G-Protein Couple Receptors, Molecular Mechanisms Of Flashcards Preview

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Flashcards in G-Protein Couple Receptors, Molecular Mechanisms Of Deck (36):

List 4 major targets of drugs

Ion channels


Define: Receptors

Molecules through which soluble physiological mediators produce their desired effects (i.e. agonists)
= binding
= conformational change
= signal transduction
= response


Name the 4 types of receptor

Ligand-gated ion channels
e.g. nicotinic acetylcholine receptors
G protein-coupled receptor
e.g. adrenoceptors, opioid receptors (morphine)
Enzyme coupled receptors
Nuclear receptors


Where are GPCRs found?

GPCRs are normally found at the cell surface
Intracellular Ca2+ made from action


Describe the structure of a GPCR

7 transmembrane domains - single polypeptide chain crosses membrane 7 times = forms a core


How do GPCRs work?

The receptor couples to G proteins to initiate signal transduction
Activated by diverse signals e.g. photons, hormones, peptides & peptidases


What are the 2 classes of GPCR?

Olfactory = sense receptors


Why are GPCRs important?

Make up over half the current drug targets
Largest family of cell surface receptor
120 orphan receptors


How many classes of GPCR are there?

A, B, C, D, E, F
Classed according to primary amino structure


What is the name of class A GPCRs?

Rhodopsin-like receptors
Easily targeted by antagonists


What is the name of class B GPCRs?

Secretin-like receptors


What is the name of class C GPCRs?

Metabotropic glutamate receptors


Describe class A rhodopsin like GPCRs

7 transmembrane domains
Short N terminus
Agonists bind with extracellular loops and transmembrane domains
e.g. β-adrenoceptors, histamine receptors


Describe class B secretin-like GPCRs

7 transmembrane domains
Larger, globular N-terminus
Agonists bind to N-terminus or extracellular loops
e.g. secretin receptors, glucagon receptors


Describe class C metabotropic glutamate GPCRs

Very large N terminus
Exist at cell surface as dimers - form obligatory dimers when agonists bind
e.g. GABA(B) receptors, metabotropic glutamate receptors


How are GPCRs activated?
i.e. how are signals conveyed from one side of the plasma membrane to the other

Agonist binds to the GPCR
Conformational change occurs = new surface charge (new AAs, new charges uncovered, new shape)
Attracts heterotrimeric G proteins which is formed of 3 sub-units (α, β, γ)
Receptor can now bind G proteins at C terminal as the receptor shape has changed
GTPase activity activated (part of heterotrimeric protein, α) = exchange of GDP to GTP = GDP is displaced (affinity)
GTP dissociates (hydrolysed by α sub-unit)
Molecules then signal downstream to effectors = response


What are the 3 types of G protein?

Stimulatory = Gαs


Describe the signalling events triggered by a stimulatory G protein

The Gαs sub-unit is now activated and GTP-bound
Adenylate cyclase (very large protein) is present in the plasma membrane (cell surface) and has a catalytic domain
The stimulatory G protein has a positive effect on the catalytic site = promotes the activity of the adenylate cyclase
This converts ATP to cAMP
Rapid build up of cAMP in cell
This activates protein kinases e.g. protein kinase A (PKA)


What do Gαs coupled receptors promote?

They promote the formation of cAMP and the activation of PKA


Describe the signalling events triggered by inhibitory G proteins

Gαi/o sub-unit is now activated and GTP bound
It inhibits the activity of the catalytic domain of adenylate cyclase
So ATP is not converted to cAMP
Therefore PKA activity is shut off


What do Gαi/o coupled receptors inhibit?

