Glycogen synthesis begins with glucose-1-phosphate formation from glucose-6-phosphate through the action of phosphoglucomutase. Glucose-l-phosphate then is converted to uridinediphosphate glucose (UDP-glucose), which can then be added to the glycogen already in storage (the glycogen primer). UDP-glucose can be added through a 1,6 linkage or a 1,4 linkage. Two high-energy bonds are used to incorporate each molecule of glucose into the glycogen. The straight chain glucose polymer comprises glucoses joined through the 1,4 linkage and is less compact than the branched chain glycogen, which has both 1,4 and 1,6 linkages. The addition of glucose to the primer glycogen with a 1,4 linkage is catalyzed by the glycogen synthase enzyme, while the 1,6 addition is catalyzed by the so-called glycogen branching enzyme, amylo (1 → 4, 1→ 6) transglucosidase. Once the liver and muscle cell achieve their full storage capacity, these enzymes are product inhibited and glycogenesis is “turned off.” Glycogen synthase is inactivated by a cAMP-dependent kinase and activated by a synthase phosphatase enzyme that is stimulated by changes in the ratio of ATP to ADR Glycogen synthesis is stimulated by the hormone insulin and suppressed by the catabolic hormones. The process does not fully cease but operates at a very low level. Glycogen does not accumulate appreciably in cells other than liver and muscle, although all cells contain a small amount of glycogen. Note that a glycogen primer is required for glycogen synthesis to proceed. This primer is carefully guarded so that some is always available when glycogen is synthesized. This means that glycogenolysis never fully depletes the cell of its glycogen content.