Receptor Mechanism II

Question 1

Describe the basic structure of a G-Protein Coupled Receptor.

Answer 1

It is a protein made up of 7 domains. It has an agonist-binding site on one side, and a G-protein interaction site on the other. Coupled to it are 3 G protein subunits: α, β and γ. The αG subunit contains ATPase, and uses ATP to go and activate an enzyme (specific to each type of αG subunit).
The β and γ subunits can have roles in stimulating K+ channels.

Question 2

Describe how the αGs subunit works.

Answer 2

When activated, the αGs subunit increases Adenylate Cyclase activity. This converts more ATP to cAMP. This in turn activated Protein Kinase A (PKA).

Question 3

Describe how the αGi subunit works.

Answer 3

When activated, the αGi subunit decreases Adenylate Cyclase activity. This means that less ATP is converted to cAMP.

Question 4

Describe how the αGq/11 subunit works.

Answer 4

When activated, the αGq/11 subunit increases Phospholipase C activity. This breaks down PIP2 (phosphatidylinositol) to IP3 (inositol triphosphate) and DAG (diacylglycerol) . The IP3 then goes to calcium release channels on the SR and stimulates an influx of Ca2+ into the cytoplasm. This activates PKC.