Glycogen - Structure and Functions of Glycogen

Glycogen, a polysaccharide is the primary storage form of glucose in the human and animal cells for future use. It is present in the form of granules in the cytosol in many cell types. It is a multi-branched polysaccharide of glucose that remains as a form of energy storage in humans, fungi, animals, and bacteria. It is stored in the liver, muscle, and skeletal cells.

Structure of Glycogen:

Glycogen can be organized in a spherical form in which the glucose chains are structured around a core protein of glycogen with 38,000 molecular weight and it looks like branches of tree originated from a center point.

A branched polymer of glucose is called glycogen. The glucose residues are associated linearly by α-1, 4 glycosidic bonds and nearly 8- 10 residues a chain of glucose branches off through α-1, 6 glycosidic linkages. Helical polymer structure is formed by the α- glycosidic bonds.

Granules in the cytoplasm are formed by hydrating glycogen with 3-4 parts of water which will be 10-40 nm in diameter. At the core of glycogen, the granule is located the protein glycogen in which is involved in glycogen synthesis. It is an analog of starch which is the important form of storage of the glucose in most plants also starch has few branches and it will be in less compact when compared to the glycogen.

Functions of Glycogen:

In human beings and animals, glycogen is found mainly in the liver and muscle cells. It is synthesized from glucose when the sugar level in the blood is high and it serves as a ready source of glucose for the tissues throughout the entire body when sugar level in the blood reduces.

Muscle Cells:

Glycogen accounts for only 1-2% of the muscles by weight. Though, given the greater mass of muscle in the body, the total amount of glycogen storage in the muscles will be greater than that of the storage in the liver. The glycogen present in the muscles is provided only to the muscle cell itself. The enzyme glucose-6-phosphate will not be expressed by the muscle cells that will be required to release the glucose into the bloodstream.

The energy is provided to the muscles during any exercise or stress is experienced by the body. It is done by the breakdown in the muscle fibers of the glucose-1 phosphate produced from glycogen and converting into glucose-6 phosphate.

Liver Cells:

In the liver cells, the glycogen makes up to 6-10% of the liver by weight. If the food taken is not digested, then the blood glucose level increases and the insulin are released from the pancreas promoting the uptake of glucose into the liver cells. The enzymes involved in the glycogen synthesis are activated by the insulin.

When the insulin and the glucose levels are high, the glycogen chains by the addition of glucose molecules are extended and this process is called glycogenesis. The glycogen synthesis ceases as the glucose level and the insulin level decreases. If there is a decrease in the blood sugar level below a certain level, the glucagon released from the pancreas gesture the liver cells to break down glycogen. The glycogenesis process occurs and the glucose is released into the bloodstream.

Hence the glycogen will serve as the main shield of blood glucose level by storing the glucose during high sugar level in the blood and releasing it when the sugar level is low. Simply glycogen breakdown for supplying glucose will not be sufficient to meet the energy needs of the body so, in addition to this glucagon, cortisol, epinephrine and norepinephrine will also stimulate the breakdown of glycogen.

Other Tissues:

Glycogen can also be found in smaller amounts in other tissues like kidney, white blood cells, and red blood cells and in addition to the muscle and liver cells. In order to provide the energy needs of the embryo, the glycogen will be used to store the glucose in the uterus. The glycogen after the breakdown will enter the glycolytic or pentose phosphate pathway or it will be released into the bloodstream.

Bacteria and Fungi:

Microorganisms like bacteria and fungi possess some mechanisms for storing the energy to deal with the limited environmental resources; here the glycogen represents the main source for the storage of energy. The nutrient limitations such as low levels of phosphorus, carbon, sulfur or nitrogen can stimulate the glycogen formation in yeast. The bacteria synthesize glycogen in response to the readily available carbon energy sources with restriction of other required nutrients. The yeast sporulation and bacterial growth are associated with glycogen accumulation.

Metabolism of Glycogen:

The glycogen haemostasis which is a highly regulated process will allow the body to release or store the glucose depending upon its energetic needs. The steps involved in glycogen metabolism are glycogenesis or glycogen synthesis and glycogenolysis or glycogen breakdown.