They inhibit the activity of adenylate cyclase
= decrease in cAMP
= PKA activity is shut off


Name the main difference between the signalling events triggered by the stimulatory and inhibitory G proteins and the events triggered by the q/11 G protein

The q/11 G protein does not act on adenylate cyclase
Stimulatory activates it, inhibitory inhibits it


Describe the signalling events triggered by q/11 G proteins

Gαq/11 causes recruitment of phospholipase C to plasma membrane
PIP2 lipids exist in the membrane (substrate for phospholipase C)
PIP2 is cleaved into 2 smaller fragments
- 1 stays in the plasma membrane = diacylglycerol (DAG)
- 2 smaller fragment = IP3
The 2 fragments activate different signalling pathways
DAG activates protein kinase C(β) (recruited to plasma membrane)
IP3 binds to receptors present on the endoplasmic reticulum = causes eflux of intracellular calcium into cell
Calcium rapidly brought back into ER by transporters


Therefore what do q/11 G proteins promote?

Gαq/11 coupled receptors promote release of intracellular calcium


Which channels do GPCRs modulate the function of?

GPCRs modulate the function of ion channels


How do GPCRs modulate the function of ion channels?

Gβγ sub-units have a negative effect on the activation of the 1/11 loop in a voltage-gated calcium channel
Induces a loss of function in the voltage-gated calcium channel and therefore there is a reduction in neurotransmitter release
e.g. GABA receptors


What is the desensitisation of GPCRs?

Molecular mechanism
Switches off activation of GPCRs - prevents their continuous activation
Distinct from downregulation (=the degradation of receptors)


Name the 2 types of desensitisation of GPCRs

Homologous desensitisation
Heterologous desensitisation


Define: Homologous desensitisation

Desensitisation effects are restricted to agonists acting through a specific receptor
i.e. if receptor A is desensitised, all receptor A's will become desensitised


Define: Heterologous desensitisation

Desensitisation effects can affect receptors that share a component of the same signalling cascase
i.e. if receptor A is desensitised, so is receptor B


Define: Downregulation

Degrading of receptors = fewer receptors in cell


Describe the process of desensitisation of GPCRs

Agonist binds to GPCR
Conformational change to receptor
Therefore GPCR kinases (GRK) joins bottom of receptor = promotes phosphorylation of receptor
Phosphorylation occurs on serine and threonine within the intracellular loop and C terminal tail
The phosphorylated form of the GPCR is now attractive to β-arrestins (stop signal transduction) which facilitate internalisation
β-arrestins bind and recruit scaffolding proteins = form a cage around multiple GPCRs and promote its internalisation into internal vesicles (endosomes)
Once in endosome, ligand, agonist and arrestins are released
Receptor becomes dephosphorylated by phosphatases
Receptor can then be recycled and returned back to surface = resensitisation
Receptor can be trafficked to a lysosome where they are degraded = downregulation


Define: Tolerance

Tolerance is the progressive reduction in the effectiveness of a drug
Usually occurring over days or hours
e.g. morphine at μ-opioid receptors


Explain if tolerance is developed at GPCRs when there has been no pre-treatment

Endogenous agonist (e.g. enkephalins) binds to the receptor and activates it
Agonist promotes the phosphorylation of receptor by GPCR kinases (GRKs), the recruitment of arrestins and therefore internalisation of receptors
Receptors are recycled and so ready to be activated again
Can keep activating with enkephalins and tolerance is not developed


Explain the development of tolerance at GPCRs (μ-opioid receptors) to chronic morphine

Morphine is a partial agonist so does not cause efficient internalisation of receptor = does not activate GRK-β arrestin pathway
Morphine binds to receptor
Activates protein kinase C (PKC)
So receptor is phosphorylated and desensitisation is caused
The receptor is not internalised very well and so cannot go through a resensitisation and recycling process
Receptor sits at surface of cell still desensitised for longer periods of time
Morphine can still bind but because of the different phosphorylation, it cannot activate the pathway = pain relief


Why do enkephalins and morphine cause different responses from the μ-opioid receptor?

Both activate the receptor but activate different signalling pathways
Enkephalins = phosphorylation by GRK, arrestins, internalisation - no tolerance developed
Morphine = PKC activated, β-arrestin pathway not activated (inefficient internalisation) - tolerance developed