The polysaccharide glycogen (page 30) is used as a store of glucose particularly in liver and muscle cells. We saw in Chapter 2 how the glycosidic bond can be hydrolyzed with the broken ends of the bond being sealed with groups from a water molecule, so that a hydrogen atom is added to one side of the broken bond and a hydroxyl group is added to the other (page 44). The enzyme glycogen phosphorylase specifically breaks the a(1 ^4) glycosidic bond in glycogen but seals the broken ends with groups from inorganic phosphate, so that a hydrogen atom is added to one side of the broken bond and a phosphate group is added to the freed glucose monomer (Fig. 13.5). The resulting glucose-1-phosphate is readily converted to glucose-6-phosphate for glycolysis. Breaking up glycogen this way is more energy efficient than simply hydrolyzing it (as happens in the intestine) since the ATP that would otherwise be required to make glucose-6-phosphate from free glucose is saved. The occasional a(1^6) links that attach side arms to the glycogen chain are broken by other enzymes.
One of the many important roles of the liver is to maintain the level of glucose in the blood—it is the most important of the circulating fuels and is the primary fuel for red
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