SEQ’S : 1st priority How glycogen is synthesized in the liver? Outline the steps of breakdown of glycogen. Enlist different glycogen storage diseases with enzyme deficient, How glycogen synthesis is regulated? How glycogen breakdown is regulated? 2nd priority
Why muscles cant form free glucose formed from glycogen degradation and provide glucose for maintaining blood glucose level? What is the role of Ca and AMP in glycogen metabolism?
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The main stores of glycogen in the body are found in skeletal muscle and liver The function of muscle glycogen is to serve as a fuel reserve for the synthesis of adenosine triphosphate (ATP) during muscle contraction. Function of liver glycogen is to maintain the blood glucose concentration, particularly during the early stages of a fast Approximately 400 g of glycogen in muscle make up 1–2% of the fresh weight of resting muscle Approximately 100 g of glycogen make up to 10% of the fresh weight of a well-fed adult liver. Glycogen is a branched-chain polysaccharide made exclusively from α-D-glucose. The primary glycosidic bond in glycogen is an α(1→4) linkage. After an average of eight to ten glucosyl residues, there is a branch containing an α(1→6) linkage Liver glycogen stores increase during the well-fed state and are depleted during a fast. Muscle glycogen is depleted during strenuous exercise and is resynthesized to replenish muscle stores Glycogen synthesis occurs in cytosol Energy for Glycogen synthesis is derived from ATP and UTP α-D-Glucose attached to uridine diphosphate (UDP) is the source of all the glucosyl residues that are added to the growing glycogen molecule.
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UDP-glucose is synthesized from glucose 1-phosphate and UTP by UDP-glucose pyrophosphorylase Glucose 6-phosphate is converted to glucose 1phosphate by phosphoglucomutase. A protein glycogenin, serve as first acceptor of glucose residues from UDP-glucose. The reaction is catalyzed by glycogenin itself via autoglucosylation. Glycogenin then catalyzes the transfer of the next few molecules of glucose from UDP-glucose, producing a short, α(1→4)linked glucosyl chain. Elongation of glycogen chains by glycogen synthase Branches are made by the action of the branching enzyme, amylo-α(1→4) → α(1→6)-transglucosidase or 4;6 transferase the primary product of glycogen breakdown is glucose 1- phosphate. in addition, free glucose is released from each α(1→6)-linked glucosyl residue. Glycogen phosphorylase shorten the chain at the nonreducing ends of the glycogen chains until four glucosyl units remain on each chain before a branch point pyridoxal phosphate (PLP) is a coenzyme for glycogen phosphorylase. Glycogen phosphorylase forms limit dextrin. oligo-α(1→4)→α(1→4)-glucan transferase activity of debranching enzyme removes the outer three of the four glucosyl residues attached at a branch (4-4 transeferase). the remaining single glucose residue attached in an α(1→6) linkage is removed hydrolytically by amylo-α(1→6)-
www.themedicalglobe.com / www.mgElearning.net Page 3 of 6 glucosidase activity of debranching enzyme, releasing free glucose. In the liver, glucose 6-phosphate is transported into the endoplasmic reticulum (ER) by glucose 6-phosphate translocase. In ER, glucose 6-phosphate is converted to glucose by glucose 6-phosphatase Lysosomal degradation of glycogen by lysosomal enzyme, α(1→4)-glucosidase (acid maltase) TYPE V: McARDLE SYNDROME (SKELETAL MUSCLE GLYCOGEN PHOSPHORYLASE) OR MYOPHOSPHORYLASE DEFICIENCY Deficiency of the liver isozyme of glycogen phosphorylase causes Type VI: Hers disease TYPE II: POMPE DISEASE (LYSOSOMAL α(1→4)-GLUCOSIDASE DEFICIENCY) Pompe dieases is Lysosomal storage disease TYPE III: CORI DISEASE (4:4 TRANSFERASE and/or 1:6 GLUCOSIDASE DEFICIENCY) In Cori Disease, Glycogen has abnormal structure TYPE Ia: VON GIERKE DISEASE (GLUCOSE 6-PHOSPHATASE DEFICIENCY) TYPE Ib: GLUCOSE 6-PHOSPHATE TRANSLOCASE DEFICIENCY Hyperlacticacidemia, hyperlipidemia, and hyperuricemia occurs in Tyoe Ia and IIb Glycogen phosphorylase in dephosphorylated form is inactive and the phosphorylated, active form
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glycogen synthase, the active form is dephosphorylated and inactivated form is phosphorylated In the well-fed state, glycogen synthase in both liver and muscle is allosterically activated by glucose 6-phosphate glycogen phosphorylase is allosterically inhibited by glucose 6phosphate and ATP, In liver free glucose is an allosteric inhibitor of glycogen phosphorylase FiAgcutirvea:tion of glycogen degradation by calcium and AMP (in muscle): 11.5 11.8 11.9 11.10 11.11
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