metabotropic receptors

Question 1

what time frame do G-protein coupled receptors work in?

Answer 1

seconds

Question 2

what is a G-protein coupled receptor?

Answer 2

when a ligand binds to them they signal by activating G-proteins in the cell
G-proteins activate second messenger pathways
G-proteins lead to downstream effects of receptor

Question 3

what is an example of G-protein coupled receptors?

Answer 3

muscarinic ACh receptors

Question 4

GPCR structure

Answer 4

• 7 transmembrane (TM) domains
• 3 intracellular, 3 extracellular loops
• N-terminus extracellular
• intracellular phosphorylation sites
• 3rd intracellular loop & C-terminus
  interacts with G-protein
• ligand binding in TM domains or
  N-terminal domain
• most are monomeric, some dimeric

Question 5

example of a sensory GPCR

Answer 5

rhodopsin

Question 6

examples of neurotransmitter GPCRs

Answer 6

cholinergic
dopaminergic
GABAb

Question 7

G-protein = GTPAse

Answer 7

when G-protien binds to GDP it is inactive (off state) however with stimulus it binds to GTP and forms active (on state) G-protien to regulate ion channels
because of intrisnic GTP hydrolysis activity it will hydrolyse its GTP into GDP (off state)

Question 8

heterotrimeric (3 subunits, alpha beta and gamma) G-proteins

Answer 8

G-proteins that couple to GPCRs, the alpha subunit has GTPase activity, not the beta or gamma

Question 9

the role of heterotrimeric G proteins in signal transduction

Answer 9

information transfer= signal to receptor to G protein to effectors
- ligand bind to receptor which causes conformational change in rceeptor which increases its affinity to G-protein so it can bind
- also conformational change in G-protein so GDP is exhanged for GTP (active in GTP-bound state)
- alpha subunit and beta-gamma subunit diffuse apart and independently activate intracellular signalling pathways
alpha subunit in GTP-bound active state activate downstream pathways as well as beta-gamma
over time alpha subunit hydrolyses GTP to GDP and lose affinity for target
alpha and beta-gamma subunits come back together to form trimeric G-protein

Question 10

resting state in heterotrimeric G proteins

Answer 10

alpha subunit is bound to GDP (off state)
target proteins also not being regulated

Question 11

what are 4 examples of effector systems for G proteins?

Answer 11

adenylate cyclase
phospholipase C
potassium channels
calcium channels

Question 12

role of adenylate cyclase

Answer 12

catalyses the production of second messenger cyclic AMP (cAMP)

Question 13

role of phopholipase C

Answer 13

catalyses the production of IP3 and diacyl glycerol (DAG) from PIP2 (second messengers)

Question 14

role of potassium channels

Answer 14

G-proteins regulate membrane potential

Question 15

role of calcium channels

Answer 15

G-proteins allow calcium ions to enter the neuron (increase intracellular calcium)

Question 16

adenylate cyclase activity

Answer 16

1. neurotransmitter binds to receptor
2. activates G protein (active alpha subunit)
3. activates enzyme adenylate cyclase which uses ATP to produce cAMP
4. produces second messenger with cAMP
5. cAMP activates protein kinase A (phosphorylates other proteins)
6. protein kinase A phosphorylates K+ channel
7. causes reduction in potasssium influx

Question 17

phospholipase C activity

Answer 17

1. alpha subunit in GTP-bound state regulates phospholipase C
2. phospholipase C cleaves a membranre phosopholipid called PIP2 into IP3 (cytosolic) and DAG (stays in plasma membrane)
3. these products are important second messengers that activates certain pathways
4. IP3 binds to calcium channels on the surface of ER= calcium flows out into cytosol which can activate calcium-sensitive proteins (e.g protein kinase C that phosphorylates multiple downstream targets)
5. DAG also has to bind to protein kinase C to avtivate it as well as increase in calcium concentration

Question 18

ion channel activity

Answer 18

active alpha subunit binds to receptors so regulates ion activity
alteration in electrical properties of the cell

Question 19

advantages of G proteins in signal transduction

Answer 19

1. simple amplification
2. signal diversification

Question 20

signal amplification

Answer 20

a single receptor may activate several alpha-subunits (activate lots of G-proteins)
- depends on factors such as how long ligand remains bound

Question 21

signal diversification

Answer 21

• receptors can interact with more than one type of G-protein
  (there are >20 different variants of alpha-subunit which all have different targets)
• G-proteins can regulate more than one effector
  -eg. Gi and Go can inhibit adenylate cyclase, open K+ channels and close Ca2+-channels
• both alpha and beta-gamma subunits can regulate target proteins (downstream of the effector you get multiple effects)

Question 22

examples of intracellular second messengers

Answer 22

3’, 5’-cylic AMP
calcium ions
1,2-diacylglycerol (DAG)
inositol 1,4,5-trsphosphate (IP3)

Question 23

3’,5’-Cyclic AMP (cAMP)

Answer 23

receptor= protein kinase A
action= protein phosphorylation

Question 24

calcium ion

Answer 24

receptor= calmodulin and other calcium binding proteins
many actions like activating calcium dependent kinases

Question 25

1,2-Diacylglycerol (DAG)

Answer 25

receptor= protein kinase C action= protein phosphorylation and activating protein kinase C

Question 26

inositol 1,4,5-trisphosphate (IP3

Answer 26

receptor= Ca-release channel (IP3 receptor) action= calcium release

Question 27

presynaptic neuromodulation- intracellular signalling following GPCR

Answer 27

facilitator neuron regulates how excitable a presynapse is 1. neuron releases 5HT, DA, Ach, peptides, etc 2. GPCR activates effector system on postsynaptic membrane 3. increased production of 2nd messenger from active G proteins (e.g adenylate cyclase to cAMP) 4. intracellular “receptor” (eg protein kinase which inhibits potassium channels) 5. phosphorylates K+ channel to close it 6. VSCC allow Ca2+ into the presynaptic neuron (sense depolarisation) which primes release of neurotransmitter as it is already excitable 7. glutamate is released 8. glutamate receptors depolarise postsynaptic neuron essentially through GPCR signalling you are effecting how active the presynapse is (more)

Question 28

postsynaptic modulation- intracellular signalling following GCPR (e.g cerebellar long term depression)

Answer 28

1. presynaptic neuron releases glutamate 2. mGluR (GPCR) activates effector system (phospholipase C) 3. increased production of 2nd messengers (DAG and IP3) 4. intracellular receptors (protein Kinase C and IP3 receptor) 5. PKC phosphorylates AMPA receptor 6. IP3 receptor mediates Ca2+ release 7. AMPAR phosphorylation plus other Ca2+- dependent processes cause AMPAR endocytosis (changing how synapse responds in future by removeing AMPAR receptors from postsynaptic membrane) 8. this reduces synaptic strength

Question 29

GPCR heterodimers

Answer 29

orginally dimers were thought to be homodimers (2 copies of same protein subunit) however those working on metabotropic GABAb receptors found them to be heterodimers (2 different GPCR that come together to form the receptor) as a subunit on its own it doesnt work this means there is a wide variety of heterodimeric G protein couples receptors in the brain e.g glutamate and serotonin subunit so binds to 2 different ligands - activation of heterodimer causes a different effect than its two subunits=expands receptor diversity signalling