Glycogen: Definition, Structure, Synthesis, Functions, Associated Diseases, Diagnosis and Treatments

Living organisms are mainly based on four main classes of biomolecules.

Carbohydrates, proteins, nucleotides, and lipids are involved in and regulate many biochemical reactions.

Of these biomolecules, carbohydrates are the most abundant biomolecules and show various functions in living organisms, such as energy transport, which are structural components.

They are also involved in the immune system in the blood clotting process, among others.

These biomolecules are carbon, oxygen, and hydrogen with the general formula C n (H 2 O) n.

They are polyhydroxy aldehydes and ketones.

Carbohydrates are made up of tens to hundreds to several thousand monosaccharide units and are also called polysaccharides.


Common polysaccharides contain glucose as a monosaccharide unit and are synthesized by plants, animals, and humans.

They mainly act as reserve food, participate in structural support (the cell wall in the plant cell and the external skeleton of insects in the form of chitin), and are metabolized to produce energy.

Polysaccharides act mainly as food preservatives in the food reserve of plants in the form of starch.

It is stored as glycogen in the liver and muscles in humans and other animals.

Glucose and other carbohydrates are the primary energy sources for the human body.

When we consume sugars and carbohydrates, we increase the available energy molecules that can be used for cellular respiration and the production of adenosine triphosphate (ATP).

Glycosidic bonds

Glycogen is a polysaccharide that is formed from excess glucose in the body.

Simple glucose molecules can form glycosidic bonds to form larger macromolecules.

As we consume sugar, either as individual molecules or in the form of starches, we break these bonds to release the glucose and monosaccharides necessary for ATP production.

Any excess glucose is stored as glycogen in the liver and muscle cells for future use if energy needs increase dramatically.

Glucose that is not used immediately is stored as glycogen in the liver and muscles through glycogenesis.

Some tissues, such as the liver and skeletal muscle, store glucose as glycogen, which can be rapidly mobilized.

For example, glycogen breaks down when blood glucose is high and releases glucose into the bloodstream. In contrast, in the case of low blood sugar, such as fasting, muscle uses its glycogen stores for energy and serves as a buffer. To maintain blood glucose levels.

Glycogen Structure

Glycogen consists of long polymeric chains of glucose units linked by an alpha bond.

A bond is formed by combining the carbonyl group and the alcoholic group.

If the carbonyl group is an aldehyde group (-CHO), it is called hemiacetal, and if there is a ketone group, it forms a hemiketal bond.

If two alkoxy groups are attached to the same carbon atom, they called it an acetal group, and the band was called an acetal bond.

In the case of glycogen, all alpha-D-glucose are linked together by an alpha-acetal bond between C1 of one monomeric unit and C4 of another monomeric unit, which is why it is called the αα-1,4-glucosidic bond.

Since glycogen is a branched polymer, branching occurs at 8-10 glucose units.

These branches are formed by an acetal bond between C1 and C6.

There are possible forms of glycogen, proglycogen having a molecular mass of around 400 kDa and macroglycogen with a very high molecular mass.

Glycogen can be organized in a spherical form in which the glucose chains are organized globularly around the central protein glycogen.

It seems that the branches of a tree originated from a central point.

Glycogen Synthesis

Excess glucose is stored in the form of glycogen.

Glycogen is stored in the liver and muscles that act as storage sites.

For all regular movements and functions, energy is first provided by glucose contained in the bloodstream. After consumption of this glucose, energy is extracted in the form of glycogen from storage sites.

The synthesis of glycogen from glucose is called glycogenesis.

The glycogen synthesis is called glycogenesis, and it is an endergonic process. Therefore, a specific energy contribution is required, provided by uridine triphosphate.

Glycogenesis is a four-step process in which each step is catalyzed by specific enzymes.

-In the first step, glucose is converted to glucose-6-phosphate in the presence of glucokinase or hexokinase enzymes.

This glucose-6-phosphate forms glucose-1-phosphate by the action of phosphoglucomutase.

Uridine triphosphate reacts with glucose-1-phosphate to form UDP-glucose and pyrophosphate in UDP-glucose pyrophosphorylase or Uridyl Transferase.

Pyrophosphate is additionally hydrolyzed into two molecules in the presence of pyrophosphatase.

UDP-glucose acts as a monomeric unit that progressively lengthens the glycogen chain with (α1â † ‘4) bound glucose in the presence of the enzyme glycogen.

  • Excess glucose is stored as glycogen at storage sites.
  • In case of low blood sugar, a glucagon hormone is released, which involves the conversion of glycogen into blood sugar, a fuel source.
  • This process is known as glycogenolysis.
  • The process of glycogenolysis is initiated by the enzyme glycogen phosphorylase, which involves the cleavage of terminal glycogen residues by inorganic phosphate.
  • This enzyme requires a covalently linked pyridoxal phosphate co-factor (vitamin B6).
  • Cleavage of the terminal glycogen residue forms glucose-1-phosphate, further converted to glucose-6-phosphate by phosphoglucomutase.
  • The complete breakdown of glycogen requires the action of a second bifunctional enzyme is known as a debranching enzyme as oligo (αα-1,4 -> αα-1,4) glucantransferase.
  • Hence the glycogen breakdown process is completed in three steps.
  • The conversion of glycogen to glucose 1-phosphate.

