Intracellular signalling pathways (1)

Question 1

List the superfamilies of cell-surface receptors and give examples

Answer 1

1. G protein-coupled (7TM) receptors - muscarinic acetylcholine receptors
2. Ligand-gated ion channels - nicotinic acetylcholine receptors
3. Receptor Tyrosine Kinases - insulin receptor

Question 2

What is an Agonist?

Answer 2

A cehmical that binds to the receptor and activates a biological response

Question 3

What is an antagonist?

Answer 3

A chemical that binds to a receptor and prevents biological response

Question 4

What can GPCR respond to?

Answer 4

1. Sensory GPCR : light, odours, taste
2. Other GPCRs : Ions, Neurotransmittters, peptide and non-peptide hormones, large glycoproteins

Question 5

What is the basic structure of GPCR?

Answer 5

• Single polypeptide chain
• 7-transmembrane (7TM) - spanning regions
• Extracellular N-terminal
• Intracellular C-terminal

Question 6

Where are the ligand binding sites on GPCR?

Answer 6

• 2-3 of transmembrane domains
• N-terminal region

Question 7

Describe the events that happen following the binding of ligand on binding domain of GPCR

Answer 7

• GPCR respond to ligand and undergo confirmational change
• G protein moves towards activated GPCR
• GPCR-G protein interaction activates G protein
• Causes GTP to exchange for GDP on G protein a subunit
• a-by complex dissociates
• each can interact with effector protein

Question 8

What causes the termination of G-protein signalling?

Answer 8

• GTPase hydrolyses GTP on a subunit to GDP
• a-GDP and By subunits reform to form inactive heterotrimeric complex
• Ga acts as a timer and on/off switch

Question 9

What is the primary determinant for G-protein receptor selection?

Answer 9

Ga subunit

Question 10

List the G-alpha subunits associated with their major adrenergic and muscarinic receptors and the enzymes they act on.

Answer 10

GPCR/G protein/Effector

Adrenergic (adrenaline and noradrenaline)

• Q : a1-adrenoreceptors/Gq/phospholipase c (+)
• I : a2-adrenoreceptors/Gi/adenylyl cyclase (-)
• # S : b1-adrenoreceptors/Gs/adenylyl cyclase (+)
  
  • glycogenesis, lipolysis

Muscarinic (acetylcholine)

• Q : M1 receptor/Gq/phospholipase c (+)
• # I : M2 adrenoreceptor/Gi/adenylyl cyclase (-)
  
  • slowing of cardiac pacemaker
• # Q : M3 receptor/Gq/phospholipase c (+)
  
  • smooth muscle conytraction

Question 11

What are the 2 toxins that inhibit the G protein function?

Answer 11

1. Pertussis
2. Cholera

Question 12

How does pertussis affect G protein function?

Answer 12

• Pertussis toxin catalyses modification of a residue on alpha subunit of Gi and Go
• modified alpha subunits remain locked in an inactive state
• unable to inhibit adenylate cyclase

Question 13

How does Cholera affect G protein function?

Answer 13

• Cholera inhibits GTPase
• prevent termination of signalling by Gs preferring GPCRs
• leads to long-lasting activation of downstrem pathways

Question 14

How is signal amplification achieved?

Answer 14

• By intracellular signalling cascade :

Binding of ligand to a receptor can lead to the activation of enzymes that can substantially amplify initial signal