Allosteric interactions are superimposed on an even more sophisticated control system involving covalent modification - phosphorylation/dephosphorylation of glycogen synthase and glycogen phosphorylase. Since the synthesis of glycogen is closely coordinated with its degradation, phosphorylation has opposite effects on the enzymatic activities of glycogen synthase and phosphorylase. This reversible phosphorylation is responsive to hormones such as insulin and epinephrine (that stimulates glycogen breakdown) and glucagon (that stimualtes its synthesis).