Glycolysis: what is it and what are its 10 phases?

Glycolysis is a chemical process that allows respiration and cellular metabolism, specifically through the breakdown of glucose.

In this article we will see in more detail what glycolysis is and what it is for, as well as its 10 phases of action.

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What is glycolysis?

The term "glycolysis" is made up of the Greek "glycos" which means "sugar", and "lysis" which means "breakdown". In this sense, glycolysis is the process by which the composition of glucose is modified to extract sufficient energy for the benefit of cells. In fact, it not only acts as a source of energy, but also affects cell activity in different ways, without necessarily generating additional energy.

For example, it produces a high yield of the molecules that allow metabolism and cellular respiration both aerobic and anaerobic. Broadly speaking, aerobic is a type of metabolism that consists of extracting energy from organic molecules from the oxidation of carbon by oxygen. In anaerobic the element used to achieve oxidation is not oxygen but sulfate or nitrate.

At the same time, glucose is an organic molecule composed of a 6-ring membrane found in the blood, and that is generally the result of the transformation of carbohydrates into sugars. In order to enter cells, glucose travels through the proteins responsible for transporting it from outside from the cell to the cytosol (intracellular fluid that is, the fluid found in the center of the cells).

Through glycolysis, glucose is converted into an acid called "pivuric" or "pyruvate" that plays a very important role in biochemical activity. This process occurs in the cytoplasm (the part of the cell that lies between the nucleus and the membrane). But for glucose to become pyruvate, a very complex chemical mechanism composed of different phases must occur.

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Its 10 phases

Glycolysis is a process that has been studied since the second decade of the 19th century, when chemists Louis Pasteur, Eduard Buchner, Arthur Harden, and William Young began to detail the mechanism of the fermentation. These studies allowed to know the development and different forms of reaction in the composition of the molecules.

It is one of the oldest cellular mechanisms, and it is likewise the fastest way to get energy and metabolize carbohydrates. For this, it is necessary for 10 different chemical reactions to occur, divided into two large phases. The first one consists of expending energy by transforming the glucose molecule into two different molecules; while the second phase is obtaining energy by transforming the two molecules generated in the previous stage.

That said, we will see the 10 phases of glycolysis below.

1. Hexokinase

The first step in glycolysis is to convert the D-glucose molecule into a glucose-6-phosphate molecule (a glucose molecule phosphorylated at carbon 6). To generate this reaction, an enzyme known as Hexokinase must participate, and it has the function of activating glucose so that it can be used in subsequent processes.

2. Phosphoglucose isomerase (Glucose-6 P isomerase)

The second reaction of glycolysis is the transformation of glucose-6-phosphate into fructose-6-phosphate. For it an enzyme called phosphoglucose isomerase must act. This is the phase of definition of the molecular composition that will allow to consolidate the glycolysis in the two stages that follow.

3. Phosphofructokinase

In this phase, fructose-6-phosphate is converted to fructose 1,6-bisphosphate, through the action of phosphofructokinase and magnesium. It is an irreversible phase, which causes glycolysis to begin to stabilize.

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4. Aldolasse

Now fructose 1,6-bisphosphate is divided into two isomeric sugars, that is, two molecules with the same formula, but whose atoms are ordered differently, thus also having different properties. The two sugars are dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP), and the division occurs due to the activity of the enzyme aldolase.

5. Triphosphate isomerase

Phase number 5 consists of reserving the glyceraldehyde phosphate for the next stage of glycolysis. For this, an enzyme called triphosphate isomerase must act within the two sugars obtained in the previous stage (dihydroxyacetone phosphate and glyceraldehyde 3-phosphate). This is where the first of the great stages that we describe at the beginning of this numbering ends, whose function is to generate energy expenditure.

6. Glyceraldehyde-3-phosphate Dehydrogenase

In this phase the obtaining of energy begins (during the previous 5 it had only been spent). We continue with the two sugars generated previously and their activity is as follows: produce 1,3-bisphosphoglycerate, by adding an inorganic phosphate to glyceraldehyde 3-phosphate.

In order to add this phosphate, the other molecule (glyceraldehyde-3-phosphate dehydrogenase) must be dehydrogenated. This means that the energy of the compound begins to increase.

7. Phosphoglycerate kinase

In this phase there is another transfer of a phosphate, to be able to form adenosine triphosphate and 3-phosphoglycerate. It is the 1,3-bisphosphoglycerate molecule that receives a phosphate group from phosphoglycerate kinase.

8. Phosphoglycerate mutase

From the above reaction 3-phosphoglycerate was obtained. Now it is necessary to generate 2-phosphoglycerate, through the action of an enzyme called phosphoglycerate mutase. The latter relocates the position of the phosphate from the third carbon (C3) to the second carbon (C2), and thus the expected molecule is obtained.

9. Enolase

An enzyme called enolase is responsible for removing the water molecule from 2-phosphoglycerate. In this way the precursor of pyruvic acid is obtained and we are nearing the end of the glycolysis process. This precursor is phosphoenolpyruvate.

10. Pyruvate kinase

Finally, a transfer of phosphorus from phosphoenolpyruvate to adenosine diphosphate occurs. This reaction occurs by the action of the enzyme pyruvate kinase, and allows glucose to finish transforming into pyruvic acid.

Bibliographic references:

• Glycolysis-10 steps explained steps by steps with diagram (2018). MicrobiologyInfo.com. Retrieved September 26, 2018. Available in https://microbiologyinfo.com/glycolysis-10-steps-explained-steps-by-steps-with-diagram/.