9 - GPCR Flashcards

Question 1

Q

What is the difference between Ionotropic and Metabotropic receptors?

Answer

A

Ionotropic = ligand gates ion channel 
Metabotropic = G-protein coupled (7TM) receptor (use second messengers)

Question 2

Q

What is the structure of GPCR’s?

Answer

A

Small membrane proteins
Structurally conserved
- 7 trans-membrane alpha-helices
- Extracellular loops: ligand binding
- Intracellular loops: G-protein binding
- Maintain similar alpha-helix configuration following activation

Question 3

Q

What is an orphaned receptor?

Answer

A

GPCR whose endogenous agonist is unknown

(most are olfactory)

Question 4

Q

What are G-proteins?

Answer

A

Guanine nucleotide (GDP, GTP) binding proteins

Question 5

Q

G-proteins have intrinsic ______ activity

Answer

A

G-proteins have intrinsic GTPase activity

Question 6

Q

How do G-proteins allow signal transduction?

Answer

A

By switching between GTP- or GDP- binding to regulate cell function

Question 7

Q

What are the two types of G-proteins?

Answer

A

Heterotrimeric (alpha, beta-gamma)
- alpha subunit - most diversity
  - BINDS GTP OR GDP
- Beta-gamma subunits - function as a unit
Monomeric (Ras superfamily)
- Homologous to alpha subunit of heterotrimeric

Question 8

Q

What anchors G-proteins to the membrane? Why?

Answer

A

Lipid anchors – allows movement close to the membrane

Question 9

Q

What are the four main classes of heterotrimeric G-proteins?

Answer

A

Galpha_s - stimulates adenylate cyclase
Galpha_i
1. includes:
  - G-alpha_o- inhibit adenylate cyclase
  - G-alpha_t - activate cGMP phosphodiesterase
G-alpha_q/11 - activate phospholipase C (PLC - cleaves PIP2 to IP3 and DAG)
G-alpha_12/13 - activate Ras (GTPase activating protein) and some tyrosine kinases

Question 10

Q

Mechanism of G-protein activation

Answer

A

Rest
- G-protein closely associated with GPCR
- Heterotrimeric
- High affinity of alpha for GDP
Activation:
- G-protein binds to GPCR
- alpha subunit loses GDP
- alpha has high affinity for GTP
- G-protein dissociate to alpha + beta-gamma
Termination
- Free alpha is an intrinsic GTPase
- Hydrolyze bound-GTP to GDP
- Heterotrimer reassembles

Question 11

Q

Addition of ______ prolongs activation

Answer

A

Addition of Gpp(NH)p prolongs activation

non-hydrolysable analog of GTP
Irreversible binding to G-protein - stays dissociated/activated

Question 12

Q

How is G-protein interaction abolished?

Answer

A

Pertussis toxin (G-alpha_i and G-alpha_o proteins)
- inactivates G-proteins
- Places an ADP-ribose onto the GTP-binding site

whooping cough inactivates Galpha i/Galpha o to prevent these signaling pathways

Question 13

Q

What is direct modulation of GPCR

Answer

A

G-protein binds directly to ion channel
- activation of GIRK and K_ATP channel
- Inhibit N- and P/Q type VDCC
- Inhibit Na+ channels
- Activate Ca++ channels (heart and muscle)
- Galpha or Gbeta-gamma can both use this pathway

VDCC = voltage-dependent calcium channel

Question 14

Q

Effects of G-protein binding directly onto the ion channel?

Answer

A

activation of GIRK and K_ATP channel
Inhibit N- and P/Q type VDCC
Inhibit Na+ channels
Activate Ca++ channels (heart and muscle)
Galpha or Gbeta-gamma can both use this pathway

VDCC = voltage-dependent calcium channel

Question 15

Q

What is indirect modulation of ion channels by G-proteins?

