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 adenyl cyclase. Caffeine does not directly interact with glycogen phosphorylase or hormone-sensitive lipase 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. Ref: O'Brien C.P. (2011). Chapter 24. Drug Addiction. In L.L. Brunton, B.A. Chabner, B.C. Knollmann (Eds), Goodman & Gilman's The Pharmacological Basis of Therapeutics, 12e