What happens after a ligand binds to a G-protein linked receptor?

• A. The receptor undergoes dimerization.
• B. GDP is released and replaced by GTP.
• C. Adenylate Cyclase is inhibited.
• D. The ion channel opens immediately.

Answer: (B)

When a ligand binds to a G-protein linked receptor, one of the key events is the exchange of GDP for GTP on the intracellular portion of the G-protein that is coupled to the receptor. This is a crucial step in the signaling pathway. The binding of the ligand causes a conformational change in the receptor that activates the G-protein. As a result, GDP, which is bound to the G-protein in its inactive state, is released and is quickly replaced by GTP. This activation triggers the G-protein to dissociate from the receptor and allows the G-protein to interact with downstream effectors in the signaling pathway, such as enzymes or ion channels, leading to a physiological response.

The other options do not accurately depict the immediate consequences of ligand binding to G-protein linked receptors. For instance, dimerization is more characteristic of receptor types like receptor tyrosine kinases and not typically associated with G-protein coupled receptors. Additionally, while adenylate cyclase can be inhibited in certain pathways, this is a downstream effect that occurs based on the type of G-protein activated (either stimulatory or inhibitory) rather than a direct consequence of ligand binding. Finally, the opening of ion channels can occur due