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Glycogen metabolism

Glycogen is used in animals as a carbohydrate reserve, from which glucose phosphates and glucose can be released when needed. Glucose storage itself would not be useful, as high concentrations within the cells would make them strongly hypertonic and would therefore cause influx of water. By contrast, insoluble glycogen has only low osmotic activity.

Glycogen balance

Animal glycogen, like amilopectin in plants, is a branched homopolymer of glucose. The glucose residues are linked by an α1à4-glucosydic bond. Every tenth or so glucose residue has an additional α1à6 bond to another glucose. These branches are extended by additional α1à4-linked glucose residues. This structure produces tree-shaped molecule structure that consists of up to 50,000 residues.
Hepatyc glycogen is never completely degraded. In general, only the non-reducing ends of the “tree” are shortened, or – when glucose is abundant – elongated. The reducing end of the tree is linked to a special protein, glycogenin. Glycogenin carries out autocatalytic covalent bonding of the first glucose at one of its tyrosine residues and elongation of this by up to seven additional glucose residues. It is only at this point that glycogen synthase becomes active to supply further elongation.
1)      The formation of glycosidic bonds between sugars is endergonic. Initially, therefore, the activated form-UDP-glucose – is synthesized  by reaction of glucose 1-phosphate with UTP.
2)      Glycogen synthase now transfers glucose residues one by one from UDP-glucose to the non-reducing ends of the available “branches”.
3)      Once the growing chain has reached a specific length(>11 residues), the branching enzyme cleaves an oligosaccharide, consisting of 6-7 residues from the end of it, and adds this into the interior of the same chain or a neighboring one with α1à6 linkage. These branches are then further extended by glycogen synthase.
4)      The branched structure of glycogen allows rapid release of sugar residues. The most important degradative enzyme, glycogen phosphorylase, cleaves residues from a non-reducing end one after another as glucose 1-phosphate. The larger the number of these ends, the more phosphorylase molecules can attack simultaneously. The formation of glucose 1-phosphate instead of glucose has the advantage that no ATP is needed to channel the released residues into the glycolysis or the PPP.
5)      Due to the structure of glycogen phosphorylase, degradation comes to a halt four residues away from each branching point. Two more enzymes overcome this point. First, a glucanotransferase moves a trisaccharide from the side chain to the end of the main chain. A 1,6-glucosidase then cleaves the single remaining residues as a free glucose and leaves behind an unbreached chain that is once again accessible to phosphorylase. 

Glycogen balance

The human organism can store up to 450 g of glycogen – one-third in the liver and almost of the remainder in muscle. The glycogen content of the other organs is low.
Hepatic glycogen is mainly used to maintain the blood glucose level in the postresorptive phase. The glycogen content of the liver therefore varies widely, and can decline to almost zero in periods of extended hunger. After that, gluconeogenesis takes over the supply for the organism. Muscle glycogen serves as an energy reserve and is not involved in blood glucose regulation. Muscle does not contain any glucose-6-phosphatase and is therefore unable to release glucose into the blood. The glycogen content of the muscle therefore does not fluctuate as widely as that of the liver. 

“Coloured atlas of biochemistry”, second edition; J. Koolman, K.H. Roehm

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