cyclase activating many cAMP molecules which activate many PKA molecules which activate many target proteins is an example of …
Signal amplification
What is the key feature of signalling transduction pathways? Describe this (4)
- 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
Give an example of signal amplification in the body (4)
- 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
