Chapter12Fall2005 - Chapter 12 Gluconeogenesis the Pentose...

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Chapter 12 – Gluconeogenesis, the Pentose Phosphate Pathway and Glycogen Metabolism GLYCOGEN METABOLISM Glycogen stored in muscle and liver cells. Important in maintaining blood glucose levels. Glycogen structure: α 1,4 glycosidic linkages with α 1,6 branches. Branches give multiple free ends for quicker breakdown or for more places to add additional units. Glycogen Degradation Glucose residues of starch and glycogen released through enzymes called starch phosphorylases and glycogen phosphorylases. Catalyze phosphorolosis : polysaccharide +P i ---> polysaccharide (n-1) + glucose 1-phosphate Pyridoxal phosphate (PLP) is prosthetic group in active site of enzyme; serves as a proton donor in active site. Allosterically inhibited by high [ATP] and high [glucose 6-phosphate]. Allosterically activated by high [AMP]. Sequentially removes glucose residues from nonreducing ends of glycogen, but stops 4 glucose residues from branch point --> leaves a limit dextran . Limit dextran further degraded by glycogen-debranching enzyme (glucanotransferase activity) which relocated the a free hydroxyl end. Amylo-1,6-glucosidase activity of debranching enzyme removes remaining residues of chain. This leaves substrate for glycogen phosphorylase. Each glucose molecule released from glycogen by debranching enzyme will yield 3 ATPs in glycolysis. Each glucose molecule released by glycogen phosphorylase will yield 2 ATPs in glycolysis. Why? ATP not needed in first step because glucose 1-phosphate already formed. phosphoglucomutase glucose 1-phosphate ----------------------> glucose 6-phosphate 1) In liver, kidney, pancreas, small intestine, glucose 6-phosphatase glucose 6-phosphate --------------------------> glucose + P i Glycogen Synthesis Not reverse of glycogen degradation because different enzymes are used.
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3 About 2/3 of glucose ingested during a meal is converted to glycogen. First step is the first step of glycolysis: hexokinase glucose --------------> glucose 6-phosphate There are three enzyme- catalyzed reactions: phosphoglucomutase glucose 6-phosphate ---------------------> glucose 1-phosphate glucose 1-phosphate ---------------> UDP- glucose (activated form of glucose)
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Chapter12Fall2005 - Chapter 12 Gluconeogenesis the Pentose...

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