Signal Transduction-G Protein Coupled Receptors Practice

A ligand binds to a G protein–coupled receptor (GPCR) on the surface of a cell. Which of the following events occurs immediately after ligand binding?

A. Activation of adenylate cyclase

B. Hydrolysis of GTP to GDP on the α-subunit

C. Exchange of GDP for GTP on the G protein’s α-subunit

D. Phosphorylation of target proteins by protein kinase A

E. Binding of the G protein to another receptor molecule

C

Describe the role of adenylyl cyclase and cyclic AMP (cAMP) in a GPCR signaling pathway. How does this contribute to signal amplification?

• In a G protein-coupled receptor (GPCR) pathway, activated G proteins stimulate the enzyme adenylyl cyclase to produce cyclic AMP (cAMP) from ATP.

• This process amplifies the signal because a single activated adenylyl cyclase molecule can generate many cAMP molecules, which then act as a "second messenger" to activate other proteins, such as Protein Kinase A (PKA), inside the cell.

A student treats cultured cells with a drug that prevents GTP from binding to G proteins. Which of the following outcomes is most likely?

A. The receptor will continuously activate adenylyl cyclase.

B. The G protein will remain inactive and signaling will stop.

C. cAMP levels will increase.

D. Protein kinase A will remain active.

B

Researchers introduce a mutation into the gene encoding a GPCR that prevents it from undergoing conformational change upon ligand binding. Predict how this mutation would affect the signaling pathway and describe one method scientists could use to measure this effect experimentally.

• A mutation that prevents a G-protein coupled receptor (GPCR) from undergoing conformational change upon ligands binding would inhibit the signaling pathway

• Scientists could use a radioligand binding assay to measure the effect of the mutation.