Structure and Function of Glycogen

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Chapter: Biochemistry : Glycogen Metabolism

The main stores of glycogen are found in skeletal muscle and liver, although most other cells store small amounts of glycogen for their own use.


STRUCTURE AND FUNCTION OF GLYCOGEN

The main stores of glycogen are found in skeletal muscle and liver, although most other cells store small amounts of glycogen for their own use. The function of muscle glycogen is to serve as a fuel reserve for the synthesis of adenosine triphosphate (ATP) during muscle contraction. That of liver glycogen is to maintain the blood glucose concentration, particularly during the early stages of a fast (Figure 11.2;). [Note: Liver glycogen can maintain blood glucose for 10–18 hours.]


Figure 11.2 Functions of muscle and liver glycogen. P = phosphate; Pi = inorganic phosphate.

 

A. Amounts of liver and muscle glycogen

Approximately 400 g of glycogen make up 1%–2% of the fresh weight of resting muscle, and approximately 100 g of glycogen make up to 10% of the fresh weight of a well-fed adult liver. What limits the production of glycogen at these levels is not clear. However, in some glycogen storage diseases ([GSDs] see Figure 11.8), the amount of glycogen in the liver and/or muscle can be significantly higher. [Note: In the body, muscle mass is greater than liver mass. Consequently, most of the body’s glycogen is found in muscle.]

 

B. Structure of glycogen

Glycogen is a branched-chain polysaccharide made exclusively from α-D-glucose. The primary glycosidic bond is an α(1→4) linkage. After an average of eight to ten glucosyl residues, there is a branch containing an α(1→6) linkage (Figure 11.3). A single glycogen molecule can have a molecular weight of up to 108 Da. These polymers of glucose exist in discrete cytoplasmic granules that also contain most of the enzymes necessary for glycogen synthesis and degradation.


Figure 11.3 Branched structure of glycogen, showing α(1→4) and α(1→6) glycosidic bonds.

 

C. Fluctuation of glycogen stores

Liver glycogen stores increase during the well-fed state and are depleted during a fast. Muscle glycogen is not affected by short periods of fasting (a few days) and is only moderately decreased in prolonged fasting (weeks). Muscle glycogen is synthesized to replenish muscle stores after they have been depleted following strenuous exercise. [Note: Glycogen synthesis and degradation go on continuously. The differences between the rates of these two processes determine the levels of stored glycogen during specific physiologic states.]


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