Structure of Glycogen and Glycogen synthesis (Glycogenesis) : Medical biochemistry

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Structure of Glycogen and Glycogen synthesis (Glycogenesis) : Medical biochemistry
Glycogen is the major storage carbohydrate in humans, corresponding to starch in plants. It is a branched polymer of α-D-glucose. Glycogen is crucial to human survival because it provides a glucose reserve that can be rapidly called on when needed. Compare it with dietary glucose, which is subject to access to food, and gluconeogenesis, which synthesizes glucose de novo (from scratch), yet in a slow manner.
Linearly linked α-1,4 glycosidic bonds connect molecules, and at approximately every tenth residue, a chain of glucose residues branches off via α-1,6 glycosidic linkage, forming the characteristic branched form
The glycosidic bonds between glucose molecules are cleaved to release glucose molecules. Branches allow for more sites of enzyme activity, so glucose can be released quickly in times of need. Also, if a large number of glucose molecules are present inside the cell, the osmotic pressure of the cell will be very high. This can cause the cell membrane to burst. But if glucose combines into one big molecule of glycogen, the problem does not occur. So the branched structure of glycogen is of high importance.
Glycogen in the Liver
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Liver glycogen functions as a short-term reserve to maintain blood glucose concentration in the fasting state through a process called glycogenolysis. After 12-18 hours of fasting, liver glycogen is almost totally depleted, and gluconeogenesis (generation of glucose from noncarbohydrate carbon substrates) kicks in.
Glycogen in Skeletal Muscle
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In contrast to liver glycogen, muscle glycogen does not directly yield free glucose. This is because muscle lacks glucose-6-phosphatase. In skeletal muscle, glucose-6-phosphate, obtained from degradation and processing of glycogen, enters glycolysis to produce energy to fuel muscle contraction. Glycogenin is an enzyme involved in converting glucose to glycogen. It acts as a primer by polymerizing the first few glucose molecules, after which other enzymes take over.
Points to ponder
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Glycogen is synthesized from glucose when blood glucose levels are high and serves as a ready source of glucose for tissues throughout the body when blood glucose levels decline.
Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis, the phosphorolytic cleavage of the 1-4 linkages of glycogen to yield glucose-1-phosphate.
The debranching enzyme 1,6-glycosidase catalyzes hydrolysis of the 1-6 glycoside bond to liberate free glucose.
The allosteric regulatory effects are exercised by glucose-6-phosphate, ATP, and AMP on glycogen phosphorylase and glycogen synthase.
Hormonal control of glycogen metabolism is mainly mediated through insulin, which promotes glycogenesis, and glucagon and epinephrine, which promote glycogenolysis.
In contrast to liver phosphorylase, only muscle phosphorylase is sensitive to regulation by AMP.
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where is glycogenolysis?