Final answer:
A G-protein must be bound to GTP (guanosine triphosphate) to be in its active state. This GTP binding occurs as part of the cell signaling processes facilitated by G-protein-linked receptors and leads to the activation of various cellular responses.
Step-by-step explanation:
For a G-protein to be in its active state, it must be bound to guanosine triphosphate, or GTP. During the cycle of cell signaling using G-protein-linked receptors, the inactive G-protein initially binds GDP. When a signaling molecule, such as a hormone, binds to the receptor, it triggers the G-protein to release GDP and pick up GTP, thereby activating it.
The activated G-protein can then go on to activate other proteins, such as adenylyl cyclase, which further propagates the signaling cascade by converting ATP to cAMP. cAMP then activates protein kinases, leading to various cellular responses. Eventually, the GTP bound to the active G-protein is hydrolyzed back to GDP, rendering the G-protein inactive, and the cycle can start again.