Glycogen Degradation

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  • 24.1 Glycogen Breakdown Requires Several Enzymes

  • 24.2 Phosphorylase Is Regulated by Allosteric Interactions and Reversible Phosphorylation

  • 24.3 Epinephrine and Glucagon Signal the Need for Glycogen Breakdown

Glycogen is a key source of energy for runners. Glycogen mobilization—the conversion of glycogen into glucose—is highly regulated.

Glycogen is a very large, branched polymer of glucose residues (Figure 24.1). Most of the glucose residues in glycogen are linked by α-1,4-glycosidic bonds, and branches at about every 10th residue are created by α-1, 6-glycosidic bonds. Glycogen is not as reduced as fatty acids are and, consequently, not as energy rich. So why isn’t all excess fuel stored as fatty acids rather than as glycogen? The readily mobilized glucose from glycogen is a good source of energy for sudden, strenuous activity. Unlike fatty acids, the released glucose can provide energy in the absence of oxygen and can thus supply energy for anaerobic activity. Moreover, the controlled release of glucose from glycogen maintains the blood-glucose concentration between meals. The circulating blood keeps the brain supplied with glucose, which is virtually the only fuel used by the brain, except during prolonged starvation.

Figure 24.1: Glycogen. (A) Glucose units joined by α-1,4 linkages are shown as straight lines. The nonreducing ends of the glycogen molecule form the surface of the glycogen granule. At the core of the glycogen molecule is the protein glycogenin (yellow, Chapter 25). Degradation takes place at this surface. (B) A cross section of a glycogen molecule shows the branching caused by the α-1,6 linkages. The glycogenin is identified as G.

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Glycogen is present in bacteria, archae, and eukaryotes. Recall that plants store glucose as starch, a similar biomolecule. Thus, storing energy as glucose polymers is common to all forms of life. In humans, most tissues have some glycogen, but the two major sites of glycogen storage are the liver and skeletal muscle. The concentration of glycogen is higher in the liver than in muscle (10% versus 2% by weight), but more glycogen is stored in skeletal muscle overall because there is more skeletal muscle in the body than there is liver tissue. Glycogen is present in the cytoplasm in the form of granules ranging in diameter from 10 to 40 nm, containing about 55,000 glucose molecules. In the liver, glycogen synthesis and degradation are regulated to maintain the concentration of glucose in the blood required to meet the needs of the organism as a whole. The glucose is parceled out from the liver during a nocturnal fast, maintaining brain function throughout the night. In contrast, in muscle, these processes are regulated to meet the energy needs of the muscle itself. Glycogen breakdown takes place to fuel the ATP needs of muscle contraction. The depletion of muscle glycogen is thought to be a major component of exhaustion—“bonking” or “hitting the wall.”