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Liver glycogen stores serve as a store of glucose for use throughout the body, particularly the central nervous system. [4] The human brain consumes approximately 60% of blood glucose in fasted, sedentary individuals. [4] Glycogen is an analogue of starch, a glucose polymer that functions as energy storage in plants.
The body's glycogen stores are consumed in about 24 hours. In a normal 70 kg adult, only about 8,000 kilojoules of glycogen are stored in the body (mostly in the striated muscles ). The body also engages in gluconeogenesis to convert glycerol and glucogenic amino acids into glucose for metabolism.
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 .
The overall reaction for the breakdown of glycogen to glucose-1-phosphate is: [1] glycogen (n residues) + P i ⇌ glycogen (n-1 residues) + glucose-1-phosphate. Here, glycogen phosphorylase cleaves the bond linking a terminal glucose residue to a glycogen branch by substitution of a phosphoryl group for the α[1→4] linkage. [1]
Unless glycogen stores are replenished during exercise, glycogen stores in such an individual will be depleted after less than 2 hours of continuous cycling [11] or 15 miles (24 km) of running. Training and carbohydrate loading can raise these reserves as high as 880 g (3600 kcal), correspondingly raising the potential for uninterrupted exercise.
Glycogen is a readily-accessible storage form of glucose, stored in notable quantities in the liver and skeletal muscle. After the exhaustion of the glycogen reserve, and for the next two to three days, fatty acids become the principal metabolic fuel. At first, the brain continues to use glucose.
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.
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 ...