Figure 1. Haworth-style projection of the structure of -d-glucose.

Figure 2. Structure of sucrose.

Figure 3. Structure of glycogen. Modified with permission from Marks DB, Marks AD and Marks CM (eds) (1996) Medical Biochemistry: A Clinical Approach, 1st edn. Baltimore MD, USA: Lippincott, Williams and Wilkens.

Figure 4. The role of glycogenin in glycogen synthesis. Modified with permission from Devlin TM (ed.) (2002) Textbook of Biochemistry with Clinical Correlations. New York: Wiley–Liss.

Figure 5. Mode of action of PKA activation. In this example glucagon binds to its¢ cell-surface receptor, thereby activating the receptor. Activation of the receptor is coupled to the activation of a receptor-coupled G-protein (GTP-binding and hydrolysing protein) composed of 3 subunits. Upon activation the alpha subunit dissociates and binds to and activates adenylate cyclase. Adenylate cyclase then converts ATP to cyclic-AMP (cAMP). The cAMP thus produced then binds to the regulatory subunits of PKA leading to dissociation of the associated catalytic subunits. The catalytic subunits are inactive until dissociated from the regulatory subunits. Once released the catalytic subunits of PKA phosphorylate numerous substrates using ATP as the phosphate donor.

Figure 6. Regulation of glycogen synthase by phosphorylation and dephosphorylation. PKA is cAMP-dependent protein kinase. PPI-1 is phosphoprotein phosphatase-1 inhibitor. Whether a factor has positive (+ve) or negative (–ve) effects on any enzyme is indicated. Briefly, glycogen synthase a is phosphorylated, and rendered much less active and requires glucose 6-phosphate to have any activity at all. Phosphorylation of glycogen synthase is accomplished by several different enzymes. The most important is synthase-phosphorylase kinase the same enzyme responsible for phosphorylation (and activation) of glycogen phosphorylase. PKA (itself activated through receptor mediated mechanisms) also phosphorylates glycogen synthase directly. The effects of PKA on PPI-1 are the same as those described above for the regulation of glycogen phosphorylase. The other enzymes shown to directly phosphorylate glycogen synthase are protein kinase C (PKC), calmodulin-dependent protein kinase, glycogen synthase kinase-3 (GSK-3) and two forms of casein kinase (CK-I and CK-II). The enzyme PKC is activated by  Ca²⁺ ions and phospholipids, primarily diacylglycerol, DAG. DAG is formed by receptor-mediated hydrolysis of membrane phosphatidylinositol bisphosphate (PIP₂).

Figure 7. Pathways involved in the regulation of glycogen synthase by epinephrine activation of -adrenergic receptors. PKC is protein kinase C. PLC- is phospholipase C-. The substrate for PLC- is phosphatidylinositol-4,5-bisphosphate (PIP₂) and the products are IP₃, inositol trisphosphate and DAG, diacylglycerol.

Figure 8. Regulation of glycogen phosphorylase activity. PKA is cAMP-dependent protein kinase. PPI-1 is phosphoprotein phosphatase-1 inhibitor. Whether a factor has positive (+ve) or negative (–ve) effects on any enzyme is indicated. Briefly, phosphorylase b is phosphorylated, and rendered highly active, by glycogen synthase-phosphorylase kinase (phosphorylase kinase for short in this figure). Phosphorylase kinase is itself phosphorylated, leading to increased activity, by PKA (itself activated through receptor mediated mechanisms). PKA also phosphorylates PPI-1 leading to an inhibition of phosphate removal allowing the activated enzymes to remain so longer. Calcium ions can activate phosphorylase kinase even in the absence of the enzyme being phosphorylated. This allows neuromuscular stimulation by acetylcholine, which results in increased intracellular free  Ca²⁺, to lead to increased glycogenolysis in the absence of receptor-mediated stimulation of PKA activity.

Figure 9. Epinephrine regulation of glycogen phosphorylase through activation of -adrenergic receptors. PLC- is phospholipase C-. The substrate for PLC- is phosphatidylinositol-4,5-bisphosphate (PIP₂) and the products are IP₃, inositol trisphosphate and DAG, diacylglycerol.