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Glycogen is found in the form of granules in the cytosol/cytoplasm in many cell types, and plays an important role in the glucose cycle. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of triglycerides . As such it is also found as storage ...
In the muscles, glycogenolysis begins due to the binding of cAMP to phosphorylate kinase, converting the latter to its active form so it can convert phosphorylase b to phosphorylase a, which is responsible for catalyzing the breakdown of glycogen. [2] The overall reaction for the breakdown of glycogen to glucose-1-phosphate is: [1]
Glycogenesis is the process of glycogen synthesis or the process of converting glucose into glycogen in which glucose molecules are added to chains of glycogen for storage. This process is activated during rest periods following the Cori cycle , in the liver , and also activated by insulin in response to high glucose levels .
In the absence of dietary sugars and carbohydrates, glucose is obtained from the breakdown of stored glycogen. Glycogen is a readily-accessible storage form of glucose, stored in notable quantities in the liver and skeletal muscle. [5] When the glycogen reserve is depleted, glucose can be obtained from the breakdown of fats from adipose tissue.
In glycogen storage diseases, however, glycogen cannot be properly broken down, leading to inappropriately decreased glucose levels in a fasting state, and thus hypoglycemia. [3] The glycogen storage diseases associated with hypoglycemia include type 0 , type I , type III , and type IV , as well as Fanconi syndrome .
Glycogen is a highly branched structure, consisting of the core protein Glycogenin, surrounded by branches of glucose units, linked together. [2] [12] The branching of glycogen increases its solubility, and allows for a higher number of glucose molecules to be accessible for breakdown at the same time. [2]
Glycogen breakdown is highly regulated in the body, especially in the liver, by various hormones including insulin and glucagon, to maintain a homeostatic balance of blood-glucose levels. [8] When glycogen breakdown is compromised by mutations in the glycogen debranching enzyme, metabolic diseases such as Glycogen storage disease type III can ...
Glycogen synthase as mentioned helps in glycogen elongation and the removal of the glucose from glycogen is aided by debranching enzymes and phosphorylase. All of these enzymes are found in the glycosome, showing that this organelle complete with glycogen as well is responsible for storing glycogen and separate from the cytosol.