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Old 12-15-2012
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Default Pharm Q

A seasoned marathon runner drinks a cup of strong black coffee about an hour before his race to enhance his performance. By which of the following mechanisms does caffeine directly contribute to enhanced performance?
A. Allosteric activation of adenylyl cyclase

B. Allosteric activation of glycogen phosphorylase

C. Allosteric activation of hormone sensitive lipase

D. Inhibition of dephosphorylation of protein kinase A

E. Inhibition of phosphodiesterase
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Old 12-15-2012
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Default my answer :)

E. Inhibition of phosphodiesterase
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E. Inhibition of phosphodiesterase
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Old 12-16-2012
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Default Yes, E. :)

The correct answer is E. Caffeine inhibits phosphodiesterase and thereby prevents cAMP from being degraded. Consequently, cAMP continues to activate protein kinase A, which results in both glycogen and triglyceride breakdown.

Caffeine has no direct or indirect effect on adenylyl cyclase (choice A).

Caffeine does not directly interact with glycogen phosphorylase (choice B) or hormone-sensitive lipase (choice C) and cannot allosterically activate these enzymes. However, because cAMP is not degraded due to the action
of caffeine on phosphodiesterase, the cAMP will continue to activate protein kinase A. In turn, activation of protein kinase A will lead to phosphorylation of glycogen phosphorylase and hormone-sensitive lipase, resulting in glycogen and triglyceride breakdown.

Protein kinase A is not phosphorylated under normal physiological circumstances, and therefore caffeine cannot inhibit its dephosphorylation (choice D).
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