Chapter 21 Biochemistry6e

Chapter 21 Biochemistry6e - Chapter 21 Biochemistry...

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Unformatted text preview: Chapter 21 Biochemistry Glycogen Metabolism-6th Edition by Berg, Tymoczko and Stryer [Note-Due to time limitations, only selected parts of this chapter will be covered.] Glycogen is not only a glucose storage molecule, it has the advantage of being readily mobilized. It is a large polymer, branched at both -1,4- and -1,6-glycosidic linkages with most of the linkages being -1,4 and about every 10 th linkage being an -1,6-glycosidic bond ( Figure 21.1 ). The -glycosidic linkages result in the polymer being open and helical. While glycogen contains less energy (it is less reduced) than the corresponding amount of fat, it is what the body prefers to maintain controlled levels of glucose in the blood and serum, it is the preferred energy source of the brain (which only changes in times of starvation) and, importantly, it is A) easily and quickly mobilized during times of stress, such as when epinephrine results in a rush of glucose to the tissues and B) it does not require oxygen (in contrast to fatty acid metabolism) to be used to generate ATP. Glycogen is stored in both skeletal muscle and the liver, with the overall amount being greater in the muscle due to its larger mass in the body. The liver is key to glycogen metabolism as this organ both synthesizes and degrades glycogen in accordance to the needs of the body, that is, the blood glucose homeostasis. Muscle, in comparison, regulates its glycogen stores to provide only for itself. Glycogen degradation is relatively simple, being comprised of 3 steps: 1-the release of glucose-1- P from glycogen, 2-the remodeling of the glycogen substrate for the further degradation of the remaining glycogen and 3-the conversion of glucose-1-P to glucose-6-P, the main source by which glucose is metabolized. This glucose-6-P has any one of three possible fates: A-the initial substrate for glycolysis, B-conversion into glucose for release into the blood and 3-entrance into the pentose-phosphate pathway in order that ribose, and its derivatives, as well as NADPH can be made ( Figure 21.3 ). Glycogen synthesis requires an activated form of glucose, namely UDP-glucose. This product is 1 formed from UTP and glucose-1-P and it (UDP-glucose) is added to the non-reducing end of an existing glycogen molecule. As with glycogen breakdown, the glycogen molecule must be remodeled for continued synthesis. Regulation of these processes is complex, with several allosteric enzymes necessary so that cellular energetic states can be properly managed. Along with hormones, these allosteric enzymes insure that the needs of the cell are met. While allosteric enzymes control the metabolism of glycogen at the cellular level, hormones control glycogen metabolism at the organismal level....
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Chapter 21 Biochemistry6e - Chapter 21 Biochemistry...

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