Principles of Biochemistry Volume 3 - A. Lehninger 1985

Selected Aspects of Human Biochemistry
Hormones
Epinephrine also inhibits glycogen synthesis

Epinephrine not only stimulates Glycogen breakdown but simultaneously inhibits its synthesis from glucose in the Liver, thereby maximizing the release of glucose into the bloodstream. As shown in Fig. 25-12, the binding of epinephrine to The surface of hepatocytes and the subsequent production of cAMP stimulate the protein kinase-catalyzed phosphorylation of glycogen synthase, converting its active dephosphorylated form into an inactive phosphorylated form (see Section 20.14). Consequently, the signaling cascade leading to the downregulation of glycogen synthase activity shares the same initial trigger as Glycogenolysis aimed at releasing free glucose into the Blood.

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Fig. 25-12. Inhibition of glycogen synthesis by epinephrine. Protein kinase, activated by cyclic AMP, catalyzes the phosphorylation of glycogen synthase, thereby converting the enzyme into a less active form (see Section 20.14).

As a result, all available glycogen, glucose-6-phosphate, and other precursors are channeled into The production of free glucose to be released into the bloodstream. This maximizes the fuel supply to the Muscles, effectively preparing the Organism for a fight-or-flight response.

Epinephrine exerts its effects not only on the liver but also on Skeletal Muscle and The Heart, where it likewise promotes glycogen breakdown by stimulating muscle phosphorylase via cAMP production. Because muscles and the heart lack glucose-6-phosphatase, the end product of glycogenolysis in these Tissues is not blood glucose, but lactic acid generated from glucose-6-phosphate through Glycolysis. Thus, The stimulation of glycogen breakdown in muscles drives up The rate of glycolysis and ATP production, enabling a rapid surge in muscular activity.