Whether a G alpha subunit is active or inactive depends on which guanine nucleotide is bound to it. Binding of GDP or GTP results in the protein switching between two conformational states. Which of the following answers best describes the structural changes that occur in a G alpha subunit due to guanine nucleotide binding? Choose one: A. Phosphorylation of GDP to GTP by the G alpha subunit moves the switch II helix region from binding to G beta/gamma to binding effectors like adenylate cyclase. B. The helical region of G alpha, called switch II, which interacts with G beta/gamma in the inactive state, is brought into the interior of the G alpha protein, reducing contact with G beta/gamma. This permits G alpha interaction with effector proteins, like adenylate cyclase, since the switch Il region is now buried. C. Epinephrine directly binds and activates G alpha to allow the subunit to bind to an effector protein. D. Dissociation of GDP for GTP with the G alpha subunit structurally shifts the switch II helix region, allowing for the association of G alpha with its effector proteins, such as adenylate cyclase.