Final answer:
In cell signaling pathways, the exchange of GDP for GTP on G-proteins activates them. The concentration of GTP is usually higher during active signaling but varies based on the cell's regulatory mechanisms. Hydrolysis of GTP to GDP by GTPase enzymes inactivates the G-protein, controlling the signaling process.
Step-by-step explanation:
Within cells, the balance between guanosine diphosphate (GDP) and guanosine triphosphate (GTP) is a critical aspect of the G-protein signaling pathway. In its inactive state, a G-protein is bound to GDP.
When a signaling molecule such as a hormone binds to a G-protein-coupled receptor on the cell membrane, the G-protein undergoes a conformational change, causing the release of GDP and the binding of GTP in its place. This exchange activates the G-protein, which then dissociates into alpha (a) and beta-gamma (By) subunits.
These subunits can trigger various cellular responses. The signal is terminated when the GTP bound to the alpha subunit is hydrolyzed back to GDP, rendering the G-protein inactive again.
The intracellular concentration of GTP is typically higher when a cell is actively responding to a signaling event. However, this concentration can vary depending on the cell type and the presence of signaling molecules.
It is important to note that the ratio of GTP to GDP is not static and is regulated by the activation state of cell signaling pathways and related enzymatic activities such as GTPase, which hydrolyze GTP to GDP.