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What does the term 'glycogen mobilisation' mean?

What does the term 'glycogen mobilisation' mean?


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I read that glycogen is a mobilised store of glucose:

Glycogen is a readily mobilized storage form of glucose. It is a very large, branched polymer of glucose residues (Figure 21.1) that can be broken down to yield glucose molecules when energy is needed. Most of the glucose residues in glycogen are linked by α-1,4-glycosidic bonds. [Source]

When looking online, mobilisation refers to something that is capable of movement.

I think glycogen mobilisation has something to do with glycogenolysis but not sure how to easily define glycogen mobilisation.

How would you define glycogen mobilisation in the easiest definition?


I think the key to understanding mobilization in this context is in the second sentence:

[Glycogen] can be broken down to yield glucose molecules when energy is needed.

The authors are using mobilize in a figurative sense to suggest that glycogen can be quickly and efficiently metabolized to suit the energy needs of the body, like an army may be mobilized to fight a war. See definition #2 on Wiktionary:

mobilize (verb)
2. (transitive) To assemble troops and their equipment in a coordinated fashion so as to be ready for war.


I think that @acvill more or less gets it, but I wanted to add a direct usage in context that makes clear the meaning.

I found this as the top hit when I google "glycogen mobilization". It has a lot of information and it is not straightforward, but it makes clear that mobilization is the process of breaking down glycogen into glucose. For instance, one slide says:

Glycogen phosphorylase usespyridoxal phosphate (PLP) a derivative of pyridixine (vitamine B6) as a coenzyme. B6 is required for the mobilization of glucose from glycogen.

As far as I can tell, "glycogen mobilization" means the mobilization of glucose from glycogen, therefore it is a special case of glucose mobilization in which the source molecule is glycogen. For example:

Glucose is a negative regulator of liver phosphorylase. - Glucose is not mobilized when glucose is abundant.

Additionally, wikipedia says:

… debranching enzymes mobilize glucose reserves from glycogen deposits in the liver.

and

glycogen debranching enzymes function in glycogen breakdown and glucose mobilization. When phosphorylase has digested a glycogen branch down to four glucose residues, it will not remove further residues. Glycogen debranching enzymes assist phosphorylase, the primary enzyme involved in glycogen breakdown, in the mobilization of glycogen stores.

So glycogen mobilization == glucose mobilization, in the context of human biology. That is admittedly kind of confusing.


Hydrolyze

To hydrolyze a bond is to break it apart with water. From the Greek words hydro and lysis, or “water break”, hydrolyze is literally just that. Water (or H2O) breaks into two parts: a positive hydrogen, H + , and a negative hydroxide, (OH) – . These charged molecules are used to split larger molecules by means of attracting different parts of a bond. By doing this a bond can be split, the hydroxide bonding to one half and the positive hydrogen to the other.

While there are a number of chemical reactions outside of biology that involve hydrolysis, there are many biological reactions that require water to hydrolyze the bonds of large molecules. Animals require water to hydrolyze sugars, lipids, and proteins. In other words, hydrolysis allows us to digest everything we eat. The following are some examples.


Gluconeogenesis Pathway

  1. Gluconeogenesis begins in either the mitochondria or cytoplasm of the liver or kidney. First, two pyruvate molecules are carboxylated to form oxaloacetate. One ATP (energy) molecule is needed for this.
  2. Oxaloacetate is reduced to malate by NADH so that it can be transported out of the mitochondria.
  3. Malate is oxidized back to oxaloacetate once it is out of the mitochondria.
  4. Oxaloacetate forms phosphoenolpyruvate using the enzyme PEPCK.
  5. Phosphoenolpyruvate is changed to fructose-1,6-biphosphate, and then to fructose-6-phosphate. ATP is also used during this process, which is essentially glycolysis in reverse.
  6. Fructose-6-phosphate becomes glucose-6-phosphate with the enzyme phosphoglucoisomerase.
  7. Glucose is formed from glucose-6-phosphate in the cell’s endoplasmic reticulum via the enzyme glucose-6-phosphatase. To form glucose, a phosphate group is removed, and glucose-6-phosphate and ATP becomes glucose and ADP.


