Final answer:
The exchange of GDP and GTP on the alpha subunit of the G-protein determines its two states, ON and OFF. When a signaling molecule binds to the G-protein-coupled receptor, the alpha subunit exchanges GDP for GTP, activating the G-protein. Hydrolysis of GTP to GDP inactivates the G-protein and switches it back to the OFF state.
Step-by-step explanation:
The two states of a G-protein, ON and OFF, are determined by the exchange of guanosine triphosphate (GTP) and guanosine diphosphate (GDP) on the alpha (a) subunit of the G-protein.
When a signaling molecule binds to the G-protein-coupled receptor, the a subunit exchanges GDP for GTP, causing the G-protein to switch from the OFF state to the ON state. This activates the G-protein, allowing it to interact with downstream signaling molecules and initiate cellular responses.
The GTP-bound a subunit can then hydrolyze the GTP to GDP, resulting in the inactivation of the G-protein and the switch back to the OFF state. The subunits of the G-protein reassociate and are ready to undergo another cycle of activation and inactivation.
Therefore, the exchange of GDP and GTP on the a subunit of the G-protein is the key determinant of the two states of G-protein, ON and OFF.