What is Glycolysis?  

Where does it take place?

Pathway and Products



What is Glycolysis?

 

The word Glycolysis is derived from the Greek words "Glykos" meaning sweet (sugar) and "Lysis" which means to split or splitting. Therefore, glycolysis (or the glycolytic pathway) may be described as the metabolic breakdown of glucose (a 6 carbon sugar) in order to release energy.

For various organisms, energy in the form of adenosine triphosphate (ATP) is required for biochemical reactions (e.g. reactions involved in muscle contraction). Here, then, glucose, the main source of energy, has to be broken down through several subsequent processes in order to release this chemical energy.

In addition to adenosine triphosphate, this metabolic pathway also releases two molecules of NADH (nicotinamide adenine dinucleotide) and pyruvate (a three-carbon molecule).

 

* Glycolysis was discovered in 1897 by Hans Buchner and Eduard Buchner, German scientists, as they sought to manufacture cell-free yeast extract.

  



Where does Glycolysis Take Place?


Glycolysis is the first phase of cellular respiration. It takes place in the cytoplasm where associated enzymes and factors are located. This process is anaerobic and therefore does not require energy. As such, it has been shown to be one of the most ancient metabolic pathways that could occur even in the simplest cells (earliest prokaryotic cells).


Metabolic pathway of glycolysis converts glucose to pyruvate via a series of intermediate metabolites by Thomas Shafee / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)Metabolic pathway of glycolysis converts glucose to pyruvate via a series of intermediate metabolites by Thomas Shafee / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)


Glycolysis Pathway and Products


Glucose Transport into the Cell


As mentioned, glucose is the main source of energy. However, given that this simple sugar may not be readily available, the body has to break down large molecules (e.g. polymeric carbohydrates like starch).

The breakdown of starch starts in the mouth where amylase is responsible for the breakdown of starch into sugars. In the small intestine, this activity is carried out by carbohydrase enzymes that continue acting on the starch molecules. 

For glycolysis to start, glucose has to be transported into the cell (from the gut and into the epithelial cells) where the process occurs. One group of transporters involved in the transport of glucose in or out of the cells is known as GLUTs (glucose transporters). These are proteins with substrate binding sites on which glucose molecules bind in order to be transported.

Following this binding (to the sites exposed to the inside or outside the cell), the transporter undergoes conformational changes that ultimately result in the molecule being transported through the lipid bilayer in or out of the cell. 



Phosphorylation I


Once the glucose has been successfully transported into the cell, a phosphoryl group is added in the presence of hexokinase type II in different types of tissues in the body or glucokinase (also known as hexokinase IV) in the liver. This reaction is commonly known as phosphorylation and involves the addition to a phosphoryl group onto the sixth (6th) carbon of the sugar molecule.

As mentioned, the glucose transporters located on the cell membrane are capable of transporting glucose in and out of the cell. However, by adding a Phosphoryl group onto this sugar molecule, it's trapped and cannot be transported out of the cell. Therefore, this step serves to trap the sugar molecule in the cell. 

 

* During the phosphorylation, ATP provides a phosphate which is added onto the sixth carbon of the sugar molecule. This converts the ATP molecule into ADP. This reaction is facilitated by either of the two enzymes mentioned above depending on the type of cells involved.


* Addition of the phosphoryl group has also been shown to make the sugar molecule more reactive, less stable as compared to the original sugar molecule/glucose, and thus ready for glycolysis.



Isomerization


Once a glucose molecule has been converted to glucose 6-phosphate through phosphorylation, it's then converted into a fructose. This step is facilitated by the enzyme phosphohexose isomerase. Here, the enzyme first opens up the glucose 6-phosphate ring so as to expose the aldehyde group which is the reactive part of the molecule.

The group is transformed into a ketose group ultimately resulting in the formation of fructose 6-phosphate. However, this molecule can be converted back to glucose 6-phosphate if need be. 



Phosphorylation II


The Fructose molecule formed during the isomerization stage undergoes phosphorylation thus making it even more reactive. This is facilitated by the enzyme phosphofructokinase I.

It's worth noting that in the fructose 6-phosphate molecule, the sixth (6th) carbon still has the phosphate that was added during the first phosphorylation step. In this step, then, the enzyme adds a phosphate group onto the first carbon of the sugar molecule. 

This results in the formation of a molecule known as fructose 1, 6-biphosphate. Unlike a bi-phosphate where the phosphate groups are next to each other in the molecule, a biphosphate molecule consists of carbon atoms between the phosphate groups. Here, carbon molecules create distance between the phosphate groups. 