This diagram shows the gluconeogenesis pathway.

2. Gluconeogenesis is a(n) ______ process.
A. Endogenous
B. Exogenous
C. Neither endogenous nor exogenous

3. What is the main body organ where gluconeogenesis takes place?
A. Kidney
B. Brain
C. Liver
D. Mitochondria


Glycogen Metabolism

Glycogen homeostasis is a highly regulated process that allows the body to store or release glucose depending on its energetic needs. The basic steps in glucose metabolism are glycogenesis, or glycogen synthesis, and glycogenolysis, or glycogen breakdown.

Glycogenesis

Glycogen synthesis requires energy, which is supplied by uridine triphosphate (UTP). Hexokinases or glucokinase first phosphorylate free glucose to form glucose-6-phosphate, which is converted to glucose-1-phosphate by phosphoglucomutase. UTP-glucose-1-phosphate uridylyltransferase then catalyzes the activation of glucose, in which UTP and glucose-1-phosphate react to form UDP-glucose. In de novo glycogen synthesis, the protein glycogenin catalyzes the attachment of UDP-glucose to itself. Glycogenin is a homodimer containing a tyrosine residue in each subunit that serves as an anchor or attachment point for glucose. Additional glucose molecules are subsequently added to the reducing end of the previous glucose molecule to form a chain of approximately eight glucose molecules. Glycogen synthase then extends the chain by adding glucose via α-1,4 glycosidic linkages.

Branching is catalyzed by amylo-(1,4 to 1,6)-transglucosidase, also called the glycogen branching enzyme. The glycogen branching enzyme transfers a fragment of six to seven glucose molecules from the end of a chain to the C6 of a glucose molecule located further inside the glycogen molecule, forming α-1,6 glycosidic linkages.

Glycogenolysis

Glucose is removed from glycogen by glycogen phosphorylase, which phosphorolytically removes one molecule of glucose from the nonreducing end, yielding glucose-1-phosphate. The glucose-1-phosphate generated by glycogen breakdown is converted to glucose-6-phosphate, a process that requires the enzyme phosphoglucomutase. Phosphoglucomutase transfers a phosphate group from a phosphorylated serine residue within the active site to C6 of glucose-1-phosphate, producing glucose-1,6-bisphosphate. The glucose C1 phosphate is then attached to the active site serine within phosphoglucomutase, and glucose-6-phosphate is released.

Glycogen phosphorylase is not able to cleave glucose from branch points debranching requires amylo-1,6-glucosidase, 4-α-glucanotransferase, or glycogen debranching enzyme (GDE), which has glucotransferase and glucosidase activities. About four residues from a branch point, glycogen phosphorylase is unable to remove glucose residues. GDE cleaves the final three residues of a branch and attaches them to C4 of a glucose molecule at the end of a different branch, then removes the final α-1,6-linked glucose residue from the branch point. GDE does not remove the α-1,6-linked glucose from the branch point phosphorylytically, meaning that free glucose is released. This free glucose could in theory be released from muscle into the bloodstream without the action of glucose-6-phosphatase however this free glucose is rapidly phosphorylated by hexokinase, preventing it from entering the bloodstream.

The glucose-6-phosphate resulting from glycogen breakdown may be converted to glucose by the action of glucose-6-phosphatase and released into the bloodstream. This occurs in liver, intestine, and kidney, but not in muscle, where this enzyme is absent. In muscle, glucose-6-phosphate enters the glycolytic pathway and provides energy to the cell. Glucose-6-phosphate may also enter the pentose phosphate pathway, resulting in the production of NADPH and five carbon sugars.


Occurrence

Glycogen is only found in animals. In humans, it is mainly found in liver and skeletal muscles in large amounts. Other cells of the body also contains glycogen in small amounts for their own uses.

