Glycogen: Structure, Functions & Uses

Key Highlights

• Glycogen is a polysaccharide of glucose that is used to store energy in fungi and animals. 
• It is a water-insoluble white amorphous powder that is rapidly destroyed by mineral acids to liberate glucose residues.
• Glycogen is stored primarily in the cells of the liver and skeletal muscle.
• The compound refers to the analogue of starch, which has a similar structure to that of amylopectin.
• Your body creates the required amount of glycogen by converting glucose through a process called glycogenesis.

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Glycogen is a polysaccharide that acts as the primary storage unit of glucose in cells of both humans and animals. The empirical formula of the compound is (C₆H₁₀O₅)_n, which was established by Kekulé in 1858.

• Important functions include the creation of phosphate for the short term and triglyceride stored in adipose for the long term. 
• The total amount of glycogen depends on factors such as the quantity of carbohydrates you eat and your exercise intensity.

Key Terms: Glycogen, Glucose, Carbohydrates, Liver, Polysaccharide, Polymer, Starch, Structure of Glycogen, Functions of Glycogen

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What is Glycogen?

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Glycogen is a form of connected glucose molecules which is mainly stored in your liver, skeletal muscles, brain, and other tissues. It is considered one of the analogs of starch that has a similar structure to amylopectin, a component of starch.

• This glycogen is converted to glucose by our body whenever we need the energy. 
• The amount of compound stored in the body depends upon the type 1 fibres, eating habits and metabolic rate.
• Muscle cells are rapidly converted to glucose by muscle cells.
• It was invented by French physiologist Claude Bernard. 
• Liver glycogen is converted to glucose for utilization throughout the body, including the central nervous system.
• This large D-glucose molecule is also found in bacteria, fungi, seaweeds, and plants.
• Glycogenesis refers to converting glucose into glycogen, whereas glycogenolysis refers to breaking glycogen for usage.
• The pancreatic islets of Langerhans release glucagon into the bloodstream when the glucose level falls below the threshold level. 

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Structure of Glycogen

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Glycogen is a branching polymer of glucose whose residues are linked linearly by α-1, four glycosidic bonds. It has an average chain length of approximately 8–12 glucose units with a molecular weight of 38,000. 

• It can be formed in a spherical shape, with glucose chains built around a core protein with 2,000-60,000 residues per molecule of glycogen.
• The α- glycosidic linkages form a helical polymer structure.
• Each glycogen forms a glucose tree centred on a glycogenin protein with three kinds of glucose chains: A, B, and C.
• A chain of glucose branches off nearly 8- 10 residues through α-1, six glycosidic links.
• Granules 10-40 nm in diameter are generated in the cytoplasm by hydrating glycogen with 3-4 parts water. 
• In liver and fat cells, it is stored in the hydrated form, which is composed of three to four parts of water associated with 0.45 millimoles of potassium per gram of glycogen.

Structure of Glycogen 

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Functions of Glycogen

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Some important functions of glycogen are as follows:

• It acts as a glucose level buffer, generated and degraded in response to changes in blood glucose levels.
• Astrocytes in the brain contain a modest quantity of glycogen, which builds up during sleep and is released when you walk. 
• Glycogen reserves also provide protection from hypoglycemia.
• At around 26 weeks of pregnancy, pulmonary cells begin to store these molecules and eventually produce lung surfactants.
• The hepatocyte releases liver glycogen, which acts as a glucose reserve, when necessary to maintain normal blood sugar levels. 
• Body fluids give roughly 40 kcal, whereas hepatic compounds can offer about 600 kcal following a fasting night.
• The regulated mechanism of glycogen hemostasis allows the body to release or store glucose depending on its energetic needs. 
• It can be found in lower amounts in other tissues, such as the kidney, white blood cells, and red blood cells. 
• Glycogen will store glucose in the uterus to meet the embryo's energy requirements.

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Uses of Glycogen

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The uses of glycogen are as follows:

• There will be roughly 4 grams of glucose in the blood at any given time. 
• When the level drops, either due to skipping meals or burning glucose during intensive activity, the insulin level drops.
• During this process, an enzyme called glycogen phosphorylase separates the glycogen molecules so that the body can get glucose when needed.
• The glucose generated from liver glycogen will be the body's primary source of energy for the next 8 to 12 hours. 
• The brain will utilize more than half of the blood glucose during inactivity and about 20% on a typical day out of all the body organs.

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Metabolism of Glycogen

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The carefully regulated mechanism of glycogen hemostasis allows the body to release or store glucose depending on its energetic needs. Glycogenesis, or glycogen synthesis, and glycogenolysis or glycogen breakdown, are two processes in glycogen metabolism.

Glycogen Synthesis or Glycogenesis

Uridine Tri-Phosphate provides the energy required for glycogenesis (UTP). The glucokinase and hexokinase phosphorylate produce glucose-6 phosphate, which is then transformed into glucose-1 phosphate by the enzyme phosphoglucomutase. 

• The activation of glucose is catalyzed by the UTP glucose-1 phosphate, which reacts with UTP to generate UDP glucose.
• Glycogen is a protein that catalyzes the attachment of UDP glucose during glycogen formation. 
• Each subunit of glycogen has a tyrosine residue that will function as a point of attachment for the glucose. 
• To construct a chain of nearly eight glucose molecules, more glucose molecules will be added to the reducing end of the previous glucose molecule. 
• The glycogen synthase expands when glucose is added by α-1, four glycosidic connections.
• The branching enzyme catalyzes the branching mediated by amyloid 1- 4 to 1- 6 transglucosidases. 
• Hence, transferring a certain segment of approximately 6-7 glucose molecules from the end of a chain to the C6 of a glucose molecule that is located farther inside the glucose molecule, forming α-1, 6 glycosidic connections.

Glycogen Breakdown or Glycogenolysis

The glycogen phosphorylase releases glucose from glycogen, producing glucose-1 phosphate by removing one molecule of glucose from the non-reducing end. 

• The enzyme phosphoglucomutase is required to convert glucose-1 phosphate to glucose-6 phosphates.
• Phosphoglucomutase transfers a phosphate from a phosphorylated serine residue within the active site to the C6 of glucose-1 phosphate.
• It will attach to the serine within the phosphoglucomutase and subsequently release the glucose-6 phosphates.
• Since glycogen phosphorylase is unable to cut glucose from branch sites, it will initiate the debranching process.
• Debranching of 1-6 glucosidase, glycogen debranching enzyme (GDE), or 4-α glucanotransferase is performed.
• The GDE will remove the branch's final three residues and attach them to C4 of a glucose molecule at the end of another branch before removing the branch's final α- 1- 6 linked glucose deposit.