Metabolism of Glycogen

• Glycogen is the storage form of glucose in animals
• Glycogen is stored in the liver (6-8%) and muscle (1-2%)
• Glycogen helps to maintain blood glucose levels between meals
• Glycogen stores increase in a well-fed state and are depleted during fasting
• Muscle glycogen serves as a fuel reserve for the supply of ATP during muscle contraction
• Glycogen is a homopolysaccharide where glucose molecules are held together by α-1,4 linkages with branching via α-1,6 linkage
• Glucokinase is found in the liver and hexokinase in muscle, converting glucose to glucose-6-phosphate
• Glycogenesis involves the synthesis of glycogen from glucose in the liver and muscle
• Excess glucose after a meal is converted into glycogen using UDPG as the carrier of glucose
• Glucose from UDPG is attached at the nonreducing end of glucose molecules of the glycogen primer
• In the absence of a glycogen primer, glycogenin can accept glucose from UDPG
• Branching enzyme transfers glucose residues to form α-1,6-linkages in glycogen
• Glycogenolysis is the breakdown of glycogen to glucose, occurring in the liver and muscle
• End product of liver glycogenolysis is glucose, while muscle glycogenolysis produces lactate during strenuous exercise
• Phosphorylase phosphorolytically splits α-1,4 glucoside bonds from the outermost chains of glycogen until 4 residues remain on either side of the α-1,6 branch point
• Allosteric regulation involves Glu-6-P activating glycogen synthase and inhibiting phosphorylase, while free glucose also acts as an inhibitor to phosphorylase
• Regulation of glycogenesis and glycogenolysis involves the active and inactive forms of glycogen synthase and phosphorylase, with cAMP levels determining their activity
• Glycogen storage diseases are genetic diseases resulting in abnormal deposition or increased quantity of glycogen in tissues
• Types of glycogen storage diseases include Von Gierke's disease, Pompe's disease, Limit dextrinosis, Anderson's disease, McArdle's disease, and Hers disease
• Von Gierke's disease presents with fasting hypoglycemia, lactic acidemia, hyperlipidemia, hyperuricemia, massive liver enlargement, and growth failure in children