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Introduction

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High-Yield Terms

  • Glycosidic linkage: covalent bond that joins a carbohydrate (sugar) molecule to another group such as another sugar typical of the bonds between glucose molecules in glycogen

  • Glycogenin: protein with self-glycosylating activity that serves as the primer molecule for initiation of glycogen synthesis

  • Glycogen synthase-phosphorylase kinase: this enzyme is critical for the regulation of the flux of glucose into and out of glycogen as it reciprocally regulates the 2 key enzymes of glycogenolysis and glycogen synthesis

  • Glycogen storage disease: disorders that result from defects in genes encoding enzymes involved in the process of glycogen synthesis or breakdown, primarily affecting muscles and liver

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Glycogen Composition

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Stores of readily available glucose, used to supply the tissues with an oxidizable energy source, are found principally in the liver, as glycogen. Glycogen is a polymer of glucose residues linked by α-(1,4)- and α-(1,6)-glycosidic bonds. A second major source of stored glucose is the glycogen of skeletal muscle. However, muscle glycogen is not generally available to other tissues, because muscle lacks the enzyme glucose 6-phosphatase (Figure 14-1).

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FIGURE 14-1:

Section of glycogen showing α-1,4- and α-1,6-glycosidic linkages. Reproduced with permission of the medical biochemistry page, LLC.

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The major site of daily glucose consumption (75%) is within the brain. The remainder of it is utilized mostly by erythrocytes, skeletal muscle, and heart muscle. The body obtains glucose either directly from the diet or from amino acids and lactate via gluconeogenesis (see Chapter 13). Glucose obtained from these 2 primary sources either remains soluble in the body fluids or is stored as the glucose polymer, glycogen. Glycogen is considered the principal storage form of glucose and is found mainly in liver and muscle, with kidney and intestines adding minor storage sites. With up to 10% of its weight as glycogen, the liver has the highest specific content of any body tissue. Muscle has a much lower amount of glycogen per unit mass of tissue, but since the total mass of muscle is so much greater than that of liver, total glycogen stored in muscle is about twice that of liver. Stores of glycogen in the liver are considered the main buffer of blood glucose levels (Table 14-1).

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TABLE 14-1:Storage of Carbohydrate in a 70-kg Human Being
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Glycogen Synthesis (Glycogenesis)

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De novo glycogen synthesis is initiated by ...

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