You are studying the responses of adipocytes in culture following their exposure to γ-irradiation. In comparison to normal adipocytes you find that when glucagon is added, the normal cells release free fatty acids to the medium but the irradiated cells do not. Binding studies show that both cell populations bind the same amount of glucagon with the same dissociation kinetics, indicating the receptors on the irradiated cells are unchanged. Given these results, which of the following proteins is most likely to be defective in the irradiated cells? A) a G_q-type G-protein B) a G_s-type G-protein C) phosphatidic acid phosphatase D) phosphatidylinositol-3-kinase (PI3K) E) protein kinase C (PKC)

Question

Quizplus · Accepted Answer

The Answer is B
EXPLANATION: When glucagon binds its receptor on adipocytes, it activates an associated G_s-type G-protein. The activated G-protein in turn activates adenylate cyclase, which catalyzes the synthesis of cAMP from ATP. The cAMP binds to the regulatory subunits of protein kinase A (PKA), resulting in release of the catalytic subunits. One of the targets of PKA is the enzyme, hormone-sensitive lipase, HSL, which is activated by PKAmediated phosphorylation. Activation of HSL results in increased release of fatty acid from stored triglycerides.

You Are Studying the Responses of Adipocytes in Culture Following