Terence (GPCR signalling) Flashcards
The importance of GPCRs
About 1000 GPCRs in humans.
They are involved in every aspect of human biology- shows diversity of ligands that can activate the receptors.
Evolutionarily conserved from yeast to man.
Pharmaceutical importance- 34% of all FDA-approved drugs target GPCRs
The roles of GPCRs
They are a major target for drugs because they regulate many human physiological processes:
- Vision- the opsin use a photo isomérisation to convert a photon of light into a cellular signal
- Taste- GPCRs mediate release of gustducin in response to bitter, umami, and sweet tasting substances
- Smell- receptors bind odourants and pheromones
- Behaviour and mood- receptors for neurotransmitters including serotonin, dopamine, GABA, and glutamate
- The immune system/inflammation- e.g. chemokine and histamine receptors
- Autonomic nervous system- both the sympathetic and parasympathetic nervous systems are regulated by GPCR pathways responsible for control of many aromatic functions of the body such as blood pressure, heart rate, and digestive processes
- Homeostasis modulation- e.g. hormones such as glucagon
- Growth and metastasis
The structure of GPCRs
Single polypeptide chain.
Contain 7 transmembrane domains.
Ligand binding- extracellular domain or buried within TM2&3. (The ligand will bind to the extracellular portion or if it is hydrophobic it will bind in the membrane).
All are glycoproteins.
Some function as dimers.
Subject to phosphorylation to regulate the activity of the receptor and receptor signalling.
The metabotropic glutamate receptor
Ribbon diagram of the metabotropic glutamate receptor.
7 TM (transmembrane) domain.
Extracellular ligand binding pocket.
G-protein bound to C terminal tail and 3rd intracellular loop. (G-protein is not part of the receptor but work by coupling to the receptor)
GPCR signalling
GPCR –> Heterotrimeric G-protein –> Effector protein –> Second messenger –> Target protein –> Response.
Heterotrimeric G-protein- 3 different subunits to make up that G-protein.
Once GPCR has bound to heterotrimeric protein a conformational change takes place. Binds to effector protein/enzyme which is a small diffusible molecule. Binds to target protein (e.g. metabolic enzyme, cytoskeletal protein, ion channel, transcription factor) and produces a response (cellular/tissue/organ response)
Heterotrimeric G-proteins
Couples the GPCR to an effector protein e.g. ion channel or enzyme.
Acts as a molecular switch and timer.
Made up of an alpha, beta, and gamma subunit.
B and y subunits can form a stable dimeric complex referred to as the by complex.
Membrane associated via the a and y subunits.
Alpha subunit binds GDP/GTP.
When GDP bound- at rest/turned off.
When GTP bound0 activated/turned on.
Alpha subunit has intrinsic GTPase activity i.e. hydrolyses GTP to GDP (can turn itself off)
GPCR signalling to effector
Hormone is an example of one of many ligands that can bind.
1. Binding of hormone induces a conformational change in receptor to allow it to interact tightly with the heterotrimeric G protein
2. Activated receptor binds to Ga subunit. Turns it on by inducing a change on GDP to GTP
3. Binding induces a conformational change in G2; bound GDP dissociates and is replaced by GTP. Alpha subunit dissociates from beta and gamma subunits. Beta and gamma subunit can now bind to effector protein
4. Hormone dissociates from receptor, Ga binds to effector, activating it or inhibiting it
5. Hydrolysis of GTP to GDP causes Ga to dissociate from effector and re-associate with Gby.
Effector often produces a second messenger which binds to target. Second messengers are small diffusible molecules that can relay signals from the effector protein to the target molecule.
G-protein families
G protein -> effector -> 2nd messenger
- Gs -> adenylyl cyclase -> increase cAMP
- Gi -> adenylyl cyclase -> decrease cAMP
- Gq -> phospholipase C -> IP3, DAG
Gs- stimulates enzyme to produce cAMP
Gi- inhibits activity of enzyme to inhibit production of cAMP
Gq- leads to production of IP3 (opens CA2+ channels) and DAC (activates protein kinase)
Gs signalling
The GPCRs that couple to the Gs member include:
- 5-HT receptors
- Beta adrenergic receptors
- Calcitonin receptor
- Cannabinoid receptor 2
- Histamine H2 receptor
- Parathyroid hormone receptor 1
Examples of GsPCRs and drugs that target them include:
- Beta 1 adrenergic receptors- propranololis a beta blocker
- Beta 2 adrenergic receptors- salbutamol is an agonist
- Histamine receptor 2- ranitidine is an antagonist
- Setotonin receptor- amitryptyline is an antagonist
GsPCR signalling
GsPCRs couple to and activate Gs.
Gs ACTIVATES a membrane bound enzyme called adenylyl cyclase.
Adenylyl cyclase catalyses the conversion of ATP to cAMP.
cAMP is a second messenger.
cAMP activates the protein kinase A (PKA) (phosphorylates proteins and changes their activity)
Gs protein is activated by Cholera toxin.
Receptor = GsPCR
G-protein = Gs
Effector = Adenylyl cyclase
Second messenger = cAMP
Gi-Protein Coupled Receptors (GiPCRs)
Examples of GiPCRs:
- Acetylcholine M2 & M4 receptors
- Adenosine A1 & A3 receptors
- Adrenergic alpha 2 receptors
- Dopamine D2, D3, D4
- Histamine H3 & H4 receptors
Examples of receptors and drugs:
- Alpha 2 adrenergic receptor- clonidine is an agonist
- Mu opioid receptor- codeine
- Serotonin receptor 5GTI- triptans
GiPCR signalling
GiPCRs couple to and activate Gi.
Gi INHIBITS the enzyme adenylyl cyclase.
Results in a decrease in the second messenger cAMP.
Gi protein is inactivated by pertussis toxin.
The by subunits often regulate ion channels.
BY subunits bind to gated ion channels. Found on presynaptic bulb on neurones and activation of them can lead to a reduction in the release of neuroadrenaline.
Once neuroadrenaline has bound it induces a conformational change. This splits the GaSU and the GB/ySU. GDP is converted to GTP. GaSU binds to active effector and Gb/ySU binds to ion channel.
Receptor = GiPCR
G-protein = Gi
Effector = Adenylyl cyclase
Second messenger = cAMP
G protein gated ion channels
Gated via a direct interaction with Gby.
Independent of 2nd messenger generation.
Slower than ligand gated ion channels.
Gby regulation of ion channels is often associated with Gai containing heterotrimetic G-proteins.
Gq alpha protein family
Receptors they couple to Gq subunits include:
- 5-HT2 setotonergic receptors
- Alpha-1 adrenergic receptor
- Histamine H1 receptor
- M1, M3 and M5 muscarinic receptors
Examples of receptors and drugs include:
- Histamine H1 receptor- loratadine is an antagonist
- Alpha 1 adrenergic receptor- doxazosin is an antagonist
- Angiotensin 2 receptor- losartan is an antagonist
GqPCR signalling
GqPCRs couple to and activate the G protein, Gq.
Gq activates the membrane bound effector enzyme, phospholipase C.
Phospholipase promotes an increase in the second messengers IP3 and DAG.
IP3 promotes an increase in the intracellular calcium concentration.
DAG binds to protein kinase C and activates it.
Receptor = GqPCR
G-protein = Gq
Effector = Phospholipase C
Second messenger = DAG (in the membrane) +IP3 (in the cytoplasm)
