Structural Biochemistry/Carbohydrates/Glycogen In General
One very familiar form of carbohydrate is glucose. Glucose is a monosaccharide and, being a sugar, it is vital in living organisms as a source of energy. And being a source of energy, there are moments when there is a large intake of glucose or moments when large amounts of glucose are used, actions which depend on what the organism is doing. So if there is an excess of glucose where is it stored and how is it stored? And if more glucose is required by the body where does it get it and how?
Glycogen is a sugar but in a polysaccharide form and is made up of many carbohydrates compared to that of glucose. However, glycogen is different because it is a storage form of energy. Glycogen is normally stored in either the muscles or the liver. So normally, glucose within the body will be stored in the form of glycogen. In other words, glycogen is a molecule constituted of numerous glucose molecules. When glycogen is broken down through a process called glycogenolysis, it is broken to make glucose which is then used by the human body or other organisms as energy.
Structure of Glycogen
Glycogen is made up of chains of glucose. The glucose, within the chains, are held together by glycosidic bonds, specifically a 1 to 4 alpha acetal connection. Typically each of the chains are formed with about 8-10 glucose molecules. Furthermore, these glucose chains bond to each other with a 1 to 6 linkage, called a branching point. Furthermore, glycogen is made up of many of these chains of glucose, where some glycogen molecules can contain up to some sources say 120,000 units of glucose while some sources say a range of 1,700 to 600,000. Therefore, with all these subunits (8-10of glucose connecting with each other, it creates an overall molecule that looks like extensive tree branches.
The formation of glycogen with glucose occurs through the help of the enzyme, glycogen synthase. Through the process of glycogenesis, this enzyme helps with the addition of glucose molecules onto the chain. However, since glycogen synthase can only add glucose molecules, there is a protein by the name of glycogenin which acts like a primer and initially starts the reaction to form glycogen.
The breaking down of glycogen into glucose is called glycogenolysis. This process is completed through the use of an enzyme called the glycogen phosphorylase. This enzyme helps cleave glucose molecules from the non-reducing ends. Similar to the glycogen synthase, the phosphorylase cannot remove glucose 4 residues from the end of one chain or branch. There is another enzyme called the "debranching enzyme" which can completely degrade these parts of glycogen molecules.
Where it is stored
Typically, up to about 10% of the mass of the liver can be composed of glycogen molecules. While in the muscle the percentage is around 1% of the mass. Therefore, the liver has a higher amount of glycogen per unit mass of tissue. However, the total mass of the muscle in the body is greater than that of the liver, and so the total amount of glycogen will exceed that of the total amount of glycogen in the liver.
Why is glycogen important?
Since glucose is one of the main components of creating energy (ATP) for the body including muscles, the glycogen stored in muscles help with the providing the necessary energy to keep the muscles moving. There is a certain enzyme, called glucose-6-phophatase, to allow glucose to be transferred into the blood, but muscle cells do not have that. Therefore, the glycogen stored in muscle is meant to be used for the muscle, and also helps when muscles are in need of glucose to function they can utilize the nearest stores of it. Without glucose and glycogen to serve as energy, a person will experience extreme fatigue. This is what usually happens to athletes, especially like marathon runners or cyclists etc. and is commonly known as "hitting the wall."
Another one of the important functions of glycogen is that it helps maintain glucose levels in the body. One way it does is in the case of when someone is eating. Initially, a person will have higher levels of glucose in the body as food is digested and so glycogen will be created, but when there is no food in the stomach glucose levels(in the blood) will decrease and so glycogen is broken down to release and maintain glucose levels.
Stanford University: School of Medicine. Biochemistry 200. Retrieved November 11, 2012, from http://cmgm.stanford.edu/biochem200/glycogen/
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Ophardy, Charles E. (2003). Virtual Chembook: Glycogen. Retrieved November 11, 2012, from http://www.elmhurst.edu/~chm/vchembook/547glycogen.html
Berg, Jeremy M., Tymoczko, John L., and Stryer, Lubert. Biochemistry. 5th edition. New York: W H Freeman; 2002.
The Center For The Improvement of Human Functioning International: Orthomolecular.org. Retrieved November 11, 2012, from http://orthomolecular.org/nutrients/glycogen.html
Bradt, Steve and Trafton, Anne. Harvard Gazette: Harvard Science. Retrieved November 11, 2012, from http://news.harvard.edu/gazette/story/2010/10/fuel-efficiency-for-marathoners/