BCH-4 Control in Carbohydrate Metabolism

BCH-4 Control in - Storage Mechanisms and Control Storage Mechanisms and Control in Carbohydrate Metabolism in Carbohydrate Metabolism How is

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Unformatted text preview: Storage Mechanisms and Control Storage Mechanisms and Control in Carbohydrate Metabolism in Carbohydrate Metabolism How is Glycogen Produced and Degraded? When we eat a meal high in carbohydrates, we have a supply of glucose that exceeds our mediate needs. immediate needs. We store excess glucose as a polymer, glycogen . In degradation of glycogen, several glucose residues can be released simultaneously. This is useful for meeting short-term demands for energy; for this the average chain length is optimized at 13 residues. Breakdown of Glycogen Glycogen is primarily found in the liver and muscle. Release of glycogen from the liver is triggered by low levels of glucose in the blood. Liver glycogen breaks down to glucose-6-phosphate, which is hydrolyzed to glucose. In muscle the glucose-6-phosphate enters the glycolytic pathway directly. Three reactions play a role in glycogen to glucose-6- phosphate. A glucose residue is cleaved from glycogen and reacts with phosphate to give glucose-1-phosphate. This is mediated by glycogen phosphorylase . Phosphoglucomutase converts glucose-1-phosphate to glucose-6-phosphate. The third reaction is the degradation of α (1 → 6) linkages. When glycogen is the substrate for glycolysis rather than glucose, there is a net gain of 3 ATP per glucose molecule, rather than 2 ATP. Formation of Glycogen from Glucose The synthesis of glycogen requires energy, which is supplied by the hydrolysis of a nucleoside triphosphate, UTP. The first stage of the synthesis involves glucose-1- phosphate converted from glucose-6-phosphate by an isomerase reacting with UTP to produce UDP- glucose and pyrophosphate ( UDP glucose phosphorylase ). To allow the reaction to happen, an exergonic reaction pyrophosphate hydrolysis takes place. Then UTP is replenished by nucleoside phosphate kinase. The addition of UDPG to a growing chain of glycogen is the next step of glycogen synthesis. This new α (1 → 4) glycosidic bond is catalyzed by glycogen Synthase . A primer is required to initiate glycogen synthesis, the hydroxyl group of a specific tyrosine of the protein glycogenin serves this purpose....
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This note was uploaded on 12/05/2009 for the course BCH 311 taught by Professor Howlett during the Fall '08 term at Rhode Island.

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BCH-4 Control in - Storage Mechanisms and Control Storage Mechanisms and Control in Carbohydrate Metabolism in Carbohydrate Metabolism How is

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