Final answer:
The G-protein alpha subunit that has bound GTP is active and can catalyze the activation of other enzymes.
Step-by-step explanation:
The G-protein alpha subunit that has bound GTP is active and can catalyze the activation of other enzymes.
When a ligand binds to a G-protein-linked receptor, it causes a shape change in the receptor, which activates the G-protein. This activated G-protein then interacts with membrane-bound enzymes, such as adenylyl cyclase, to catalyze the conversion of ATP to cAMP. cAMP, in turn, activates protein kinases that can transfer a phosphate group from ATP to a substrate molecule, thereby activating it.
For example, in the presence of a ligand, the alpha subunit of the G-protein may activate adenylyl cyclase, leading to the production of cAMP. This cAMP can then activate other proteins, such as transcription factors, which can affect gene expression.