In a normal person, 400 grams of glycogen is present in skeletal muscles making 1-2% of the resting muscle mass. The liver of a well-fed man contains around 100 grams of glycogen that makes around 10% of its weight.

The glycogen stores present in liver fluctuate with the blood glucose levels. Its amount increases in well-fed state and depletes during fasting. On the other hand, the glycogen stores in muscles are almost constant. They only undergo little changes in the case of strenuous exercise and are not affected by fasting. However, starvation depletes glycogen stores of both liver and skeletal muscles.


Summary

Glycogen is the glucose storage molecule found in animals only. The glycogen metabolism in the animals includes glycogenesis, glycogenolysis and glycolysis.

Glycogenesis is the synthesis of glycogen from glucose residues. The following are the important point that should be kept in mind.

  • All the glucose residues in glycogen are provided by UDP-glucose which is made for glucose-1-phosphate and UTP.
  • The chain elongation is carried out by glycogen synthase enzyme that requires a primer, to begin with.
  • The primer is made on glycogenin protein that forms the core of glycogen granules.
  • Branches are present after every 8 to 12 residues that are introduced by a special enzyme called branching enzyme.
  • Energy is provided for this process in the form of ATP and UTP.

The glycogen thus formed is broken down to release glucose during fasting by the process of glycogenolysis. It involves the following

  • Glucose residues are removed from the linear chain by glycogen phosphorylase in the form of glucose-1-phosphate.
  • The breakdown starts from the non-reducing end of the chain.
  • The branches are removed by the debranching enzyme.
  • The branch points are removed in the form of glucose-6-phosphate.
  • Glucose-6-phosphate can be converted to glucose and released into the blood only by the hepatocytes.
  • Degradation also takes place in lysosomes to some extent.

Once the glucose molecules are released into the blood, they are utilized by the cells for obtaining energy. Glycolysis is the process that generates energy by breaking down glucose molecules in the presence or absence of oxygen. The following are some important points regarding glycolysis

  • One glucose molecule gives two ATP and two NADH2 molecules at the end of glycolysis.
  • The first five reactions are energy investment phase while the next five are energy generation phase.
  • One glucose molecule is broken down into two pyruvate molecules.
  • The further processing of pyruvate depends on the availability of oxygen.

Glial Glycogen Metabolism☆

Concluding Remarks

Glycogen represents the major energy store of the brain. During decades of research, glycogen has been perceived as an emergency reserve used in case of energy supply deficiency, but more recent data, obtained with the development of new techniques, now points to a physiological role for glycogen in relation to neuronal activation. Nevertheless, some questions as to glycogen physiological processes still need further exploration. In this context, the advent of noninvasive NMR spectroscopic methods may allow the development of very useful experimental approaches to investigate glycogen function in humans. The quite exclusive localization of glycogen in glia, and its mobilization upon neuronal activation, underline the tight metabolic cooperation occurring between neurons and astrocytes.


What Is the Function of Glycogen?

Glycogen is a polysaccharide that is the storage form of glucose in the human body. Glucose is an important biomolecule that provides energy to cells throughout the entire human body. Humans derive glucose from the foods that they eat. When they are running low on glucose, glycogen can be utilized as a glucose source.

In humans, glycogen is stored and produced by the hepatocytes in the liver. The main function of glycogen is as a secondary long-term energy-storage molecule. The primary energy-storage molecules are adipose cells. Glycogen is also stored in muscle cells. Muscle glycogen is converted into glucose by the muscle cells whenever muscles are overworked and tired. Glycogen from the liver is converted into glucose to be used mainly by the central nervous system, which includes the brain and spinal cord.

In the liver, blood glucose from the foods that humans eat reaches the liver via the portal vein. There, insulin stimulates the liver cells, which stimulates glycogen synthase. This enzyme stimulates the synthesis of glycogen in the liver therefore, glycogen in the liver is formed from the food that humans eat. Muscle-cell glycogen is chemically identical to liver glycogen. However, it functions as an immediate source of glucose for muscle cells. When muscles are tired, they may convert glycogen to glucose to continue to function properly. However, liver glycogen does not convert into glucose unless the body is deprived of food.


