After studying this chapter, you should be able to:
Describe the structure of glycogen and its importance as a carbohydrate reserve.
Describe the synthesis and breakdown of glycogen and how the processes are regulated in response to hormone action.
Describe the various types of glycogen storage diseases.
Glycogen is the major storage carbohydrate in animals, corresponding to starch in plants; it is a branched polymer of α-d-glucose (see Figure 15–12). It occurs mainly in liver and muscle, with modest amounts in the brain. Although the liver content of glycogen is greater than that of muscle, because the muscle mass of the body is considerably greater than that of the liver, about three-quarters of total body glycogen is in muscle (Table 18–1).
TABLE 18–1Storage of Carbohydrate in a 70-kg Person ||Download (.pdf) TABLE 18–1 Storage of Carbohydrate in a 70-kg Person
| ||Percentage of Tissue Weight ||Tissue Weight ||Body Content (g) |
|Liver glycogen ||5.0 ||1.8 kg || 90 |
|Muscle glycogen ||0.7 ||35 kg ||245 |
|Extracellular glucose ||0.1 ||10 L || 10 |
Muscle glycogen provides a readily available source of glucose-1-phosphate for glycolysis within the muscle itself. Liver glycogen functions as a reserve to maintain the blood glucose concentration in the fasting state. The liver concentration of glycogen is about 450 mmol/L glucose equivalents after a meal, falling to about 200 mmol/L after an overnight fast; after 12 to 18 hours of fasting, liver glycogen is almost totally depleted. Although muscle glycogen does not directly yield free glucose (because muscle lacks glucose-6-phosphatase), pyruvate formed by glycolysis in muscle can undergo transamination to alanine, which is exported from muscle and used for gluconeogenesis in the liver (see Figure 19–4). Glycogen storage diseases are a group of inherited disorders characterized by deficient mobilization of glycogen or deposition of abnormal forms of glycogen, leading to liver damage and muscle weakness; some result in early death.
The highly branched structure of glycogen (see Figure 15–12) provides a large number of sites for glycogenolysis, permitting rapid release of glucose-1-phosphate for muscle activity. Endurance athletes require a slower, more sustained release of glucose-1-phosphate. The formation of branch points in glycogen is slower than the addition of glucose units to a linear chain, and some endurance athletes practice carbohydrate loading—exercise to exhaustion (when muscle glycogen in largely depleted) followed by a high-carbohydrate meal, which results in rapid glycogen synthesis, with fewer branch points than normal.
GLYCOGENESIS OCCURS MAINLY IN MUSCLE & LIVER
Glycogen Biosynthesis Involves UDP-Glucose
As in glycolysis, glucose is phosphorylated to glucose-6-phosphate, catalyzed by hexokinase in muscle and glucokinase in liver (Figure 18–1). Glucose-6-phosphate is isomerized to glucose-1-phosphate by phosphoglucomutase. The enzyme itself is phosphorylated, and the phosphate ...