GPCR

Question 1

What is an agonist?

Answer 1

Bind to receptor and activates it - leads to signal transduction

Question 2

What is an antagonist?

Answer 2

Bind to receptor and block activity (block normal ligand binding/signal transduction)

Question 3

What are some examples of the wide range of stimuli for GPCR

Answer 3

Sensory GPCR - light, odour, taste
Ions - H+, Ca2+
Neurotransmitters - Ach, glutamate
Hormones - glucagon, adrenaline
Large glycoproteins - thyroid stimulating hormone

Question 4

What is the common structure of GPCR?

Answer 4

Single polypeptide chain that contains 7 transmembrane spanning domains with an extracellular N and intracellular C termini.

Question 5

Activated GPCR can interact with G protein, causing …

Answer 5

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

Question 6

What is a G protein?

Answer 6

A guanine nucleotide binding protein.

Heterotrimeric

Question 7

What happens to terminate G protein signalling?

Answer 7

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

Question 8

What are adrenoceptors and how many subtypes are there?

Answer 8

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

Question 9

What is the effect of adrenaline/noradrenaline on beta adrenoreceptors?

Answer 9

Activates adenylyl cyclase

Question 10

What is the effect of adrenaline/noradrenaline on alpha 1 adrenoreceptors?

Answer 10

Activates phospholipase C

Question 11

What is the effect of adrenaline/noradrenaline on alpha 2 adrenoreceptors?

Answer 11

Inhibits adenylyl cyclase

Question 12

What are muscarinic receptors and what type of GPCR are they?

Answer 12

GPCRs that bind acetyl Choline. 4 main types

M2/M4 are Gi receptors
M1/M3 are Gq receptors

Question 13

What is the effect of acetyl choline on M1/M3 muscarinic receptors?

Answer 13

Activates phospholipase C

Question 14

What is the effect of acetyl choline on M2/M4 muscarinic receptors?

Answer 14

Inhibits adenylyl cyclase

Question 15

How does cholera toxin (CTx) affect GPCRs?

Answer 15

CTx prevents the alpha subunits GTPase activity therefore the signal is overactive - cannot be turned off

Question 16

How does pertussis toxin (PTx) affect GPCRs?

Answer 16

PTx prevents GDP to GTP activation the the alpha subunit of Gi receptors therefore the GPCR pathway cannot be activated to transduce signals

Question 17

What are the two main categories of GPCR effectors?

Answer 17

Enzymes - e.g. adenylyl cyclase or phospholipase C

Ion channels - e.g. Voltage gated Ca2+ channels (VGCC)

Question 18

What is adenylyl cyclase stimulated by?

Answer 18

Gs receptor pathway

Question 19

What are the examples of Gs receptors?

Answer 19

Beta adrenoreceptors
D1 dopamine receptors
H2 histamine receptors

Question 20

What is adenylyl cyclase inhibited by?

Answer 20

Gi receptor pathway

Question 21

What does adenylyl cyclase catalyse?

Answer 21

The conversion of ATP to cAMP

Question 22

Give three examples of Gi receptors

Answer 22

Alpha 2 adrenoceptors
D2 dopamine receptors
Gamma-opioid receptors

Question 23

How many subunits does protein kinase A (PKA) have?

Answer 23

2 catalytic subunits

2 regulatory subunits (where cAMP binds)

Question 24

What does cAMP binding to PKA trigger?

Answer 24

Causes release of catalytic subunits which are then available to phosphorylate target proteins in the cells

Question 25

What is normal intracellular calcium concentration?

Answer 25

100 nM

Question 26

What is normal ER/SR calcium concentration?

Answer 26

2-3 x10^-4 M

Question 27

What is normal extracellular calcium concentration?

Answer 27

1-2 mM

Question 28

Which three channels allow Ca influx across plasma membrane?

Answer 28

Voltage gated calcium channels (VGCC)
Ligand gated calcium channels (LGCC)
Store operated calcium channels (SOC)

Question 29

When do VGCC open?

Answer 29

In response to depolarisation, calcium flows down concentration gradient very rapidly

Question 30

What are LGCC activated by?

Answer 30

Neurotransmitters

Question 31

What is the role of SOCs?

Answer 31

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)

Question 32

What are the channels/receptors responsible for calcium release from intracellular stores?

Answer 32

Gq type GPCR
IP3 receptors
Ryanodine receptors (RYR)

Question 33

What is the ligand of the ryanodine receptor?

Answer 33

Calcium

Role in calcium induced calcium release (CICR)

Question 34

What are the four main mechanisms of lowering cytoplasmic calcium?

Answer 34

Plasma membrane calcium ATPase (PMCA)
Na/Ca exchanger (NCX)
Smooth/Sacco endoplasmic reticulum calcium ATPase (SERCA)
Calcium buffers/binding proteins

Question 35

What is the role of the plasma membrane calcium ATPase (PMCA)?

Answer 35

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

Question 36

What’s the function of the Na/Ca exchanger (NCX)?

Answer 36

Utilises large electrochemical gradient of sodium, exchanges 3 Na for every 1 Ca pumped out.
Lower affinity for calcium but highly expressed (high capacity)

Question 37

What happens to the NCX in a highly polarised cell?

Answer 37

NCX switches around - to reduce intracellular sodium concentration

Question 38

What’s the function of SERCA?

Answer 38

Uses 1 ATP to pump 1 Ca from cytosol into stores - rapid establishment of basal intracellular calcium concentration

Question 39

What does phospholipase C catalyse?

Answer 39

Activated by Gq receptor pathways.

Converts PIP2 into IP3 and DAG

Question 40

What’s the function of IP3?

Answer 40

Ip3 binds to IP3 receptors in the endoplasmic reticulum triggering influx of Ca into cytosol.

Question 41

What is the function of DAG?

Answer 41

DAG and Ca bind to protein kinase C (PKC) which can phosphorylate its downstream targets

Question 42

Give three examples of Gq coupled receptors?

Answer 42

Alpha 1 adrenoreceptors
M1/M3 muscarinic receptors
H1 histamine receptors

Question 43

What is inotropy of the heart?

Answer 43

Force with which the heart contracts

Question 44

Give five key roles of calcium and examples

Answer 44

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

Question 45

What is the function of F1F0 ATPase (ATP synthase)?

Answer 45

Reverse of active transport - uses hydrogen ions gradient to synthesise ATP from ADP and PI

Question 46

Explain why a mutation in GTPase activity of the alpha subunit could be gain of function OR loss of function?

Answer 46

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.