Glycogenesis – Cycle, Steps, Significance (Vs Gluconeogenesis)

The foods we eat are turned into glucose and released as energy to be able to use by the body. The molecule of glucose that is stored in the important organs of the body is called glycogen.

It is stored in various parts of the body such as the kidneys, liver, brain, and muscles. It is only released if the glucose in the blood is used up for all physical activities. Once the body runs out of glucose supply, additional energy is immediately released in the form of glycogen. (1, 2)

image explains the process of glycogen synthesis

Picture 1: The image explains the process of glycogen synthesis.

Image Source: slidesharecdn.com

What is glycogen?

Glycogen is a polysaccharide (homopolymer) deposited in the tissues and stored as a carbohydrate. During hydrolysis, glycogen is converted into glucose. (1)

 

What is glycogenesis?

It is a process by which glycogen is formed from glucose. Glycogen is synthesized accordingly as per the demand of energy. If there is a sufficient amount of insulin in the body, excess glucose will not be used and will only be stored in the form of glycogen.

If the body runs out of insulin, the stored glucose will be released to supplement the body’s need for energy in the form of glycogen and through the process of glycogenesis. (1, 2, and 3)

 

liver serves as the storage facility for glucose in the form of glycogen

Picture 2: The liver serves as the storage facility for glucose in the form of glycogen.

Image Source: nlm.nih.gov

 

What is the purpose of Glycogenesis?

The primary purpose of glycogenesis is to make sure the body does not run out of glucose. Glucose is important as it is the body’s primary source of energy. Without an adequate supply of glucose in the body, vital organs will eventually shut down. (3, 4)

 

What triggers Glycogenesis?

It is the hormone insulin that triggers glycogenesis. This particular hormone has a huge impact on the metabolism of glucose in the liver cells. It stimulates glycogenesis and at the same time inhibit the breakdown of glycogen into glucose (glycogenolysis). (4, 5, and 6)

image and diagram contains the glycogenesis pathway, which includes a total of six steps

Picture 3: The image contains the glycogenesis pathway, which includes a total of six steps.)

Image Source: researchgate.net

 

The glycogenesis pathway/ Steps of glycogenesis

The pathway of glycogenesis includes a series of steps that result in complex glycogen formation in the cytoplasm of the liver and cells of the muscles. The steps of glycogenesis are as follows:

  1. Glucose phosphorylation – In the initial phase, glucose is phosphorylated into glucose-6-phosphate, a usual reaction in glycolysis. It is catalyzed by glucokinase (liver) and hexokinase (muscle).
  2. Conversion of Glc-6-P to Glc-1-P – An enzyme Phosphoglucomutase will catalyze the conversion of Glucose-6-P is converted to Glc-1-Phosphate.
  3. UTP (uridine triphosphate) attaches to Glc-1-P – The third step focuses on the reaction of glucose-1-P to UTP thereby forming active nucleotide UDP-Glc (Uridine diphosphate glucose). The one responsible for such reaction is the enzyme UDPGlc Pyrophosphorylase.
  4. UDP-Glc attaches to glycogen primer – A small fragment of already existing glycogen will serve as a primer in order to stimulate the synthesis of glycogen. The glucose from UDP-Glc will be accepted by glycogenin. The initial glucose unit is attached to the hydroxyl group of tyrosine of glyogenin. The first molecule of glucose is transferred to glycogenin, which will then takes up for glucose residues forming a fragment of primer. It will be the one to accept all glucose molecules.
  5. Glycogen synthase synthesizes glycogen – Glycogen synthase transfers glucose from UDP-Glc to glycogen (non-reducing end) forming alpha 1,4-linkages.  The same enzyme catalyzes the synthesis of the unbranched molecule with alpha-1,4-glycosidic linkages.
  6. The formation of glycogen braches – The final step is the formation of glycogen branches caused by the effect of branching enzyme, which transfers a small fragment of about five to eight residues of glucose from the non-reducing end of the glycogen chain to another glucose residue linked by alpha-1,6 bond. This action causes the formation of a new non-reducing end. The final result is the elongation and branching out of the glycogen chain. (2, 5, 6, 7, and 8)

The process of glycogenesis utilizes two molecules of ATP. One molecule is needed for glucose phosphorylation and another molecule is needed to convert UDP to UTP.

 

Regulatory enzyme:

  • Glycogen synthase
  • Glycogen phosphorylase
  • Glycogen phosphorylase kinase (5, 8, and 9)

What is the difference between gluconeogenesis and Glycogenesis?

Glycogenesis is the formation of glycogen from glucose whereas gluconeogenesis is the formation of glucose from smaller molecules. (3, 4)

 

Regulation of glycogenesis in the body is determined by the following hormones

The formation of glycogen primarily depends on the level of glucose in the blood as well as the level of glycogen in the liver and muscle tissues. The activities of hormones in the body also affect the level and release of glycogen. These enzymes include the following:

  1. Adrenaline – It inhibits glycogen synthase and the activation of glycogen phosphorylase.
  2. Insulin – Its effect is opposite to that of adrenaline. Insulin binds to protein primmer and will be converted to a non-phosphorylated form with the aid of glycogen synthase. As a result, the blood sugar level is decreased even after consuming carbohydrate-rich foods. If the body is not effectively producing insulin, a patient suffers from a medical condition called diabetes.
  3. Calcium ions – It activates glycogen phosphorylase and at the same time inhibit glycogen synthase. (1, 7, 9, and 10)

 

Medical conditions caused by glycogen storage

  • Cori disease
  • Pompe disease
  • Von Gierke disease
  • McArdle disease (10)

 

Summary

Glycogenesis is the formation of glycogen, which serves as an energy reservoir. It is mainly found in the muscles and liver cells of humans and animals.

It is synthesized from glucose when there is an abundant supply of glucose in the blood. If the supply of glucose in the blood is deficient, glycogen will be released and will be used as a source of glucose for tissues throughout the body.

 

References:

  1. https://en.wikipedia.org/wiki/Glycogenesis
  2. http://chemistry.elmhurst.edu/vchembook/604glycogenesis.html
  3. https://www.checkdiabetes.org/glycogenesis/
  4. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/glycogenesis
  5. https://themedicalbiochemistrypage.org/glycogen.php
  6. https://www.ncbi.nlm.nih.gov/books/NBK21190/
  7. https://www.amboss.com/us/knowledge/Glycogen_metabolism
  8. http://www.innovateus.net/science/what-glycogenesis
  9. https://www.biology-online.org/dictionary/Glycogenesis
  10. https://biologydictionary.net/glycogen/

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Glycogenesis – Cycle, Steps, Significance (Vs Gluconeogenesis)
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