Glycogen

Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals,[2] fungi, and bacteria.

Protein, broken down into amino acids, is seldom used as a main energy source except during starvation and glycolytic crisis (see bioenergetic systems).

[12] The amount of glycogen stored in the body mostly depends on oxidative type 1 fibres,[13][14] physical training, basal metabolic rate, and eating habits.

[4] Glycogen is an analogue of starch, a glucose polymer that functions as energy storage in plants.

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.

[3] Glycogen is a non-osmotic molecule, so it can be used as a solution to storing glucose in the cell without disrupting osmotic pressure.

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

After a meal has been digested and glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops.

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).

This is in contrast to liver cells, which, on demand, readily do break down their stored glycogen into glucose and send it through the blood stream as fuel for other organs.

[27] During anaerobic activity, such as weightlifting and isometric exercise, the phosphagen system (ATP-PCr) and muscle glycogen are the only substrates used as they do not require oxygen nor blood flow.

Muscle glycogen can supply a much higher rate of substrate for ATP synthesis than blood glucose.

[30][4] Due to its high supply rate and quick ATP synthesis, during high-intensity aerobic activity (such as brisk walking, jogging, or running), the higher the exercise intensity, the more the muscle cell produces ATP from muscle glycogen.

[32] However, research by Besford et al[33] used small angle X-ray scattering experiments accompanied by branching theory models to show that glycogen is a randomly hyperbranched polymer nanoparticle.

This has been subsequently verified by others who have performed Monte Carlo simulations of glycogen particle growth, and shown that the molecular density reaches a maximum near the centre of the nanoparticle structure, not at the periphery (contradicting a fractal structure that would have greater density at the periphery).

"Note sur la formation physiologique du sucre dans l’economie animale."

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.

Various inborn errors of carbohydrate metabolism are caused by deficiencies of enzymes or transport proteins necessary for glycogen synthesis or breakdown.