LOs:
- A basic understanding of how monomeric G-proteins are activated & inactivated using GEFs & GAPs
- Structure of trimeric G proteins and of G-protein coupled receptors
- Activation of a GPCR & how this activates a trimeric G protein
- Examples of 2nd messengers & how cAMP is produced
- The effect of cAMP of PKA
- Basic structures of RTKs
- How RTKs are activated & are able to activate signal transduction pathways
What binds to G-protein coupled receptors initially?
Ligand (first messenger)
What happens to G-protein coupled receptors when ligand binds to it?
Activates an intracellular G-protein which in turn activates an enzyme which changes the conc of an intracellular second messenger (small molecule e.g. cAMP)
What is the structure of a G-protein coupled receptor?
- 7 transmembrane alpha helical regions
- 4 extracellular regions (on the outside of the cell between alpha regions)
- 4 cytosolic regions (loop into cell, same as above)
What are 4 examples of stimuli (ligands) in GPCR systems?
- Adrenaline
- Light
- fMet peptide
- ACh
For adrenaline (as a GPCR stimulus) name the:
Receptor, effector, 2nd messenger & response in the system:
Stimulus = adrenaline
Receptor = beta-adrenergic
Effector = adenylate cyclase
2nd messenger = cAMP
Response = glycogen breakdown
For light (as a GPCR stimulus) name the:
Receptor, effector, 2nd messenger & response in the system:
Stimulus = light
Receptor = rhodopsin
Effector = cGMP phosphodiesterase
2nd messenger = cGMP
Response = photo-reception
For fMet peptide (as a GPCR stimulus) name the:
Receptor, effector, 2nd messenger & response in the system:
Stimulus = fMet peptide
Receptor = chemotactic receptor
Effector = phospholipase C
2nd messenger =IP3 (Ca2+) & DAG
Response = chemotaxis
For ACh (as a GPCR stimulus) name the:
Receptor, effector, 2nd messenger & response in the system:
Stimulus = ACh
Receptor = muscarinic ACH receptor
Effector = K+ channel
2nd messenger = K+
Response = slowing pacemaker activity
What state are G-protein receptors in when GDP is bound?
“Inactive” - the system is considered off
What state are G-protein receptors in when GTP is bound?
“Active” - the system has been switched on
The active G-protein has intrinsic GTPase activity
What are the 2 types of GTP-binding proteins?
- Monometric
- Trimetric
How G-proteins regulate how fast they act?
By how quickly they switch out GDP for GTP
GTP activates the receptor - therefore the faster it is switched in the more the receptor will react
What are GAPs and what do they do?
GTPase activating proteins
Inactivate the G protein by stimulating hydrolysis of GTP to GDP
What are GEFs and what do they do?
Guanine nucleotide exchange factors (GEFs)
Activate the G protein by stimulating it to release GDP
What is the trimeric structure of G proteins?
3 subunits:
Alpha, beta & gamma
How are the beta & gamma subunits in a G-protein arranged?
Beta & gamma are very tightly bound
What is the role of the beta and gamma unit?
To encourage the breakdown of GTP to GDP
Where are G-proteins attached to?
The cytoplasmic face of the plasma membrane
Describe the process of G protein relay signals:
- G protein attached to cytoplasmic face of the plasma membrane
- Alpha & beta subunits have covalently attached lipid molecules that aid binding to the plasma membrane
- Activation of G protein, via receptor activation
- Inactivation of G protein by GTP hydrolysis by alpha-subunit
(beta-gamma complex may dissociate)
What is the process of G protein receptors creating a second messenger system?
- Binding of ligand to GPCR changes the conformation of the receptor
- Allows GDP to be exhanged for GTP (acts as a GEF)
- alpha & beta-gamma subunits may dissociate (but not always)
- Activated G protein elicits a second messenger system
What are DAG and IP3 derived from?
A lipid
What does DAG generally do?
Floats around & stimulates protein kinase C
What does IP3 generally do?
Diffuse thru cytosol & reach endoplasmic reticulum, will allow calcium to diffuse into the cell
