Question

immediately after a membrane receptor protein binds to its ligand, which of the following processes occurs?
○ cytoplasmic enzymes are activated by phosphorylation.
○ an intracellular g protein is activated.
○ the membrane receptor protein undergoes a conformational change.
○ the receptor/ligand complex enters the cytoplasm by endocytosis.

Explanation:

When a membrane receptor protein binds to its ligand, the first event is a conformational change in the receptor protein itself. Activation of cytoplasmic enzymes by phosphorylation (first option) or activation of G - proteins (second option) are downstream events that occur after the receptor has undergone a conformational change. The receptor/ligand complex does not typically enter the cytoplasm by endocytosis immediately after binding (fourth option); this is a different process for some receptors but not the immediate event after ligand binding. So the immediate event after ligand binding to a membrane receptor is the conformational change of the receptor.

Answer:

The membrane receptor protein undergoes a conformational change.