Figure 25.6 Insulin inactivates glycogen synthase kinase. Insulin triggers a cascade that leads to the phosphorylation and inactivation of glycogen synthase kinase and prevents the phosphorylation of glycogen synthase. Protein phosphatase 1 (PP1) removes the phosphoryl groups from glycogen synthase, thereby activating the enzyme and allowing glycogen synthesis. Abbreviation: IRS, insulin-