4 - 7 Pass Receptors - Gray

Question 1

Give 2 examples of 7 pass receptor families.

Give any similarities of 7 pass receptor families

Answer 1

• GPCRs and B adrenergic / rhodopsin family

- no sequence similarity between families but lots of structural similarities

Question 2

give the overall mode of action of these 7 pass receptor families

Answer 2

facilitates signal transduction across the membrane to the cytosol, leads to activation of G protein -> effector protein eg enzyme that generates 2nd messenger or ion channel

Question 3

draw the strucutre of a GPCR, state which structures bind things etc

Answer 3

343 - 4

• 7 TM a helices
• cytosolic loops interact and activate G proteins
• helices 3,5,6 contribute to ligand specificity
• H5/6 bind to ligand and on binding move relative to each other
• this movement causes hydrophilic cytoplasmic loop C3 to alter its conformation (this alters the specificity for the G proteins - ie can bind Gs or Gi proteins)
• C3 binds and activates the G proteins

Question 4

Give some examples of the types of ligands that can bind to GPCRs

Answer 4

neurotransmitters, hormones, rhodopsin, odours -> odorant receptors

Question 5

Give an example of a molecule binding to a 7 pass receptor and state the name of this R. state any specific interactions that arise from this binding

Answer 5

• adrenaline binding to B2 adrenergic receptor found on smooth muscle cells (more specifically the H3,5,6)
• catechol ring of adrenaline makes interaction with Helix 6

Question 6

describe the G protein that is involved in GPCRs. state the different forms of this G protein that can exist

Answer 6

heterotrimeric G protein
3 subunits - aBy
Ga subunit can bind GTP (ON) and GDP (OFF)
G protein can be both stimulatory and inhibitory. Gs and Gi have same By domains but different a domains.

Question 7

draw a diagram and explain how ligand binding to GPCR will stimulate/inhibit the effector protein

Answer 7

343 - 4 word

Question 8

What is the difference between the G proteins in RTK signalling and GPCR signalling?

Answer 8

• G proteins downstream of RTK signalling = monomeric G proteins (Ras)
• G proteins downstream of GPCR signalling= heterotrimeic G proteins that can be inhibitory or stimulatory

Question 9

Describe ALL aspects of adrenaline (epinephrine) signalling

Answer 9

• adrenal glands secretes the adrenaline hormone which can bind to a number of different receptor sub-types on a variety of cell types
  Binding to B adrenergic receptors on….
• heart muscle cells (B1) increases contraction rate, increases heart rate, increases blood flow
• hepatic and adipose cells (B2) causing release of glucose and fatty acids
• smooth muscle cells of the small intestine and bladder (B2) -> butterflies
  Binding to a2 adrenergic Rs on…
• muscle cells lining blood vessels cuts off circulation to skin, intestine, kidneys.
• supplies energy for movement of major locomotor muscles in response to bodily stress eg fight, flight, frolic

Question 10

Describe how the differences in sensitivity of blood vessels to adrenaline is achieved (vasoconstriction and vasodilation)

Answer 10

VASOCONSTRICTION; 
- controlled by a1 Rs 
- less sensitive and more of them 
VASODILATION; 
- controlled by B2 Rs 
- less of them but higher affinity 

• at low [adrenaline], bind to B2 adrenergic Rs enabling vasodilation
• at higher [adrenaline] more can bind to a1 (higher numbers of these present), -> vasoconstriction
• shows different effects of same Ligand binding to different receptors

Question 11

What does stimulating B adrenergic Rs lead to? (broadly speaking)

Answer 11

rise in intracellular 2nd messenger cAMP

Question 12

What does cAMP do? (broadly speaking)

Answer 12

modifies rates of many enzyme catalysed reaction eg Protein Kinase A

Question 13

What is adenylate cyclase, where is it located and how can its activity be regulated?

Answer 13

• integral membrane protein that converts ATP -> cAMP + PPi
• activated through interaction with Gsa and inhibited through interaction with Gia
• 2 catalytic domains that interact with these G proteins

Question 14

What do inhibitory G proteins do?

Answer 14

inhibit activity of their effector proteins when in GTP bound form

Question 15

name some different types of Rs that activate either Gs or Gi proteins and state their effects on adenylate cyclase

Answer 15

• B1/B2 receptors activate Gs stimulatory protein. binds to and activates adenylate cyclase
• a2 receptors activate Gi inhibitory protein. assume it inhibits activity of adenylate cyclase?

