Signal Transduction Flashcards
Describe the structure of GPCRs
Single polypeptide chain
7-transmembrane domains
Extracellular N-terminal, intracellular C-terminal
What are the 3 superfamilies of receptors?
Ligand gated ion channels
Receptors with intrinsic enzyme activity
G-protein coupled receptors (GPCRs)
Where on GPCRs can ligands bind?
To the N-terminal region (eg peptides, glutamate)
To the 3rd cytoplasmic loop- between the 2nd and 3rd transmembrane domains (eg ACh, adrenaline)
Give an example of a Beta adrenoceptor agonist and its use
Salbutamol, anti-asthmatic
Give an example of a u-opioid receptor agonist and its use
Morphine, anaesthesia
Give an example of a beta adrenoceptor antagonist and its use
Propranolol/atenolol, hypertensive drugs
Give an example of a D2 dopamine receptor antagonist and its use
Haloperidol, anti-schizophrenic
Give some examples of mutations to GPCRs
Loss of function mutation to rhodopsin causes Retinitis Pigmentosa.
Loss of function mutation to V2 vasopressin receptor causes Nephrogenic Diabetes Incipidus
Gain of function mutation to LH receptor causes Familial Male Precocious Puberty
What stimuli can GPCRs respond to?
Sensory GPCRs sense light, odours, taste
Other GPCRs sense to changes in extracellular ions, neurotransmitters, hormones, large glycoproteins
How do GPCRs change cellular activity?
Agonist binding results in a conformational change, which allows interaction with a Guanine-nucleotide Binding Protein, a G-protein.
Describe the structure of a G-protein
They are heterotrimeric, with alpha, beta and gamma subunits.
GDP is bound to the alpha subunit
Describe the events that follow GPCR G-protein interaction
This activates the G-protein.
The GDP on that alpha subunit is exchanged for GTP by Guanine Nucleotide Exchange Factors (GEF).
The complex dissociates into alpha-GTP and free beta-gamma subunits which interact with effector proteins
How is the signal terminated?
Interaction with effector proteins lasts until the alpha subunit GTPase activity hydrolyses the GTP to GDP.
The alpha-GDP and beta-gamma subunits reform a heterotrimeric complex which is inactive.
How can magnitude of the signal be regulated?
By regulation of the GTPase activity by GTPase Activating Protein (GAP).
This alters the time that the subunits interact with effector proteins before reforming the heterotrimeric complex.
How can G-protein diversity lead to a specific cellular response?
Activated GPCRs preferentially act with specific types of G protein (mainly determined by alpha subunit type) and the G-protein subunits interact with specific effector proteins.
Extracellular signals work via a specific GPCR, activating a single/sub population of G-proteins and effectors, bringing about a specific response.
What are the different types of G-protein and what do they stimulate/inhibit?
- Gs, stimulates adenylyl cyclase to produce cAMP
- Gi, inhibits adenylyl cyclase to reduce cAMP levels
- Gq, stimulates phospholipase C, increased PIP2
- Gt, stimulates cyclic GMP phosphodiesterase
Which G-protein is activated by adrenaline/noradrenaline on beta-adrenoceptors and what physiological response is brought about?
Gs proteins, stimulates adenylyl cyclase, increases cAMP, leading to increased glycogenolysis/lipolysis
Which G-protein is activated by adrenaline/noradrenaline on alpha2-adrenoceptors and what physiological response is brought about?
Gi proteins, inhibit adenylyl cyclase, reduce cAMP levels, reduced glycogenolysis/lypolysis
Which G-protein is activated by adrenaline/noradrenaline on alpha1-adrenoceptors and what physiological response is brought about?
Gq proteins, stimulates phospholipase C, smooth muscle contraction
Which G-protein is activated by light on rhodopsin receptors and what physiological response is brought about?
Gt proteins, stimulate cyclic GMP phosphodiesterase, leading to visual excitation
Which G-protein is activated by ACh on M2/M4 muscarinic receptors and what physiological response is brought about?
Gi, inhibits adenylyl cyclase but stimulates K channels, leading to slowing of cardiac pacemaker
Which G-protein is activated by ACh on M1/M3 muscarinic receptors and what physiological response is brought about?
Gq, stimulates phospholipase C, leading to smooth muscle contraction
Explain the use of cholera and pertussis toxins in experimental manipulation of the G-protein cycle
The toxins are enzymes that ADP-ribosylate specific G-proteins
Cholera toxin eliminates the GTPase activity of the G-alpha subunit, making it irreversibly activated.
Pertussis interferes with the GDP/GTP exchange on the G-alpha subunit, making it irreversibly inactivated.
How does Adenylyl Cyclase activation bring about a cellular response?
Adenylyl cyclase hydrolyses ATP to give cyclic AMP, this activates cAMP-dependent protein kinase (PKA) which phosphorylates other proteins within the cell to affect activity.