 

* As was the case with the first phosphorylation, the second phosphorylation also requires an ATP molecule to provide a phosphate. The process has used two ATP molecules so far. 


* Unlike fructose 6-phosphate, which can be stored as glycogen, fructose 1, 6-biphosphate cannot be stored. At this stage, it's said to have committed to glycolysis and therefore cannot go back. This also further destabilizes the molecule so that it can be easily broken down in the next stage. 


A diagram of cellular respiration including glycolysis, Krebs cycle (AKA citric acid cycle), and the electron transport chain by RegisFrey / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)A diagram of cellular respiration including glycolysis, Krebs cycle (AKA citric acid cycle), and the electron transport chain by RegisFrey / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)


Splitting Fructose 1.6-Biphosphate

 

This stage of glycolysis involves the breakdown of the molecule into two 3 carbon molecules. While the two molecules have 3 carbons each, they are not identical. Here, the fructose molecule, fructose 1, 6-biphosphate, is first opened up in order to expose the carbon bond to be cleaved.

Therefore, it's necessary to open up the cyclic form of the fructose molecule into the chain form. Once it has been opened up, the enzyme Aldolase then acts on the carbon bond thus cleaving the molecule to produce two 3 carbon molecules. 

One of the molecules is known as dihydroxyacetone phosphate (DHAP) which contains 3 carbons and a phosphoryl group on one of the carbons. The other 3 carbon molecule is known as glyceraldehyde 3-phosphate (G3P) and also consists of 3 carbons and a phosphoryl group.

While glyceraldehyde 3-phosphate lies directly in the glycolytic pathway and can proceed onto the next step, dihydroxyacetone phosphate first has to be converted to glyceraldehyde-3-phosphate before it can proceed onto the next step of this stage of glycolysis. 

 

* In this stage, as already mentioned, the fructose molecule (Fructose 1, 6-bisphosphatase) is cleaved to produce two 3 carbon molecules. The fact that the two molecules are different is very important given that it allows for the proper regulation of cell metabolism in general.

While glyceraldehyde-3-phosphate is directly involved in the production of ATP energy, dihydroxyacetone phosphate is not. This means that the conversion of dihydroxyacetone phosphate into glyceraldehyde-3-phosphate will largely depend on the needs of the cell. 


In a scenario where there is already too much ATP in the cell, then there is no reason for the continued production of ATP. As a result, glycolysis does not need to continue. The enzyme triose-phosphate isomerase can convert the glyceraldehyde-3-phosphate into dihydroxyacetone phosphate which can then be transformed into triglycerides before being stored as fats.

However, in a scenario where more ATP is required (e.g. during running which requires more energy), then the equilibrium has to shift to the right. This means that rather than converting glyceraldehyde-3-phosphate to dihydroxyacetone phosphate, the enzyme triose-phosphate isomerase has to convert dihydroxyacetone phosphate into glyceraldehyde-3-phosphate which can then be used to produce ATP energy. 

 

* In the cell, the dihydroxyacetone phosphate is the predominant molecule (about 96 percent at equilibrium). This allows it to be the main source of glyceraldehyde-3-phosphate thus allowing the equilibrium to shift to the right as more ATP is required. 

 

* Dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P) are isomers of each other. While they have the same formula, the atoms are arranged differently which in turn means that they have different properties. In the presence of the enzyme Triose-phosphate isomerase, they can be readily interconverted from one to the other. 


In order to convert the dihydroxyacetone phosphate (a ketone) into glyceraldehyde 3-phosphate (an aldose), the enzyme has to transfer the hydrogen located on the first carbon of the dihydroxyacetone phosphate to the second carbon of the glyceraldehyde 3-phosphate. In doing so, it rapidly converts the ketose to aldose through a redox reaction where hydrogen is transferred from one carbon of the former molecule to the second carbon of the second molecule.



Conversion of Glyceraldehyde-3-phosphate to pyruvate

 

This is the last stage of glycolysis and involves the conversion of glyceraldehyde-3-phosphate into pyruvate, ATP and NADH. In this stage of the glycolysis pathway, the glyceraldehyde-3-phosphate from the second stage is first converted into 1, 3 bisphosphoglycerate (also known as 1, 3-bisphosphoglyceric acid). 

In this reaction, the enzyme glyceraldehyde 3-phosphate dehydrogenase is involved in the addition of an orthophosphate (Pi) onto the glyceraldehyde 3-phosphate (on the third carbon of the molecule) to form 1, 3-bisphosphoglycerate. 

