Glycolysis is the entire set of processes that the body executes automatically. As is known, man needs a lot of energy to be able to carry out all his daily activities, for this he must maintain a good diet based on vegetables, proteins, fruits and above all, have the incorporation of one of the most important energy sources, for example, glucose. Glucose enters the body through food and in different chemical forms that will later be converted into others, this happens from different metabolic processes.
What is Glycolysis
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Glycolysis represents the way in which the body initiates the breakdown of glucose molecules to obtain a substance that can provide energy to the body. This is the metabolic pathway responsible for oxidizing glucose, in order to acquire energy for the cell. It represents the most immediate way to capture this energy, in addition, it is one of the routes that is generally chosen within the carbohydrate metabolism.
Among its functions is to generate high-energy molecules NADH and ATP as the cause of the origin of cellular energy in fermentation and aerobic respiration processes.
Another function that glycolysis performs is the creation of pyruvate (a basic molecule within cellular metabolism) which passes into the cycle of cellular respiration as an element of aerobic respiration. In addition, it generates 3 and 6 carbon intermediates, which are commonly used in different cellular processes.
Glycolysis is made up of 2 stages, each one is made up of 5 reactions. Stage number 1 comprises the first five reactions, then the original glucose molecule is converted into two 3-phosphoglyceraldehyde molecules.
This stage is generally called the preparative stage, that is, it is here when glucose is divided into two molecules of 3 carbons each; incorporating two phosphoric acids (two glyceraldehyde 3 phosphate molecules). It is also possible that glycolysis occurs in plants, generally this information tends to be explained in glycolysis pdf.
Discovery of glycolysis
In 1860 the first studies related to the enzyme of glycolysis were carried out, which were elaborated by Louis Pasteur, who discovered that fermentation occurs thanks to the intervention of various microorganisms, years later, in 1897, Eduard Buchner discovered an extract cell that could cause fermentation.
In 1905 another contribution was made to the theory, as Arthur Harden and William Young determined that the cellular fractions of molecular mass are necessary for fermentation to take place, however, these masses must be high and heat sensitive, that is, they must be enzymes.
They also claimed that a cytoplasmic fraction with a low molecular mass and heat resistance is needed, that is, coenzymes of the ATP, ADP and NAD + type. There were more details that were confirmed in 1940 with the intervention of Otto Meyerhof and Luis Leloir who joined him a few years later. They had some difficulties determining the fermentation pathway, including the short life span and low concentrations of intermediates in glycolytic reactions that always ended up being rapid.
Furthermore, the glycolysis enzyme was shown to occur in the cytosol of eukaryotic and prokaryotic cells, but in plant cells, glycolytic reactions were in the calvin cycle, which occurs within chloroplasts. Phylogenetically ancient organisms are included in the conservation of this pathway, it is for them that it is considered one of the oldest metabolic pathways. Once this summary glycolysis is finished, you can talk extensively about its cycles or phases.
Glycolysis cycle
As mentioned previously, there are a series of phases or cycles in glycolysis that are of utmost importance, these are the energy expenditure phase and the energy benefit phase, which can be explained as a glycolysis scheme or simply by listing each of the glycolysis reactions. These, in turn, are broken down into 4 parts or fundamental elements that will be explained in detail below.
Energy expenditure phase
It is a phase that is responsible for transforming a glucose molecule into two glyceraldehyde molecules, however, for this to happen, 5 steps are needed, these are hexokinase, glucose-6-P isomerase, phosphofructokinase, aldolase and triose. phosphate isomerase, which will be detailed below:
- Hexokinase: in order to increase the energy of glucose, glycolysis must generate a reaction, this is the phosphorylation of glucose. Now, for this activation to take place, a reaction catalyzed by the hexokinase enzyme is needed, that is, a transfer of a phosphate group from ATP, which can be added from a phosphate group to a series of molecules that are similar to glucose, including mannose and fructose. Once this reaction occurs, it can be used in other processes, but only when necessary.
- Glucose-6-P isomerase: this is a very important step because it is here where the molecular geometry that will affect the critical phases in glycolysis is defined, the first is the one that adds the phosphate group to the reaction product, the second is when the two glyceraldehyde molecules are going to be created, which, finally, will be the precursors of pyruvate. Glucose 6 phosphate is isomerized to fructose 6 phosphate in this reaction and this is done through the enzyme glucose 6 phosphate isomerase.
- Phosphofructokinase: in this process of glycolysis, the phosphorylation of fructose 6 phosphate is carried out at carbon 1, in addition, the expenditure of an ATP is carried out through the enzyme phosphofructokinase 1, better known as PFK1.
Due to all the above, phosphate has a low hydrolysis energy and an irreversible process, finally obtaining a product called fructose 1,6 bisphosphate. The irreversible quality is imperative because it makes it a control point for glycolysis, that is why it is placed in this and not in the first reaction, because there are other substrates apart from glucose that manage to enter glycolysis.
