The Metabolism of Glycogen in Animals

In a wide range of organisms, excess glucose is converted to polymeric forms for storage—glycogen in vertebrates and many microorganisms, starch in plants. In vertebrates, glycogen is found primarily in the liver and skeletal muscle; it may represent up to 10% of the weight of liver and 1% to 2% of the weight of muscle. If this much glucose were dissolved in the cytosol of a hepatocyte, its concentration would be about 0.4 m, enough to dominate the osmotic properties of the cell. When stored as a long polymer (glycogen), however, the same mass of glucose has a concentration of only 0.01 [M. Glycogen is stored in large cytosolic granules. The elementary particle of glycogen, the ^-particle, about 21 nm in diameter, consists of up to 55,000 glucose residues with about 2,000 nonreducing ends. Twenty to 40 of these particles cluster together to form a-rosettes, easily seen with the microscope in tissue samples from well-fed animals (Fig. 15-2) but essentially absent after a 24-hour fast.

The glycogen in muscle is there to provide a quick source of energy for either aerobic or anaerobic metabolism. Muscle glycogen can be exhausted in less than an hour during vigorous activity. Liver glycogen serves as a reservoir of glucose for other tissues when dietary glucose is not available (between meals or during a fast); this is especially important for the neurons of the brain, which cannot use fatty acids as fuel. Liver glycogen can be depleted in 12 to 24 hours. In humans, the total amount of energy stored as glycogen is far less than the

Glycogen Granules Hepatocyte

FIGURE 15-2 Glycogen granules in a hepatocyte. Glycogen is a storage form of carbohydrate in cells, especially hepatocytes, as illustrated here. Glycogen appears as electron-dense particles, often in aggregates or rosettes. In hepatocytes the glycogen is closely associated with tubules of the smooth endoplasmic reticulum. Many mitochondria are also present.

FIGURE 15-2 Glycogen granules in a hepatocyte. Glycogen is a storage form of carbohydrate in cells, especially hepatocytes, as illustrated here. Glycogen appears as electron-dense particles, often in aggregates or rosettes. In hepatocytes the glycogen is closely associated with tubules of the smooth endoplasmic reticulum. Many mitochondria are also present.

amount stored as fat (triacylglycerol) (see Table 23-5), but fats cannot be converted to glucose in mammals and cannot be catabolized anaerobically.

Glycogen granules are complex aggregates of glyco-gen and the enzymes that synthesize it and degrade it, as well as the machinery for regulating these enzymes. The general mechanisms for storing and mobilizing glycogen are the same in muscle and liver, but the enzymes differ in subtle yet important ways that reflect the different roles of glycogen in the two tissues. Glyco-gen is also obtained in the diet and broken down in the gut, and this involves a separate set of hydrolytic enzymes that convert glycogen (and starch) to free glucose.

The transformations of glucose discussed in this chapter and in Chapter 14 are central to the metabolism of most organisms, microbial, animal, or plant. We begin with a discussion of the catabolic pathways from glycogen to glucose 6-phosphate (glycogenolysis) and from glucose 6-phosphate to pyruvate (glycolysis), then turn to the anabolic pathways from pyruvate to glucose (gluconeogenesis) and from glucose to glyco-gen (glycogenesis).

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