What event would activate a G protein?

• A. Hydrolysis of GTP to GDP
• B. Hydrolysis of GDP to GTP
• C. Phosphorylation of GDP to GTP
• D. Phosphorylation of GTP to GDP
• E. Replacement of GDP with GTP

Answer: (E)

Verified step by step guidance

1. Understand the role of G proteins: G proteins are molecular switches that play a crucial role in transmitting signals from outside the cell to the inside. They are activated by the binding of GTP and inactivated when GTP is hydrolyzed to GDP.
2. Identify the process of activation: Activation of a G protein involves the exchange of GDP for GTP. This exchange is facilitated by a receptor or other signaling molecule that causes the G protein to release GDP and bind GTP.
3. Clarify the concept of nucleotide exchange: The activation of G proteins is not through phosphorylation or hydrolysis but through the replacement of GDP with GTP. This exchange alters the conformation of the G protein, enabling it to interact with downstream effectors.
4. Recognize the significance of GTP binding: When GTP binds to the G protein, it triggers a conformational change that activates the protein, allowing it to propagate the signal within the cell.
5. Conclude with the correct event: The activation of a G protein is specifically due to the replacement of GDP with GTP, which is the key event that initiates the signaling cascade.