Insulin binds with the insulin receptor in the cell surface. The binding results in the recruitment of certain glucose transporters GluTs. GluTs facilitate the entry of glucose into the cell.
Liver and brain cells are insulin-independent cells , meaning glucose can enter these cells without the prior stimulation of insulin. Liver cells may not require insulin for glucose uptake but insulin still has an effect on them.
Insulin activates the enzyme hexokinase that phosphorylates glucose in order to trap it within the cell. It also activates certain enzymes involved in glycogen synthesis, e. Thus, insulin tells the liver to convert glucose into glycogen by glycogenesis.
The pancreas releases glucagon when blood glucose level turns low. This hormone acts by increasing the amount of glucose in the blood. It does so by activating the enzymes involved in glycogenolysis and gluconeogenesis in the liver. It tells the hepatocytes to depolymerize glycogen to release glucose. Although fatty acids are much more energy-rich than glycogen, glycogen remains to be the preferred form of energy storage compounds in animals.
The excess glucose is stored in glycogen granules especially in the cells of the liver, muscle, and adipose tissues. Glycogen is non-osmotic whereas glucose is osmotic. Thus, if the excess glucose is not stored as glycogen, it can cause disruption in the osmotic pressure , and eventually cause cell damage or cell death. Glycogen is an accessible source of glucose. In the muscle and fat cells, glycogen provides them glucose that they can metabolize locally. Since these cells lack the enzyme glucose 6-phosphatase , the glucose is used internally and is not shared with other cells.
In contrast, the liver cell has glucose 6-phosphatase that can dephosphorylate the trapped glucose, and thereby, allows glucose to be mobilized out of the liver cell. When there is not enough glucose circulating in the bloodstream, the pancreas secretes glucagon that stimulates the liver cells to undergo glycogenolysis and release free glucose into the bloodstream.
Hence, glycogen helps in maintaining normal blood glucose levels. Glucose is an essential fuel. It is the chief energy source preferred by the brain. Furthermore, glucose, unlike fatty acids, can supply energy even during an anaerobic oxygen-deprived activity.
Glycogenosis is the condition characterized by the inability of the body to metabolize glycogen properly. There are different types of glycogenosis depending on the enzyme deficiency involved. Each of these types involves a different dysfunctional gene, and therefore a different enzyme deficiency. These types are as follows:. The blood sugar level is regulated by two hormones. The mechanism behind this type of negative feedback control is described in this tutorial. Failure to regulate blood sugar levels could lead to physiological disorders and diseases, such as diabetes.
Read this tutorial to learn more Read More. The human body is capable of regulating growth and energy balance through various feedback mechanisms. Get to know the events of absorptive and post-absorptive states. This tutorial also describes the endocrine and neural control of compounds such as insulin and glucagon. It also deals with the regulation of growth, heat loss, and heat gain. Proteins have a crucial role in various biological activities.
Get to know how proteins are able to perform as enzymes, cofactors, or regulators. In this tutorial, you will also know the common metabolic pathways of biomolecules, such as glucose and other carbohydrates, fats, proteins and amino acids, and essential nutrients Muscle cells are specialized to generate force and movement. 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 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. In animals, it is found in practically all cells and, in mammals, it is most abundant in the liver and skeletal muscle. In the liver, several beta granules arrange in a broccoli-like fashion to form the so-called alpha granules.
Glycogen is also found in lysosomes. It is absent in plants, where starch is the storage form of glucose. Therefore, the polymerization of glucose represents a universal mechanism for energy storage. From a human nutrition point of view, glycogen has little significance as after an animal has been killed it is mostly broken down to glucose and then to lactic acid. It should be noted that the acidity consequently to lactic acid production gradually improves the texture and keeping qualities of the meat.
Glycogen was discovered in by the French physiologist Claude Bernard , considered the founder of experimental medicine. In the second half of the last century, studies on glycogen metabolism led to several significant discoveries, such as:.
Individual glycogen molecule is a branched polymer of D-glucose in the pyranose form, namely, a stiff six-membered heterocyclic ring, five carbons and one oxygen, with chair conformation.
The central priming protein glycogenin and phosphate groups are covalently bound to the polysaccharide chain. Note: in disaccharides, oligosaccharides, and polysaccharide the non-reducing end is the end that lacks a free anomeric carbon atom.
Having the same types of bonds, the primary structure of glycogen resembles that of amylopectin, which, with amylose, is one of the two polymers of D-glucose units composing starch. However, compared to amylopectin where branches occur every glucose units, glycogen is more branched, and the branches are smaller.
Unlike proteins and nucleic acids, polysaccharides are synthesized without a template , resulting from the addition of monosaccharides or oligosaccharides to the growing structure. In addition, because branches occur without a precise localization, molecules with the same mass will not necessarily have the same structure. Hence, for each type of molecules there are different chemical structures.
Moreover, glycogen isolated from different biological sources exists as a population of molecules of different sizes. Therefore, the best way to describe its chemistry is to define the distribution of the molecular masses, and the average frequency with which branches occurs and their average length.
Finally, it should be emphasized that glycogen is not a static entity but constantly vary over the course of its existence. As glucose is a chiral molecule, it exists as a pair of enantiomers, indicated according to the Fischer convention as D-glucose, the most widespread in nature and the monomeric unit of glycogen and starch, and L-glucose. The folding into three-dimensional structures of macromolecules such as proteins, nucleic acids and polysaccharides is governed by the same principles: the monomeric units, namely, amino acids, nucleotides, and monosaccharides, with their more-or-less rigid structure, are joined by covalent bonds to form one dimensional polymers that spontaneously fold into three-dimensional structures stabilized by noncovalent interactions such as:.
These interactions can occur within macromolecules or between macromolecules, as in supramolecular complexes such as cellulose or multienzyme complexes. Hence, some conformations will be more stable than others. For amylose and glycogen, the most stable 3D structure is a tightly coiled helix stabilized by interchain hydrogen bonds. Due to the action of glycogenin, and then of glycogen synthase and branching enzyme EC 2.
Moreover, considering that each tier has a thickness of 3. This does not mean that these molecules are all accessible to glycogen phosphorylase because the enzyme stalls four residues from the branch point. The intervention of the debranching enzyme, whose activity is slower than glycogen phosphorylase activity, removes the branch and allows glycogenolysis to proceed.
Why is 13 th tier not possible? The 13 th tier seems to be not possible because of the steric hindrance due to high density of glucose units on the molecule surface. Such an high density of glucose residues would lead to insufficient space for the interaction between the catalytic region of glycogen metabolism enzymes, and then of glycogen synthase, too, and the growing chains.
The structure of the glycogen molecule includes the protein glycogenin , which is covalently bonded to the polysaccharide chain. Glycogenin initiates the synthesis of glycogen by autoglycosylation, catalyzing the addition of glucose units to a specific tyrosine residue. This primer chain then acts as substrate for glycogen synthase. In addition, binding to actin filaments, glycogenin anchors the oligosaccharide primer chain to the cytoskeleton.
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