Gly

Glycogen
Glycogen.png
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Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. It is the main storage form of glucose in the human body and is primarily found in the liver and skeletal muscle tissues.

Structure

Glycogen is a large, branched polymer of glucose residues. The primary structure of glycogen consists of linear chains of glucose molecules linked by α(1→4) glycosidic bonds. Branches occur at approximately every 8 to 12 glucose units along the chain and are created by α(1→6) glycosidic bonds. This highly branched structure allows for rapid release of glucose when it is needed by the body.

Synthesis

The process of glycogen synthesis is known as glycogenesis. It occurs primarily in the liver and muscle cells and involves several key enzymes:

• Glycogen synthase: This enzyme catalyzes the addition of glucose units to the growing glycogen chain.
• Branching enzyme: This enzyme introduces α(1→6) branches into the glycogen molecule.

Glycogenesis begins with the conversion of glucose to glucose-6-phosphate by the enzyme hexokinase or glucokinase. Glucose-6-phosphate is then converted to glucose-1-phosphate, which is activated by uridine triphosphate (UTP) to form UDP-glucose. Glycogen synthase then adds UDP-glucose to the growing glycogen chain.

Degradation

The breakdown of glycogen is known as glycogenolysis. This process releases glucose-1-phosphate from the glycogen molecule, which can then be converted to glucose-6-phosphate. The key enzymes involved in glycogenolysis are:

• Glycogen phosphorylase: This enzyme cleaves α(1→4) glycosidic bonds, releasing glucose-1-phosphate.
• Debranching enzyme: This enzyme removes branches from the glycogen molecule, allowing further degradation by glycogen phosphorylase.

In the liver, glucose-6-phosphate can be converted to free glucose by the enzyme glucose-6-phosphatase, allowing it to enter the bloodstream and maintain blood glucose levels.

Function

Glycogen serves as a readily available source of energy. In the liver, glycogen is used to maintain blood glucose levels, especially between meals or during fasting. In muscle tissue, glycogen provides a quick source of energy during physical activity.

Regulation

The synthesis and degradation of glycogen are tightly regulated by hormonal and allosteric mechanisms. Key hormones involved in glycogen metabolism include:

• Insulin: Promotes glycogen synthesis by activating glycogen synthase and inhibiting glycogen phosphorylase.
• Glucagon: Stimulates glycogenolysis in the liver, increasing blood glucose levels.
• Epinephrine: Activates glycogenolysis in both liver and muscle tissues during stress or exercise.

Allosteric regulation involves the binding of metabolites such as glucose-6-phosphate, ATP, and AMP to enzymes involved in glycogen metabolism, modulating their activity.

Clinical Significance

Disorders of glycogen metabolism can lead to various glycogen storage diseases, which are characterized by the accumulation or deficiency of glycogen in tissues. Examples include:

• Von Gierke's disease: A deficiency of glucose-6-phosphatase, leading to hypoglycemia and hepatomegaly.
• Pompe disease: A deficiency of acid alpha-glucosidase, causing glycogen accumulation in lysosomes.
• McArdle's disease: A deficiency of muscle glycogen phosphorylase, resulting in exercise intolerance.

See Also

• Carbohydrate metabolism
• Glycolysis
• Gluconeogenesis

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