Glycogen

Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria.

Glucose for metabolism is partially stored as a polymer, in plants mainly as starch and amylopectin and in animals as glycogen.

Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria.

Examples include storage polysaccharides such as starch and glycogen, and structural polysaccharides such as cellulose and chitin.

In humans, glycogen is made and stored primarily in the cells of the liver and skeletal muscle.

Its other roles in metabolism include the regulation of glycogen storage, decomposition of red blood cells and the production of hormones.

Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria.

Approximately 4 grams of glucose are present in the blood of humans at all times; in fasted individuals, blood glucose is maintained constant at this level at the expense of glycogen stores in the liver and skeletal muscle. As a meal containing carbohydrates or protein is eaten and digested, blood glucose levels rise, and the pancreas secretes insulin.

Glucose is stored in skeletal muscle and liver cells in the form of glycogen; in fasted individuals, blood glucose is maintained at a constant level at the expense of glycogen stores in the liver and skeletal muscle.

Glycogen is the analogue of starch, a glucose polymer that functions as energy storage in plants.

Glycogen, the glucose store of animals, is a more highly branched version of amylopectin.

It has a structure similar to amylopectin (a component of starch), but is more extensively branched and compact than starch.

Its counterpart in animals is glycogen, which has the same composition and structure, but with more extensive branching that occurs every eight to 12 glucose units.

Due to the way glycogen is synthesised, every glycogen granule has at its core a glycogenin protein.

Glycogenin is an enzyme involved in converting glucose to glycogen.

As a meal containing carbohydrates or protein is eaten and digested, blood glucose levels rise, and the pancreas secretes insulin.

In these tissues the absorbed glucose is converted into either glycogen via glycogenesis or [[Fatty acid metabolism#Glycolytic end products are used in the conversion of carbohydrates into fatty acids|fats]] (triglycerides) via lipogenesis, or, in the case of the liver, into both.

As a meal containing carbohydrates or protein is eaten and digested, blood glucose levels rise, and the pancreas secretes insulin.

Polysaccharides serve for the storage of energy (e.g. starch and glycogen) and as structural components (e.g. cellulose in plants and chitin in arthropods).

Insulin acts on the hepatocytes to stimulate the action of several enzymes, including glycogen synthase.

Glycogen synthase (UDP-glucose-glycogen glucosyltransferase) is a key enzyme in glycogenesis, the conversion of glucose into glycogen.

In response to insulin levels being below normal (when blood levels of glucose begin to fall below the normal range), glucagon is secreted in increasing amounts and stimulates both glycogenolysis (the breakdown of glycogen) and gluconeogenesis (the production of glucose from other sources).

Glycogenolysis is the breakdown of glycogen (n) to glucose-1-phosphate and glycogen (n-1).

Glycogen is found in the form of granules in the cytosol/cytoplasm in many cell types, and plays an important role in the glucose cycle.

When the blood glucose level is too high, glucose can be stored in the liver as glycogen.

Blood glucose from the portal vein enters liver cells (hepatocytes).

Brown lipofuscin granules are also observed (with increasing age) together with irregular unstained areas of cytoplasm; these correspond to cytoplasmic glycogen and lipid stores removed during histological preparation.

Glycogen phosphorylase is the primary enzyme of glycogen breakdown.

Glycogen phosphorylase breaks up glycogen into glucose subunits (see also figure below):

The human brain consumes approximately 60% of blood glucose in fasted, sedentary individuals.

The brain stores glucose in the form of glycogen, albeit in significantly smaller amounts than that found in the liver or skeletal muscle.

Glucagon, another hormone produced by the pancreas, in many respects serves as a countersignal to insulin.

Glucagon causes the liver to convert stored glycogen into glucose, which is released into the bloodstream.

In response to insulin levels being below normal (when blood levels of glucose begin to fall below the normal range), glucagon is secreted in increasing amounts and stimulates both glycogenolysis (the breakdown of glycogen) and gluconeogenesis (the production of glucose from other sources).

Other means include the degradation of glycogen (glycogenolysis) and fatty acid catabolism.

The glycogen branching enzyme catalyzes the transfer of a terminal fragment of six or seven glucose residues from a nonreducing end to the C-6 hydroxyl group of a glucose residue deeper into the interior of the glycogen molecule.

Glycogen branching enzyme is an enzyme that adds branches to the growing glycogen molecule during the synthesis of glycogen, a storage form of glucose.

A debranching enzyme is a molecule that helps facilitate the breakdown of glycogen, which serves as a store of glucose in the body, through glucosyltransferase and glucosidase activity.

After glycogen phosphorylase catalyzes the phosphorolytic cleavage of a glucosyl residue from the glycogen polymer, the freed glucose has a phosphate group on its 1-carbon.

In addition to these two metabolic pathways, glucose 6-phosphate may also be converted to glycogen or starch for storage.

Energy for glycogen synthesis comes from uridine triphosphate (UTP), which reacts with glucose-1-phosphate, forming UDP-glucose, in a reaction catalysed by UTP—glucose-1-phosphate uridylyltransferase.

In glycogenolysis, it is the direct product of the reaction in which glycogen phosphorylase cleaves off a molecule of glucose from a greater glycogen structure.

Energy for glycogen synthesis comes from uridine triphosphate (UTP), which reacts with glucose-1-phosphate, forming UDP-glucose, in a reaction catalysed by UTP—glucose-1-phosphate uridylyltransferase.

UDP-glucose is a precursor of glycogen and can be converted into UDP-galactose and UDP-glucuronic acid, which can then be used as substrates by the enzymes that make polysaccharides containing galactose and glucuronic acid.

These are collectively referred to as glycogen storage diseases.

A glycogen storage disease (GSD, also glycogenosis and dextrinosis) is a metabolic disorder caused by enzyme deficiencies affecting either glycogen synthesis, glycogen breakdown or glycolysis (glucose breakdown), typically within muscles and/or liver cells.