Which of the following correctly describes the role of GDP and GTP in the activation of G protein-coupled receptors?

• A. GTP is hydrolyzed to GDP, which activates the receptor.
• B. GDP binds to the receptor, causing it to activate protein G.
• C. GTP is synthesized by the receptor to activate protein G.
• D. GDP is replaced by GTP in the alpha subunit of protein G, activating the protein.

Answer: (D)