GPCR Flashcards
What is an agonist?
Bind to receptor and activates it - leads to signal transduction
What is an antagonist?
Bind to receptor and block activity (block normal ligand binding/signal transduction)
What are some examples of the wide range of stimuli for GPCR
Sensory GPCR - light, odour, taste Ions - H+, Ca2+ Neurotransmitters - Ach, glutamate Hormones - glucagon, adrenaline Large glycoproteins - thyroid stimulating hormone
What is the common structure of GPCR?
Single polypeptide chain that contains 7 transmembrane spanning domains with an extracellular N and intracellular C termini.
Activated GPCR can interact with G protein, causing …
Following conformational change, GDP is exchange for GTP in the alpha subunit of the G protein - active. The alpha subunit then dissociated got the beta-gamma complex, and can interact eight an effector protein
What is a G protein?
A guanine nucleotide binding protein.
Heterotrimeric
What happens to terminate G protein signalling?
The alpha subunit contains intrinsic GTPase activity that hydrolyses GTP back to GDP (releasing Pi). Alpha (GDP) subunit and beta-gamma complex then reform the inactive heterotrimeric G protein
What are adrenoceptors and how many subtypes are there?
GPCRs that bind adrenaline/noradrenaline
There are two main groups of adrenergic receptors, α and β, with several subtypes.
There are alpha 1, alpha 2 and beta adrenoreceptors.
Alpha 1 is a Gq coupled receptor
Alpha 2 is a Gi coupled receptor
β receptors (subtypes β1, β2 and β3)are Gs receptors
What is the effect of adrenaline/noradrenaline on beta adrenoreceptors?
Activates adenylyl cyclase
What is the effect of adrenaline/noradrenaline on alpha 1 adrenoreceptors?
Activates phospholipase C
What is the effect of adrenaline/noradrenaline on alpha 2 adrenoreceptors?
Inhibits adenylyl cyclase
What are muscarinic receptors and what type of GPCR are they?
GPCRs that bind acetyl Choline. 4 main types
M2/M4 are Gi receptors
M1/M3 are Gq receptors
What is the effect of acetyl choline on M1/M3 muscarinic receptors?
Activates phospholipase C
What is the effect of acetyl choline on M2/M4 muscarinic receptors?
Inhibits adenylyl cyclase
How does cholera toxin (CTx) affect GPCRs?
CTx prevents the alpha subunits GTPase activity therefore the signal is overactive - cannot be turned off
How does pertussis toxin (PTx) affect GPCRs?
PTx prevents GDP to GTP activation the the alpha subunit of Gi receptors therefore the GPCR pathway cannot be activated to transduce signals
What are the two main categories of GPCR effectors?
Enzymes - e.g. adenylyl cyclase or phospholipase C
Ion channels - e.g. Voltage gated Ca2+ channels (VGCC)
What is adenylyl cyclase stimulated by?
Gs receptor pathway
What are the examples of Gs receptors?
Beta adrenoreceptors
D1 dopamine receptors
H2 histamine receptors
What is adenylyl cyclase inhibited by?
Gi receptor pathway
What does adenylyl cyclase catalyse?
The conversion of ATP to cAMP
Give three examples of Gi receptors
Alpha 2 adrenoceptors
D2 dopamine receptors
Gamma-opioid receptors
How many subunits does protein kinase A (PKA) have?
2 catalytic subunits
2 regulatory subunits (where cAMP binds)
What does cAMP binding to PKA trigger?
Causes release of catalytic subunits which are then available to phosphorylate target proteins in the cells
What is normal intracellular calcium concentration?
100 nM
What is normal ER/SR calcium concentration?
2-3 x10^-4 M
What is normal extracellular calcium concentration?
1-2 mM
Which three channels allow Ca influx across plasma membrane?
Voltage gated calcium channels (VGCC)
Ligand gated calcium channels (LGCC)
Store operated calcium channels (SOC)
When do VGCC open?
In response to depolarisation, calcium flows down concentration gradient very rapidly
What are LGCC activated by?
Neurotransmitters
What is the role of SOCs?
Important in accessing extracellular calcium when SER stores are low). Activated by calcium sensing protein in the SER.
Very slow.
Important in smooth muscle (prolonged state of contraction)
What are the channels/receptors responsible for calcium release from intracellular stores?
Gq type GPCR IP3 receptors Ryanodine receptors (RYR)
What is the ligand of the ryanodine receptor?
Calcium
Role in calcium induced calcium release (CICR)
What are the four main mechanisms of lowering cytoplasmic calcium?
Plasma membrane calcium ATPase (PMCA)
Na/Ca exchanger (NCX)
Smooth/Sacco endoplasmic reticulum calcium ATPase (SERCA)
Calcium buffers/binding proteins
What is the role of the plasma membrane calcium ATPase (PMCA)?
Uses 1 ATP molecule to pump 1 molecule of Ca into extracellular space.
High affinity for calcium (optimised by binding to calmodulin) but low capacity
What’s the function of the Na/Ca exchanger (NCX)?
Utilises large electrochemical gradient of sodium, exchanges 3 Na for every 1 Ca pumped out.
Lower affinity for calcium but highly expressed (high capacity)
What happens to the NCX in a highly polarised cell?
NCX switches around - to reduce intracellular sodium concentration
What’s the function of SERCA?
Uses 1 ATP to pump 1 Ca from cytosol into stores - rapid establishment of basal intracellular calcium concentration
What does phospholipase C catalyse?
Activated by Gq receptor pathways.
Converts PIP2 into IP3 and DAG
What’s the function of IP3?
Ip3 binds to IP3 receptors in the endoplasmic reticulum triggering influx of Ca into cytosol.
What is the function of DAG?
DAG and Ca bind to protein kinase C (PKC) which can phosphorylate its downstream targets
Give three examples of Gq coupled receptors?
Alpha 1 adrenoreceptors
M1/M3 muscarinic receptors
H1 histamine receptors
What is inotropy of the heart?
Force with which the heart contracts
Give five key roles of calcium and examples
Metabolism - bone metabolism, glycogenolysis
Hormonal regulation - formation/degradation of cAMP/cGMP and triggers hormone release
Membrane linked functions - excitation -contraction/secretion coupling, AP generation
Contractile/motile systems - muscle myofibrils, cilia and flagella, microtubules/microfilaments
Intracellular signalling function - protein kinase/phosphatase, gene expression, apoptosis
What is the function of F1F0 ATPase (ATP synthase)?
Reverse of active transport - uses hydrogen ions gradient to synthesise ATP from ADP and PI
Explain why a mutation in GTPase activity of the alpha subunit could be gain of function OR loss of function?
loss of function - alpha subunit cannot deactivate, therefore would remain activated, carry on producing second messenger and initiating cellular response (Therefore doesn’t associate with beta and gamma again)
gain of function - alpha subunit deactivated by GTPase more quickly therefore less adenylyl cyclase activation therefore less second messenger produced.