Glycogenesis or Glycogen Synthesis:

The glycogenesis requires energy that is supplied by Uridine Tri-Phosphate (UTP). glucokinase or hexokinase first phosphorylate the free glucose to form glucose-6 phosphate which will be then converted to glucose-1 phosphate by the phosphoglucomutase. The UTP glucose-1 phosphate catalyzes the activation of glucose in which the glucose-1 phosphate and UTP react to form UDP glucose.

The protein, glycogen catalyzes the attachment of UDP glucose, itself in the glycogen synthesis. Glycogenin contains a tyrosine residue in each subunit that will serve as an attachment point for the glucose further glucose molecules will be then added to the reducing end of the previous glucose molecule in order to form a chain of nearly eight glucose molecules. By adding glucose through α-1, 4 glycosidic linkages the glycogen synthase then extends.

The branching catalyzed by amyloid 1- 4 to 1- 6 transglucosidases is called as the glycogen branching enzyme. A fragment of 6- 7 glucose molecules gets transferred from the glycogen branching enzyme from the end of a chain to the C6 of a glucose molecule that is situated further inside of the glucose molecule and forms α-1, 6 glycosidic linkages.

Glycogenolysis or Glycogen Breakdown:

The glucose will be detached from glycogen through the glycogen phosphorylase which will eliminate one molecule of glucose from the non-reducing end by yielding glucose-1 phosphate. The glycogen breakdown that generates glucose- 1 phosphate is converted to glucose- 6 phosphates and this is the process that requires the enzyme phosphoglucomutase.

Phosphoglucomutase will transfer a phosphate group from a phosphorylated serine residue within the active site to C6 of glucose- 1 phosphate and it will be attached to the serine within the phosphoglucomutase and then the glucose- 6 phosphates will be released.

Glycogen phosphorylase will not be able to cut glucose from branch points, so the debranching will require 1- 6 glucosidase, glycogen debranching enzyme (GDE) or 4- αglucanotransferase which will have glucosidase activities and glucosyltransferase. Nearly four residues from a branch point, the glycogen phosphorylase will be unable to remove the glucose residues.

The GDE will cut the final three residues of the branch and it will attach them to C4 of a glucose molecule at the end of another branch and then eliminate the final α- 1- 6 linked glucose deposit from the branch point.

Glycogen and Diet:

The food is taken, and the activities done can influence the production of glycogen and the way the body will function. With a low- carb diet, the primary source for glucose synthesis i.e. the carbohydrate will be suddenly restricted.

During the start of a low- carb diet, the glycogen stores will be severely depleted which will result in symptoms of mental dullness and fatigue. Then when the body starts to adjust and renew its glycogen stored then the body will return to the normal stage. Any weight loss effort can trigger this effect to some extent.

At the starting of a low- carb diet, the body will experience a huge drop in weight which will plateau and may even increase after a period of time. This is mainly because of the glycogen which will be composed mainly of water that will be 3- 4 times the weight of glucose itself.

The rapid depletion of glycogen at the beginning of the diet will trigger the rapid loss of water weight. Then, when the glycogen stores are renewed, the water weight returns causing weight loss to halt. It is necessary to keep in mind that this is caused by the temporary gain in water weight and not the fat and the fat loss can continue in spite of this short-term plateau effect.

During exercise, the body undergoes glycogen depletion and most of the glycogen will be depleted from the muscle. So while doing exercise, the persons can use carbohydrate loading which means the consumption of large amounts of carbohydrates in order to increase the capacity for the storage of glycogen. Glycogen is different from the hormone glucagon and it also plays an important role in carbohydrate metabolism and blood glucose control.

How Glycogen is used:

At any time, there will be nearly 4 grams of glucose in the blood. When the level declines, either because of missing any meals or during exercise when the glucose is burnt the insulin level will drop. During this, an enzyme called glycogen phosphorylase will break the glycogen separately in order to supply glucose to the body when it needs.

For the next 8- 12 hours, the glucose derived from the liver glycogen will be the main source of energy for the body. Out of all the body organs, the brain will use more than half of the blood glucose during inactivity and nearly 20% of it during an average day.