Which of the following enzymes is stimulated by glucagon?
Correct Answer: Glycogen phosphorylase
Description: Before you staed analyzing all of the answer choices you should have reminded yourself that glucagon increases serum glucose. So an enzyme stimulated by glucagon might be involved in either the breakdown of glycogen to glucose (glycogenolysis) or in the creation of glucose from noncarbohydrate precursors (gluconeogenesis). Glycogen phosphorylase catalyzes the first step in glycogenolysis; it makes sense that it would be stimulated by glucagon. Acetyl-CoA carboxylase catalyzes the first step in fatty acid synthesis, an anabolic process that would be stimulated by insulin, not glucagon. As its name implies, glycogen synthase is involved in the synthesis of glycogen. Glucagon (and epinephrine) stimulate the phosphorylation and inactivation of glycogen synthase. HMG-CoA reductase is the key enzyme involved in the synthesis of cholesterol. Since this is an anabolic process that occurs in the well-fed state, you would expect it to be stimulated by insulin and inhibited by glucagon (which it is). Ref: Bender D.A., Mayes P.A. (2011). Chapter 14. Carbohydrates of Physiologic Significance. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.
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