Given that the process also requires the presence of the co-enzyme (Nicotinamide adenine dinucleotide) NAD+, it's reduced to NADH by addition of a hydrogen ion from the glyceraldehyde 3-phosphate. Therefore, the entire reaction results in the production of 1, 3-bisphosphoglycerate, two (2) NADH molecules, and an extra hydrogen ion. Unlike Glyceraldehyde 3-phosphate, 1, 3-bisphosphoglycerate consists of two Phosphoryl groups 

 


In the next step of this stage, a Phosphoryl group is transferred from the 1, 3-bisphosphoglycerate to an ADP molecule resulting in the production of an ATP molecule and 3-phosphoglycerate. This reaction, commonly known as substrate-level phosphorylation, is catalyzed by the enzyme phosphoglycerate kinase.

It's worth noting that this step involves two molecules of 1, 3-bisphosphoglycerate. For this reason, two ADP molecules are involved in the reaction resulting in the production of two (2) molecules of ATP. 

 

* As previously mentioned, the first stage of glycolysis uses a total of two ATP molecules. However, by the time we get to the substrate-level phosphorylation reaction, two ATP molecules are produced. Therefore, at this particular step, the total net of ATP produced is zero given the process has only given back the two ATPs that were initially used.

 

Through the action of the enzyme phosphoglycerate mutase (in the presence of 2, 3-biphosphoglycerate), 3-phosphoglycerate, the molecule produced in the previous step, is transformed into 2-phosphoglycerate. Here, a phosphoryl group located on the third carbon of the molecule (3-phosphoglycerate) is moved to the second carbon of the molecule thereby converting the molecule into 2-phosphoglycerate. 

Through the conversion of the 3-phosphoglycerate, it becomes a little more reactive (by being more unstable) as 2-phosphoglycerate. In turn, the 2-phosphoglycerate molecules are converted to phosphoenolpyruvate by the enzyme enolase.

This step is particularly important as it results in the production of a molecule (phosphoenolpyruvate/PEP) that can effectively transfer a phosphoryl molecule required to produce another ATP molecule. 

This is a dehydration reaction that not only results in the formation of phosphoenolpyruvate but also a water molecule. Here, the enzyme removes a hydroxyl molecule located on the first carbon and hydrogen from the second carbon to form a water molecule. 




 

* Typically, with regards to enzymes, a mutase transfers a group located on one location of a molecule to another location on the molecule thereby changing its properties.

 

* As compared to the 2-phosphoglycerate, the phosphoenolpyruvate (an enol) has a high phosphoryl-transfer potential which makes the reaction very important.

  

In the last step of the glycolytic pathway, a pyruvate molecule in addition to a molecule of ATP is produced. This reaction is catalyzed by pyruvate kinase in the presence of ADP. A hydrogen ion is also important for the reaction given that it replaces the phosphoryl group located on the phosphoenolpyruvate molecule thus allowing the group to be added to the ADP molecule. As a result, the reaction produces a pyruvate molecule as well as ATP molecules.

Here, because two (2) 3- phosphoglycerate are involved in the reaction, then two molecules of ATP and 2 molecules of pyruvate are produced.  Whereas ATP is formed through the addition of a phosphoryl group onto the ADP molecule, the pyruvate molecule is formed by replacing the phosphoryl group with a hydrogen ion. 

 

* The fate of pyruvate is largely dependent on the presence or absence of oxygen. In the absence of oxygen (anaerobic), the pyruvate is reduced (gains hydrides) to lactic acid while NADH is oxidized and converted to 2 NAD+ by Lactase Dehydrogenase (LDH).

Although the acid (lactic acid) can be converted back to glucose in the liver or used to produce ATP, it can result in blood becoming more acidic by reducing the pH. In the presence of oxygen, the pyruvate is normally converted to acetyl-CoA and consequently enters the Krebs cycle where it's involved in the production of additional energy. 

 

* In general, glycolysis results in the production of a total of two ATP molecules. 

 

See also:  Pentose Phosphate Pathway, Anaerobes, Glycosomes

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References

Berg JM, Tymoczko JL, and Stryer L. (2002). Glycolysis Is an Energy-Conversion Pathway in Many Organisms: Biochemistry. 5th edition.

Berg JM, Tymoczko JL, and Stryer L. (2002). Glycolysis and Gluconeogenesis. 

David A. Bender. (2014). Introduction to Nutrition and Metabolism, Fifth Edition. 

Raheel Chaudhry; Matthew Varacallo. (2019). Biochemistry, Glycolysis.

Robert A Harris and Edwin T Harper. (2001). Glycolytic Pathway. 

 

 

Links

 

https://www.news-medical.net/life-sciences/What-is-Glycolysis.aspx


https://www.sciencedirect.com/topics/neuroscience/glucose-transporter



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