This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
[{'content': 'Action of a a subunit of G protein is ? \nSelect and explain. \nA. Break down of GTP to GDP, B. Conversion of GDP to GTP, C. Internalization of receptors, D. Binding of agonists', 'role': 'user'}, {'content': 'Answer: A \nExplanation: a subunit of G protein has intrinsic GTPase activity to breakdown GTP to GDP terminating the effector activation. G protein can be either stimulatory (Gs) or inhibitory (Gi). Stimulatory or innhibitory activity resides in the a subunit. Cholera toxin and peussis toxin acts on a subunit (by ADP ribosylation) inhibiting its GTPase activity - |cAMP - pathological effects. Cholera toxin acts on a subunit of Gs. Peussis toxin acts on a a subunit of Gi. REF: Ganong&;s Medical physiology 26th edition Pgno: 58.62', 'role': 'assistant'}]