CS - G-protein coupled receptors

Question 1

Structure of G-protein coupled receptors (4)

Answer 1

~ 7 transmembrane alpha-helical regions
~ G-proteins interact with alpha-helices 5 + 6
~ cytoplasmic loops
~ GDP/GTP binding domain

Question 2

Examples of GPCR (5)

Answer 2

~ beta-adrenergic (epinephrine) receptor
~ alpha1-adrenergic receptor
~ acetylcholine receptor (endothelial cells)
~ odorant receptor (nose)
~ rhodopsin (rod cells)

Question 3

Epinephrine/adrenaline

Answer 3

~ produced in adrenal medula
~ fight or flight hormone
~ increase glycogen breakdown
~ increase release of fatty acids
~ beta-adrenergic receptor

Question 4

Beta-Adrenergic pathway (6)

Answer 4

1. adrenaline binds to beta-adrenergic receptor (part of Gs)
2. binding initiates the G-protein cycle
3. G-protein becomes Ga’s form
4. ‘ON’ G-protein activates adenylate cyclase
5. adenylate cyclase converts ATP –> cAMP
6. cAMP activates protein kinase A (PKA)

Question 5

Gs

Answer 5

heterotrimeric G-protein associated with the beta-adrenergic receptor and activation of adenylate cyclase
~ three subunits:
-alpha = g-protein
- beta + gamma

Question 6

G-protein cycle (4)

Answer 6

1. G-protein is in its OFF position = GDP bound + part of Gs complex
2. GTP is exchanged for GDP which removes the beta and gamma subunits, activating the G-protein = ON
3. H2O added = GTP –> GDP
4. beta and gamma subunits rejoin and G-protein is in OFF position

Question 7

Ga’s form

Answer 7

~ only the alpha subunit
~ GTP/ON form
~ have intrinsic GTPase activity

Question 8

Protein kinase A + cAMP

Answer 8

~ kinase inhibition complex has 4 cyclic nucleotide binding domains and two PKAs
~ needs 4 cAMP to activate the two enzymes which disassociate form the complex
~ PKA are bound to complex using a pseudo,subtrate sequence

Question 9

Beta-Adrenergic pathway - TERMINATION

Answer 9

1. Ga’s intrinsic GTPase activity
   - disassociates from adenylate cyclase THEN reforms Gs
2. dissociation of signal from receptor
3. desensitisation via phosphorylation of receptor

Question 10

Desensitisation of signal mechanism (GPCR)

Answer 10

1. signal binds to receptor
2. G-protein receptor kinase adds phosphate groups to the cytoplasmic tails of the receptor = creates binding sites
3. Beta-arrestin binds to the newly-formed binding sites

Question 11

Which G-protein receptor kinase is involved in the beat-adrenergic pathway?

Answer 11

BARK = Beta-adrenergic receptor kinase

Question 12

Ga’i

Answer 12

~ different form of Ga’s subunit

~ has an inhibitory effect on adenylate cyclase

Question 13

How many types of Ga’ subunits are there?

Answer 13

20, each with specific associated effectors and effects on its 2nd messengers
~ 2nd messengers are v specific and have v diff. functions

Question 14

Regulation of cAMP levels - stimulatory pathway

Answer 14

~ Ga’s = simulators pathway

~ hormones = adrenaline, glucagon + ACTH

Question 15

Regulation of cAMP levels - inhibitory pathway

Answer 15

~ Ga’i - inhibitory pathway
~ hormones = PGE1 + adenosine
~ has reverse effect of adenylate cycle than Ga’s

Question 16

alpha-Adrenergic receptor pathway

Answer 16

1. adrenaline binds to alpha-adrenergic receptor (part of Gq)
2. binding initiates the G-protein cycle
3. G-protein becomes Ga’q form
4. ‘ON’ G-protein activates phospholipase C
5. increases production of IP3 and DAG

Question 17

Role of DAG and IP3

Answer 17

DAG - activates PKC
IP3 - open Ca2+ channels in endoplasmic reticulum
- Ca2+ ions activate PKC
~ both help KPC to bind to the membrane