Receptor Mechanisms II

Question 1

Outline the structure of a GPCR

Answer 1

• 7 transmembrane domains
• Only made up of 1 protein
• Agonist binding site is amino terminus embedded within the membrane
• G protein interaction site found on other side of membrane - able to bind to the G protein

Question 2

What is the G protein made up of and what does it bind?

Answer 2

• Made up of 3 subunits(i.e a trimer) - a large α subunit that binds GDP in absence of agonist
• Alpha subunit is also bound to β and γ subunits, which behave as membrane anchors and keep the G protein bound to the membrane

Question 3

What happens upon binding of the agonist to a GPCR?

Answer 3

• The alpha subunit binds to GTP and GDP dissociates from the subunit. Alpha subunit then dissociates from β and γ subunits.
• Upon binding of the agonist, the alpha subunit will bind and activate an enzyme
• Upon activation, GTP is hydrolysed to GDP
• Alpha subunit returns to β and γ subunits

Question 4

Describe how the Gαs subunit works.

Answer 4

• Gαs subunit stimulates an increase in Adenylate Cyclase activity.
• Increases the conversion of ATP to cAMP
• Activates Protein Kinase A (PKA).
• Leads to further opening of ion channels and further activation of enzymes.

Question 5

Describe how the Gαi subunit works.

Answer 5

• The Gαi subunit decreases Adenylate Cyclase activity through inhibition.
• Less ATP is converted to cAMP.
• Reduced activation of Protein Kinase A.
• Reduced opening of ion channels and reduced enzyme activation i.e reduced response from effectors

Question 6

Describe how the Gαq/11 subunit works.

Answer 6

• The Gαq/11 subunit stimulates an increase in Phospholipase C activity.
• Causes cleavage of PIP2 to IP3 and DAG.
• IP3 then passes through the membrane and binds to complementary receptors in the SR and stimulates an influx of Ca2+ (from internal calcium stores) into the cytoplasm to cause contraction
• DAG remains in the lipid membrane and causes activation of Protein Kinase C, causing the opening of ion channels

Question 7

Acetylcholine binds to both nicotinic and muscarinic receptors.
However, the response for nicotinic receptors is much faster upon binding when compared to muscarinic receptors. Suggest a reason for this

Answer 7

• Nicotinic receptors are ligand-gated ion channel receptors whilst muscarinic receptors are examples of GPCRs
• LGICRs form aqueous pathways allowing for immediate movement of ions whilst GPCRs are more dependent on the activation of secondary messengers and biochemical cascades to elicit biological responses

Question 8

Outline the process of signal amplification

Answer 8

• The idea that the binding of a single ligand can cause the activation of further secondary messengers
• EXAMPLE: Binding of one adrenaline molecule to adrenoceptors causes the formation of 10,000 molecules of cAMP