The overall reaction for the breakdown of glycogen to glucose-1-phosphate is: [1]

glycogen(n residues) + Pi ⇌ glycogen(n-1 residues) + glucose-1-phosphate

Here, glycogen phosphorylase cleaves the bond linking a terminal glucose residue to a glycogen branch by substitution of a phosphoryl group for the α[1→4] linkage. [1]

Glucose-1-phosphate is converted to glucose-6-phosphate (which often ends up in glycolysis) by the enzyme phosphoglucomutase. [1]

Glucose residues are phosphorolysed from branches of glycogen until four residues before a glucose that is branched with a α[1→6] linkage. Glycogen debranching enzyme then transfers three of the remaining four glucose units to the end of another glycogen branch. This exposes the α[1→6] branching point, which is hydrolysed by α[1→6] glucosidase, removing the final glucose residue of the branch as a molecule of glucose and eliminating the branch. This is the only case in which a glycogen metabolite is not glucose-1-phosphate. The glucose is subsequently phosphorylated to glucose-6-phosphate by hexokinase. [1]

Glycogenolysis takes place in the cells of the muscle and liver tissues in response to hormonal and neural signals. In particular, glycogenolysis plays an important role in the fight-or-flight response and the regulation of glucose levels in the blood.

In myocytes (muscle cells), glycogen degradation serves to provide an immediate source of glucose-6-phosphate for glycolysis, to provide energy for muscle contraction.

In hepatocytes (liver cells), the main purpose of the breakdown of glycogen is for the release of glucose into the bloodstream for uptake by other cells. The phosphate group of glucose-6-phosphate is removed by the enzyme glucose-6-phosphatase, which is not present in myocytes, and the free glucose exits the cell via GLUT2 facilitated diffusion channels in the hepatocyte cell membrane.

Glycogenolysis is regulated hormonally in response to blood sugar levels by glucagon and insulin, and stimulated by epinephrine during the fight-or-flight response. Insulin potently inhibits glycogenolysis. [2]

In myocytes, glycogen degradation may also be stimulated by neural signals. [3]

Parenteral (intravenous) administration of glucagon is a common human medical intervention in diabetic emergencies when sugar cannot be given orally. It can also be administered intramuscularly.


Oligosaccharide Synthesis

Like glycogen synthesis, oligosaccharide synthesis also requires the initial step of coupling the sugar with a nucleotide. In mammals, a major disaccharide is lactose, which is the linkage of a galactose and a glucose, and the formation is catalyzed by lactose synthase. However, before the lactose synthase is able to act, the galactose must first be in the form of a UDP-galactose. Similarly, in plants, the major disaccharide is sucrose, formed by the linkage of UDP-glucose and fructose-6-phosphate. This results in sucrose-6-phosphate, which is then readily dephosphorylated to sucrose. These kinds of mechanisms are also used in the glycosylation of proteins and lipids, which will be discussed primarily in the protein processing and trafficking chapter.

Mutation of galactose-1-phosphate uridylyltransferase or mutations of other enzymes in this pathway (uridylyl transferase mutations are most common and usually most severe) can lead to galactosemia, a human genetic disease whose symptoms begin in infancy and may include mental retardation, liver damage, jaundice, vomiting, and lethargy. The cause of these symptoms is generally a buildup of galactose-1-phosphate, especially in the liver and nervous tissue. Fortunately, with early diagnosis, the symptoms can be prevented by avoiding milk products (lactose).

The major hexose species besides glucose are fructose, mannose, and galactose. Interconversion between these hexoses can occur via intermediates, as demonstrated in glycolysis (glucose-6-P to fructose-6-P). Mannose-6-P can be converted to fructose-6-P by phosphomannose isomerase. Galactose can be converted similarly, to galactose-1-P and then to glucose-1-P. The galactose to glucose conversion can also take place by epimerization of UDP-Glucose to UDP-galactose via intermediate redox using NAD + / NADH.



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