Glycogen Definition

Dehydration synthesis is the chemical process of joining monosaccharide units that produce water molecules as a byproduct. The process synthesizes starch and displaces a hydroxyl radical from one glucose and a proton from another glucose and connects both by a glycosidic bond. The water molecule is formed by the detached proton and hydroxyl radical.

In animals, the liver cells and skeletal muscle cells synthesize glycogen naturally. Kidneys, some glial cells in the brain, and white blood cells also produce a small amount of glycogen. During pregnancy, glycogen is also stored in the uterus and helps in the nourishment of the embryo. Glycogen is mainly synthesized from glucose by the process of glycogenesis.

A metabolic process of synthesizing glycogen from glucose is called glycogenesis. It is mainly produced to maintain a high level of glucose in the bloodstream. The major sites of glycogen production in animals are liver and muscle cells. About 10% glucose by weight resides in the liver whereas 2% glucose in skeletal muscles. However, due to the greater mass of muscles as compared to the liver, more glucose is reserved in muscle cells.

In the liver cell, the phosphorylation reaction occurs and produces glucose-6-phosphate by enzyme glucokinase. In other cells, glucose is phosphorylated by the enzyme hexokinase that cannot leave the cell. The process includes the conversion of short glucose polymers especially exogenous glucose into long polymers that can be stored in the cell. The glycogen breaks into glucose when the body needs energy. The process is known as glycogenolysis.

The polymerization of glucose is bloomed by the enzyme glycosyltransferase. Before the main enzyme, the addition of glucose is catalyzed by glycogen synthase. The catalytic activity of glycogenin enables an initial chain of three glucose residues. Thus, glycogenin acts as a primer. The glycogen synthase can act only if there are enough glucose residues, or it only extends the chain formed by the primer.

The glycogens convert into glucose when the body requires energy. The process of conversion of glycogen into glucose is called glycogenolysis. It is considered the opposite process of glycogenesis. In the liver, the stored glycogen breaks into glucose precursors. A single glucose molecule is cut off from the glycogen molecule and produces glucose-1-phosphate which is later transformed into glucose-6-phosphate. The glucose-6-phosphate will either take participate in glycolysis or be processed by the pentose phosphate pathway or get converted into free glucose to maintain blood glucose levels.

The polysaccharides are converted into simple monosaccharides by the process of hydrolysis or also known as saccharification. The process uses a molecule of water. In humans, the digestion of carbohydrates includes enzymes such as salivary amylase, pancreatic amylase, and maltase. The digestion starts from the mouth where the salivary amylase breaks glycogen and starch into simpler forms.

The bolus travels through the gastrointestinal tract. In the stomach, the digestive activity of salivary amylase is inhibited by acidic gastric content. In the small intestine, the partially digested carbohydrates are degraded by pancreatic amylase, which converts complex sugars into simple carbohydrates by cleaving the glycosidic bonds. Enzymes isomaltase, maltase, sucrase, and lactase are secreted by the small intestine that converts polysaccharides into maltose.

Maltose is then converted into two glucose units by the enzyme maltase. Similarly, sucrose and lactose are digested by enzymes sucrose and maltase. These monosaccharides are then absorbed by the epithelial cells and released into the bloodstream, which is then diffused into different cells of the body. The excess glucose is stored in the liver cells.

The glycogen is degraded into the cytosol or inside the lysosome. In the cytosol, enzymes glycogen phosphorylase and glycogen debranching enzyme break down the glycogen. The enzyme glycogen phosphorylase cleaves α-(1, 4) glycosidic bond and releases glucose 1-phosphate. Glycogen debranching enzyme unites the glycosidic linkage and releases free glucose molecules, which are transported outside the cell into the bloodstream. In the lysosome, the enzyme acid α-glucosidase or acid maltase helps in the degradation of glycogen into free glucose.

The glucose uptake in insulin-stimulated cells is regulated by insulin. When the blood glucose level is high, insulin is released from the pancreatic cell into the bloodstream and binds with the insulin receptor present in the cell surface. As a result, certain glucose transporters (GluTs) are recruited that facilitate the entry of glucose into the cell.

Glucose can enter into liver and brain cells without the stimulation of insulin because they are insulin-independent cells. However, insulin affects glycogen synthesis in liver cells. It activates certain enzymes such as phosphofructokinase, and glycogen synthase. Another hormone is glycogen that also released from the pancreas when the blood glucose level is low. The hormone increases the glucose level in the blood by activating some enzymes involved in glycogenolysis.