- Aldolase: this enzyme manages to break fructose 1,6 bisphosphate into two 3-carbon molecules called trioses, these molecules are called dihydroxyacetone phosphate and glyceraldehyde 3 phosphate. This break is made thanks to an aldol condensation which, by the way, is reversible.
This reaction has as its main characteristic a free energy of between 20 and 25 Kj / mol and this does not occur under normal conditions, even less spontaneously, but when it comes to intracellular conditions, the free energy is small, this is because there is a low concentration of substrates and it is precisely this that makes the reaction reversible.
- Triose phosphate isomerase: in this glycolysis process, there is a standard and positive free energy, this generates a process that is not favored, but generates a negative free energy, this makes the formation of G3P in the favored direction. In addition, it must be taken into account that the only one that can follow the remaining steps of glycolysis is glyceraldehyde 3 phosphate, so the other molecule generated by the dihydroxyacetone phosphate reaction is converted into glyceraldehyde 3 phosphate.
There are two advantages in the phosphorylation of glucose, the first is based on making glucose become a reactive metabolic agent, the second is that it is achieved that glucose 6 phosphate cannot cross the cell membrane, very different from glucose Since it has a negative charge provided by the phosphate group to the molecule, in this way, it makes it more difficult to cross. All this prevents the cell's energetic substrate from being lost.
Furthermore, fructose has allosteric centers that are sensitive to concentrations of intermediates such as fatty acids and citrate. In this reaction, the enzyme phosphofructokinase 2 is released, which is responsible for phosphorylating at carbon 2 and regulating it.
In this step, only ATP is consumed in the first and third step, in addition, it should be remembered in the fourth step, a molecule of glyceraldehyde-3-phosphate is generated, but in this reaction, a second molecule is generated. With this it should be understood that, from there, all the following reactions occur twice, this is due to 2 glyceraldehyde molecules generated from that same phase.
Energy benefit phase
While ATP energy is consumed in the first phase, in this phase, glyceraldehyde becomes a molecule with more energy, so finally a final benefit is obtained: 4 ATP molecules. Each of the glycolysis reactions is explained in this section:
- Glyceraldehyde-3-phosphate dehydrogenase: in this reaction, glyceraldehyde -3-phosphate is oxidized using NAD +, only then can a phosphate ion be added to the molecule, which is carried out by the enzyme glyceraldehyde 3-phosphate dehydrogenase in 5 steps, in this way, increases the total energy of the compound.
- Phosphoglycerate kinase: in this reaction, the enzyme phosphoglycerate kinase manages to transfer the phosphate group of 1,3 bisphosphoglycerate to an ADP molecule, this generates the first ATP molecule in the energy benefits pathway. Because glucose is transformed into two glyceraldehyde molecules, 2 ATP is recovered in this phase.
- Phosphoglycerate mutase: what happens in this reaction is the change in the position of phosphate C3 to C2, both are very similar and reversible energies with variations in free energy that is close to zero. Here the 3 phosphoglycerate obtained from the previous reaction is converted to 2 phosphoglycerate, however, the enzyme that catalyzes this reaction is phosphoglycerate mutase.
- Enolase: this enzyme gives the formation of a double bond in 2 phosphoglycerate, this causes a water molecule that had been formed by hydrogen from C2 and OH from C3 to be eliminated, thus resulting in phosphoenolpyruvate.
- Pyruvate kinase: here the dephosphorylation of phosphoenolpyruvate takes place, it is then that the enzyme pyruvate and ATP are obtained, an irreversible reaction that occurs from pyruvate kinase (an enzyme that, by the way, is dependent on potassium and magnesium.
Products of glycolysis
Because the metabolic direction of the intermediates in the reactions depends on the cellular needs, each intermediary can be considered as products of the reactions, then, each product would be (in order according to the reactions previously explained) as follows:
- Glucose 6 phosphate
- Fructose 6 phosphate
- Fructose 1,6 bisphosphate
- Dihydroxyacetone phosphate
- Glyceraldehyde 3 phosphate
- 1,3 bisphosphoglycerate
- 3 phosphoglycerate
- 2 phosphoglycerate
- Phosphoenolpyruvate
- Pyruvate
Gluconeogenesis
It is an anabolic path in which glycogen synthesis occurs through a simple precursor, this is glucose 6 phosphate. Glycogenesis occurs in the liver and muscle, but occurs to a lesser extent in the latter. It is activated through insulin in response to high glucose levels, which can occur after eating foods that contain carbohydrates.
The gluconeogenesis is created by incorporating repeated glucose units, which come in the form of UDP-glucose to a splitter glycogen that previously existed and that is based on the glycogenin proteins, which is formed by two chains autoglicosilan and that, in addition, they can link their chains to an octamer of glucose.
