Final Answer:
In the G protein, the activation of the alpha subunit requires a nucleotide exchange (GDP to GTP).
Step-by-step explanation:
G proteins are molecular switches that play a crucial role in signal transduction across cell membranes. The G protein consists of three subunits: alpha (α), beta (β), and gamma (γ). The alpha subunit, when bound to guanosine diphosphate (GDP), is in an inactive state. Upon activation by a receptor, the GDP is exchanged for guanosine triphosphate (GTP), leading to a conformational change in the alpha subunit.
This activated form of the alpha subunit can then interact with effector proteins to initiate intracellular signaling cascades. The G protein cycle involves the subsequent hydrolysis of GTP to GDP, returning the alpha subunit to its inactive state.