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Glycogen is a highly branched glucose polymer that constitutes the principal storage form of carbohydrate in animals and is functionally analogous, though structurally distinct, from plant starch. In mammals, glycogen is predominantly localized in the liver and skeletal muscle. Hepatic glycogen, present at the highest concentration per gram of tissue, plays a central role in maintaining systemic glucose homeostasis, particularly during fasting. In contrast, skeletal muscle contains the largest total glycogen reserve due to greater mass and utilizes these stores to meet the energetic demands of contraction. Smaller glycogen pools are present in additional tissues, including the kidney, heart, brain, and glial cells, where localized metabolic requirements are supported.[1][2][3] Structurally, glycogen is composed of α-D-glucose residues linked by α(1→4) glycosidic bonds with α(1→6) linkages at branch points occurring approximately every 8 to 12 residues. This extensive branching enhances solubility and generates multiple nonreducing ends, facilitating rapid glucose mobilization through the coordinated action of glycogen phosphorylase and debranching enzymes. Glycogen metabolism is tightly regulated by integrated hormonal and allosteric mechanisms. Insulin promotes glycogenesis primarily through activation of glycogen synthase and inhibition of glycogen phosphorylase, whereas glucagon in the liver and epinephrine in both liver and muscle stimulate glycogenolysis via protein kinase A (PKA) signaling pathways activated by cyclic adenosine monophosphate (cAMP). Disruptions in enzymes governing glycogen synthesis, degradation, or regulation give rise to glycogen storage diseases (GSDs), a heterogeneous group of metabolic disorders characterized by tissue-specific manifestations such as hypoglycemia, hepatomegaly, myopathy, and cardiomyopathy.[4]