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Hormones bind to cell-surface receptors, activating the alpha unit of the stimulatory G protein (Gs.) The activation stimulates exchange of GDP bound to Gs-alpha for GTP. GTP bound to the alpha unit is slowly hydrolyzed to GDP, thereby deactivating the cascade.
Adenylate cyclase stimulation takes place after receptors bind hormones because:
A. of the exchange of GTP to GDP bound to Gs-alpha
B. of the exchange of GDP for GTP bound to Gs-alpha
Answer is A because:
when hormone binds to receptor, Gs-alpha releases GDP and binds GTP. As long as it has GTP bound, it stimulates adenylate cyclase activity to produce cAMP (supporting choice A and eliminating choice B.)
Putting it into a sentence with their answer, "stimulation takes place after receptors bind hormones because GTP is exchanged for GDP, deactivating the cascade." That makes absolutely no sense because it's not saying why, it's just saying what would happen next...and if you want to say what happens next, why wouldn't you first mention GDP exchange for GTP which happens before GTP hydrolysis to GDP?
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