Question 16

Describe GCPR downstream signalling. draw a diagram

Answer 16

• elevated cAMP allows PKA to dissociate from inactive complex
• PKA phosphorylates metabolic enzymes and CREB (cAMP response element binding protein) (CREB = TF)

Question 17

state the effects of GPCR downstream signalling on 2 subsets of cells, stating the types of adrenergic Rs that are activated in the process (draw a diagram)

Answer 17

HEPATIC CELLS;
- adrenaline binds to B2 adrenergic receptor
- Gs - GTP -> adenylate cyclase -> high cAMP
343 - 4 word

SMOOTH MUSCLE CELLS;
- adrenaline to B2 R
- PKA inactivates myosin light chain kinase (enzyme)
- normally MLCK binds Ca therefore forming cross links with actin and contracting
because this enzyme inactivated, cannot form cross links therefore no contraction -> relaxation

Question 18

describe how GPCRs are inactivated. draw a diagram

Answer 18

• PKA P the receptor which desensitises it (feedback)
• BARK phosphorylates ligand bound B adrenergic receptors. leading to B arrestin binding and endocytosis -> recycling or degradation (this occurs if the R continues to be activated)

Question 19

What are agonists and antagonists ?

Answer 19

agonists - mimics ligand by binding receptor and causes normal response
antagonist - competes with the natural ligand for binding (competitive inhibitor), binding does NOT result in activation

Question 20

Draw the strucutre of the B adrenergic receptor ligands stating the roles of the functional groups

Answer 20

• side chain NH group determines affinity for receptor

- catechol ring required for adenylate cyclase activation

Question 21

What makes the structure seen in Q21 word 343 -4 a good antagonist?

Answer 21

• because it has the NH group, strong binding still occurs however altered catechol ring therefore no AC activation

Question 22

what is the alternative name for agonists to adrenaline?

Answer 22

catecholamines

Question 23

name one B adrenergic agonist and state its medical use

Answer 23

TURBUTALINE;
- used in asthma treatment
- targets B2 receptors of smooth muscle cels in bronchial passages therefore stimulating relaxation
(better than just giving adrenaline because specifically targets B2)

Question 24

name one B adrenergic antagonist and state its medical use

Answer 24

BETA BLOCKERS eg practolol;

• B1 antagonist that slows heart contractions
• on cardiac muscle cell, B1 binding normally increases heart rate on ligand binding

Question 25

Name some diseases associated with G protein activation. draw a diagram to illustrate one of these processes

Answer 25

CHOLERA TOXIN;

• enters cells lining gut and modifies Gsa
• ADP ribosylates Gsa
• Gsa-GTP still binds and activates adenylate cyclase but cannot hydrolyse GTP therefore enzyme locked on
• levels of cAMP rise leading to efflux of Na ions and water from blood to the intestinal lumen
• dehydration and diarrhoea result

PERTUSSIS TOXIN;

• causes Whooping cough through modifications to Gia
• ADP ribosylation of Gia means that adenylate cyclase activity can not be inhibited
• increased cAMP
• massive secretion of fluid from infected cells

Question 26

under what family is the rhodopsin GPCR under?

Answer 26

B adrenergic receptor family

Question 27

what is the function of rhodopsin?

Answer 27

visual pigment in rods (responsible for monochromatic vision) and cones (attached to other proteins to give colour vision)
- converts light signal to action potential in the optic nerve

Question 28

where is rhodopsin located, how much rhodopsin is present per rod cell

Answer 28

rhodopsin found in the membrane discs packed within the rod cell
around 10^6-7 rhodopsin molecules per cell giving large sa for maximal absorption of light

Question 29

draw and describe the structure of rhodopsin

Answer 29

retinal chromophore (ligand) bound to the opsin protein (7 pass receptor)

Question 30

describe the process of generating an action potential through rhodopsin (draw a diagram of this)

Answer 30

• retinal chromophore absorbs photon. causes retinal to alter its conformation from cis -> trans
• causes conformational change of the opsin protein so can bind its G protein transducin and activate it
• GTP displaces the GDP bound to Gta, Gta - GTP dissociates from GBy and binds to and activates phosphodiesterase
• converts cGMP -> GMP
• reduces cytosolic conc of cGMP results in closing of the cGMP gated ion channel preventing influx of Ca2+ / Na+
• hyper polarisation of the membrane and prevents neurotransmitter release
• the lack of neurotransmitter releases = action potential and visual signal
• as the rhodopsin is now unstable, retinal dissociates and is converted back to cis form through enzymes and can rebind opsin -> rhodopsin

Question 31

draw a diagram showing how rod cells react to increasing amounts of light and describe this process

Answer 31

(rhodopsin only responsible for monochromatic vision in DARK light)

• under normal, low light; opsin activated which activates Gta
• light activated opsin is phosphorylated by rhodopsin kinase (conversion of ATP -> ADP). slightly reduced ability to activate Gta
• under higher light, leads to greater P of Gta
• under increasing light = more P until arrestin proteins bind and blocks Gta activation
• triggers endocytosis of the R and recycling occurs therefore blocking further signalling
• transducin then moves out of the membrane discs to other parts of the cell

Question 32

summarise all these GPCRs

Answer 32

343 - 4 word