Which statement regarding the control of glycogen metabolism by cyclic AMP is TR

Question

Which statement regarding the control of glycogen metabolism by cyclic AMP is TRUE? A. Cyclic AMP binds allosterically to both glycogen phosphorylase and glycogen synthase, activating the former and inactivating the latter. B. Cyclic AMP stimulates the phosphorylation of glycogen phosphorylase and glycogen synthase, activating both. C. Cyclic AMP stimulates the phosphorylation of glycogen phosphorylase and the dephosphorylation of glycogen synthase, activating the former and deactivating the latter. D. Cyclic AMP stimulates the phosphorylation of both glycogen phosphorylase and glycogen synthase, activating the former and deactivating the latter. E. Cyclic AMP increases the intracellular concentration of calcium ions which stimulates the dephosphorylation of both glycogen phosphorylase and glycogen synthase.

Explanation / Answer

Answer: (D) Cyclic AMP stimulates the phosphorylation of both glycogen phosphorylase and glycogen synthase, activating the former and deactivating the latter.

Glycogen breakdown and synthesis are reciprocally regulated by a hormone-triggered cAMP cascade acting through protein kinase A. In addition to phosphorylating and activating phosphorylase kinase, protein kinase A adds a phosphoryl group to glycogen synthase, which leads to a decrease in enzymatic activity. This important control mechanism prevents glycogen from being synthesized at the same time that it is being broken down.

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