GlucoseGlucose 6-phosphateFructose 6-phosphateGlyceraldehyde 3-phosphatePyruvateGlycogen (glucose)nRibose 5-phosphateCHAPTER 15 - GLYCOGEN METABOLISM AND GLUCONEOGENESISIntroductionCertain tissues, such as the brain and red blood cells, rely on glucose for fuel. Serumglucose levels must be maintained at about 5 mM.Serum glucose is maintained by dietary sources, glycogen breakdown, and synthesis fromnoncarbohydrate precursors via gluconeogenesis (see Figure 1). Glucose is polymerized toglycogen (animal starch) to avoid osmotic imbalances and stored primarily in the muscle andliver. Muscle tissue maintains a store of glycogen for its own use. The liver stores glycogenstores glycogen and breaks it down to glucose for export to other tissues. Notice in the schemebelow that glycogen metabolism intersects glycolysis at glucose-6-phosphate. Keep in mindthat, since glycogen is stored in a muscle cell for use only in that cell, glucose-6-phosphate isnever hydrolyzed to glucose in muscle cells since once glucose is formed it can leave the cell(glucose-6-phosphate cannot leave the cell).Glycogen breakdown can be thought of as a mini-pathway that intersects glycolysis at glucose-6-phosphate. As is typically the case, there is a complementary, biosynthetic pathway that linksthe same endpoints. As is also typical, some of the enzymes are utilized by both catabolic andbiosynthetic pathways, but at one point at the minimum, different enzymes are employed.
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