Remodeling of the glycogen substrate for further degradation:

  • Formation of glucose-6-phosphate from glucose-1-phosphate for increased metabolism.
  • Two hormones, glucagon from the pancreas and epinephrine from the adrenal glands, regulate the process of glycogenolysis.
  • Both processes are related to glycogen, glycogenesis, and glycogenolysis, regulated by two enzymes, one involved in the synthesis of glycogen, which is glycogen synthase, and the other in the degradation of glycogen, which is glycogen phosphorylase.
  • The action of both enzymes is regulated by circulating hormones, including insulin, glucagon, and epinephrine, and by the level of energy and metabolites available to the cell.
  • The end product of glycogenolysis, glucose 6-phosphate, can be involved in several pathways.
  • It acts as a precursor to glycolysis.
  • Involve in processing the pentose phosphate pathway to form NADPH and ribose derivatives.
  • It can be converted into glucose during the need for energy, which provides power during various metabolic processes.

Glycogen functions

  • Hepatic glycogen acts as a glucose reserve that releases hepatocytes when necessary to maintain normal blood sugar levels. There is approximately 40 Kcal in body fluids, while liver glycogen can provide about 600 Kcal after an overnight fast.
  • Glucose from glycogen stores remains within cells in skeletal and cardiac muscles and is used as an energy source for muscle work.
  • The brain includes a small amount of glycogen in astrocytes. It accumulates during sleep and is mobilized when walking. Glycogen stores also ensure a moderate degree of protection against hypoglycemia.
  • It has a specialized role in fetal type II lung cells. These cells accumulate glycogen at approximately 26 weeks gestation and then synthesize lung surfactant.

Glycogen storage disorders

The used glycogen is stored as soon as we eat something to avoid depletion of the glycogen level.

In low glycogen, the body begins to break down proteins to use them as fuel sources harmful to the body.

Glycogen deficiency and abnormal functions cause many storage disorders.

Since the formation and breakdown of glycogen involve many enzymes, there are many different types of glycogen storage disorders. Each disorder is related to the lack or malfunction of other enzymes.

For example, the problem associated with the production of those enzymes that participate in glycogen synthesis can produce abnormal units of glycogen.

Similarly, defects with enzymes related to glycogenolysis can lead to low glucose in the body, which is known as hypoglycemia, accumulation of glycogen in the muscles and liver, and responsible for innate errors of metabolism.

Glycogen storage disorders primarily affect the liver and muscles with other body parts such as the kidney, heart, blood vessels, nervous system, and intestine.

Glycogen storage diseases

Glycogen is the form of sugar that your body stores in your liver and muscles for future energy needs.

Glycogen storage diseases are complex genetic conditions in which specific enzymes, which are involved in creating glycogen or breaking it down to sugar for your body to use, are either missing or not working correctly.

This can lead to liver, heart, muscle, and breathing problems.

There are several different types of glycogen storage diseases, the most common of which are:

  • Von Gierke disease (type I).
  • Pompe disease (type II).
  • Forbes-Cori disease (type III).
  • Andersen’s disease (type IV).
  • McArdle disease (GSD V).
  • Glycogen storage diseases VI.
  • Glycogen storage diseases IX.
  • Glycogen storage diseases 0.

This class of diseases is most often diagnosed in babies, but it can also be interpreted in adults.

Glycogen storage disease complications

Complications vary depending on the type of glycogen storage disease; however, they can include:

  • Liver problems
  • Low blood sugar
  • Gastrointestinal issues such as inflammatory bowel disease.
  • Growth and developmental delays.
  • Lung problems.
  • Heart problems.
  • Additional complications can include muscle disease, blood disorders, and kidney problems.

Diagnosis of glycogen storage diseases

It is based on family history and medical tests to diagnose glycogen storage diseases.

The following tests can be ordered:

Blood test

It can be used to monitor liver, kidney, and muscle health and to ensure adequate blood sugar levels.

Genetic testing

It can discover the presence of genetic changes that cause disease.

It is used to check for specific markers of diseases and hereditary traits.

Tissue biopsies

Tissue samples taken from the liver and muscle are tested for disease or abnormal cell function.

Imaging tests

Contrast-enhanced ultrasound, CT, and MRI create detailed images of organs and vessels’ size, structure, and function.


This non-surgical alternative to a liver biopsy uses ultrasound to check for liver stiffness due to scarring, called liver fibrosis.

Monitoring of preventive diseases

Living with glycogen storage disease means closely monitoring the results of laboratory tests and periodic tests and examinations to diagnose complications when they arise.

Severe forms of glycogen storage disease can damage the heart and lungs and cause infections.


Dietary treatments

The effects of some forms of glycogen storage disease can be reversed by maintaining healthy levels of vitamins, minerals, and enzymes for proper growth and development.

Depending on the condition, special diets may include meals high in carbohydrates and starch, frequent meals to maintain blood sugar levels, cornstarch therapy to avoid low blood sugar, or limiting the foods you eat. The body cannot decompose.

Physical and occupational therapy

People with glycogen storage disorders often work with physical and occupational therapists to build strength and promote proper development.

These therapies can help with motor skills to perform the tasks of daily living.

Speech therapy

Weakened muscles and developmental delays related to glycogen storage disorders can affect speech.

Speech therapy teaches children how to do the correct mouth movements to improve their words and language acquisition.


Surgery may be necessary if the liver, heart, or digestive tract are affected by the disease.

If severe damage occurs, organ transplants may be recommended.