Advanced Placement (AP) Biology Practice Exam

Which signal transduction pathway is activated when GTP displaces GDP?

1. G-protein linked receptor pathway

2. Tyrosine kinase receptor pathway

3. Ion channel receptor pathway

4. Receptor tyrosine phosphatase pathway

The correct answer is: G-protein linked receptor pathway

The signal transduction pathway activated when GTP displaces GDP is associated with G-protein linked receptors. This mechanism involves a G-protein, which is a guanosine nucleotide-binding protein that can exist in an inactive form bound to GDP. When a ligand binds to a G-protein coupled receptor (GPCR), it causes a conformational change in the receptor, enabling it to facilitate the exchange of GDP for GTP on the associated G-protein. The binding of GTP activates the G-protein, leading to the dissociation of its subunits and the subsequent activation of downstream signaling pathways. In contrast, the pathways related to tyrosine kinase receptors, ion channel receptors, and receptor tyrosine phosphatases function through different mechanisms that do not involve the direct exchange of GDP for GTP as a primary step in their activation. Tyrosine kinase receptors usually operate through phosphorylation of tyrosine residues, ion channel receptors change conformation in response to ligands allowing ions to flow, and receptor tyrosine phosphatases remove phosphate groups from tyrosines on substrates, none of which are initiated by GTP binding as in G-protein linked pathways. This specificity for GTP and its role in signaling makes the G-pro