Answer

A

G-protein regulate second messengers that alter ion channel activity
- beta-adrenergic activation of L-type VDCC
  - via cAMP-dependent phosphorylation
- Muscarinic receptor inhibition of M-current
  - via decrease in PIP2 levels

Question 16

Q

2 examples of how G-protein regulates second messengers to alter ion channel activity (Indirect modulation)

Answer

A

beta-adrenergic activation of L-type VDCC
- via cAMP-dependent phosphorylation
Muscarinic receptor inhibition of M-current
- via decrease in PIP2 levels

Question 17

Q

Provide evidence for direct modulation by G-protein

Answer

A

Membrane-delimitation
- Beta-gamma complex remains very closely associated with the membrane
Cell-attached mode
- ACh mimics effect when pipette-applied but not when bath-applied
- response is restricted to small part of the cell

Question 18

Q

Beta-adrenergic receptors increase ________ current

Answer

A

Beta-adrenergic receptors increase L-type VDCC current

activation of G_s pathway
Channel phosphorylation

Question 19

Q

Evidence of indirect modulation?

Answer

A

Cell-attached mode : bath-application mimics effect of agonist
- Second messengers can diffuse through cytoplasm
- Can signal to the entire cell from a local input

Question 20

Q

How is rhodopsin (in the retina) indirectly modulated?

Answer

A

Photon hits rhodopsin and activates tranducin (Galpha_t)
alpha-transducin activates cGMP phosphodiesterase
- Hydrolyses cGMP on non-inactivating Na+ channe;
Na+ channel closes
Photoreceptor hyperpolarizes = stops releasing glutamate
Loss of transmitter signal activates downstream neuron

Question 21

Q

What is the advantage of indirect modulation?

Answer

A

Different GPCR’s can produce the same change in an ion channel
- K+ channels in hippocampal neuron can be modulated by GABA_B and/or 5-HT_1A receptor
Diffusable messengers allow signaling to entire neuron from a local point
Control
- Each GPCR can act at more than one effector to amplify the desired response
  - 5-HT_1A receptor in dorsal raphe neurons can activate K+ channels and inhibit VDCCs
- Direct ion channel modulation localizes and restricts the response to small part of the neuron

Question 22

Q

Advantages of Indirect Modulation:

Different GPCR’s can produce the same change in an ion channel
- K+ channels in hippocampal neuron can be modulated by _____ and/or______ receptor
Diffusable messengers allow __________
Control
- Each GPCR can act at more than one effector to ___________
  - 5-HT_1A receptor in dorsal raphe neurons can activate _______ and inhibit ________
- Direct ion channel modulation localizes and restricts the response to small part of the neuron

Answer

A

Different GPCR’s can produce the same change in an ion channel
- K+ channels in hippocampal neuron can be modulated by GABA_B and/or 5-HT_1A receptor
Diffusable messengers allow signaling to entire neuron from a local point
Control
- Each GPCR can act at more than one effector to amplify the desired response
  - 5-HT_1A receptor in dorsal raphe neurons can activate K+ channels and inhibit VDCCs
- Direct ion channel modulation localizes and restricts the response to small part of the neuron

Question 23

Q

What is the significance of GPCR’s for neurons?

Answer

A

GPCR-mediated effects are slower than those from ionotropic receptors but are longer-lasting

Question 24

Q

What is the implication of having longer lasting effects on neurons (as from GPCR mediated effects)

Answer

A

changes to neuron behaviour can be induced for an extended period
- 100s of msec to hours
- Ionotropic - signaling stops when NT is cleared
- Metabotropic - signal is prolonged due to delayed onset/offset of second messenger synthesis/catalysis

Question 25

Q

What class of receptors are important for neuromodulation?

Answer

A

NPY receptors

Question 26

Q

What are the 5 subtypes of NPY receptors?>

Answer

A

Y1
Y2
Y4
Y5
Y6

Question 27

Q

NPY receptors couple to _______ proteins

Answer

A

NPY receptors couple to G_<u>i</u>/G_<u>o</u> proteins