G-protein coupled receptors Flashcards
What mechanism do all GPCR’s act through?
7 pass transmembrane G-protein coupled mechanism
Which cytosolic loops are involved in signal transduction?
Helix 3, 5 and cytosolic loops interact with coupled G protein.
What is an example of a ligand for a GPCR?
Adrenaline/ retinal
What two types of receptors does adrenaline bind?
beta2-adrenergic receptor and alpha2-adrenergic receptor
How does ligand biding to GPCR shift cytosolic loops?
Helix 5 and 6 move relative to eachother changing conformation of C3 (cytosolic loop 3)
C3 binds and activates G protein
where is beta2-adrenergic receptor found?
Found on smooth muscle cells. Binds on the heart muscle cells to increase heart contraction rate, increases blood flow to tissues
also binds hepatic and adipose tissues. triggers release of fatty acids and sugars.
where is alpha2-adrenergic receptor found?
Found in muscle cells lining blood vessels. Cuts off circulation to skin kidneys and intestines. Supplies energy to major locomotor muscles in response to bodily stress
What is the result of adrenaline binding a b2adrenergic receptor?
Adrenaline binding activates Gsalpha g-protein which stimulates adenylate cyclase activity, increases cytosolic conc of cAMP
What is the result of adrenaline binding a a2adrenergic receptor?
binding stimulates release of Gialpha g-protein which inhibits adenylate cyclase decreasing cytosolic cAMP conc
Which receptor has a higher affinity for adrenaline?
beta-2-adrenergic
How does the GTPase molecular switch work in GPCRs?
ligand binding shifts cytosolic loop 3 in receptor. This shift is passed on to the coupled G protein which causes GDP to dissociate from Ga, and for GTP to bind Ga.
Ga-GTP dissociates from the complex and binds to membrane targets such as membrane bound enzymes or ion channels (in adrenaline binds adenylate cyclase)
What are the downstream effects of GPCR signaling in hepatic cells?
Gsalpha-GTP activates adenylate cyclase to increase cytosolic cAMP levels.
Increased cAMP allows protein kinase A to dissociate from its inactive complex.
Activated protein kinase A phosphorylates metabolic enzymes (eg glycogen breakdown enzymes) and phosphorylates transcription factor CREB.
Also inactivates myosin light chain kinase in smooth muscle cells.
How are GPCR’s inactivated?
PKA phosphorylates the receptor which desensitises it (feedback)
BARK phosphorylates ligand bound beta receptors leading to B-arrestin binding and endocytosis.
What is an agonist (ligand analogue?)
An agonist mimics a ligand by binding the receptor and causing the normal response? (mimetic)
What is an example of an agonist of adrenaline in medical field?
B2 selective agonists eg turbutaline used as treatment for asthma - bind to b2 receptors on smooth muscle cells lining bronchial passages and stimulate muscle relaxation.
What is an antagonist (ligand analogue)?
Antagonist is competitive inhibitor. Binds receptor in place of ligand but does not cause activation
What is an example of an antagonist of adrenaline used in medical field?
beta blockers (practolol) bind to beta1 receptors on heart muscle cell. Normal ligand binding would increase heart rate but antagonist binding decreases heart rate.
What are the key features of a b-adrenergic ligand analogue?
Side chain containing NH groups determines affinity for the receptor.
A catechol ring is required for adenylate cyclase activation (ie agonist)
what is an example of a light sensitive GPCR?
Rhodopsin (in the beta-adrenergic family)
What is the ligand for Rhodopsin?
Retinal (technically part of the molecule)
What type of molecule is retinal?
Retinal is a chromophore. The structure is altered by light.
What happens when light is detected on Rhodopsin?
Light causes cis to trans isomerisation of retinal. This shifts cytosolic loop on opsin part of the protein.
This activates the coupled G protein
What G protein is coupled to rhodopsin?
Gt = G transducin
What is the effector enzyme of G transducin in rhodopsin pathway?
PDE (phosphodiesterase)
What is the action of the effector enzyme in the Rhodopsin pathway?
PDE converts cGMP back to GMP. This reduces cytosolic levels of cGMP blocking cGMP gated ion channels.
This prevents release of the neurotransmitter glutamine.
What is, in short, the optical signal sent by the Rhodopsin pathway?
The lack of release of the neurotransmitter glutamine is the optical signal.
How is the rhodopsin pathway turned off?
Phosphorylated by the rhodopsin kinase to downregulate ability to activate Gt.
What happens to Rhodopsin in very bright light?
Arrestins bind rhodopsin and there is no activation of Gt - Gt is endocytosed to other parts of the cell.
