The glycolysis process includes 10 steps. Among them 7 are reversible and three steps are irreversible.
What are the three irreversible steps of glycolysis
The three irreversible steps are the 1st, 3rd, and 10th steps.
01: Phosphorylation of glucose :
In the first step of the glycolysis process, D-glucose is turned into glucose-6-phosphate using ATP as a phosphate donor in the reaction. Here, glucose is phosphorylated in the process of glycolysis. Phosphorylation is the process through which a phosphate group is added to a molecule which is derived from ATP. Hexokinase requires Mg or any other metal ions from its activity, just like all other kinases. Atoms of Mg^2+ helps to shield the negative charges produced from the phosphate groups from the ATP molecule.
In the reaction of phosphorylation, ATP turns into ADP with the help of an enzyme which is known as “hexokinase”. It works in many eukaryotic cells and prokaryotic cells, and an additional enzyme “glucokinase” found in the liver is also involved in this reaction. The function of glucokinase is to remove glucose from the blood, to store glucose as glycogen in the liver for further purposes to be used in the process.
“Hexokinase” is an enzyme that catalyzes different ring structures like glucose 6 carbon. In the presence of this particular enzyme, the reaction is proceeded and facilitated in the required way. In the reaction, energy loss is happening in the form of heat.
Thus, glucose-6-phosphate is formed in the process. Glucose-6-phosphate is an important component of the junction of many metabolic pathways.
03: Phosphorylation of fructose 6-phosphate :
This step is considered as the committed step in the process of glycolysis due to its reaction and it’s a contribution to metabolism rather than storing glucose and then transforming it into any other compound. The produced product in the above step, fructose-6-phosphate, again goes through reaction to transform itself into fructose 1,6-biphosphate. Here, phosphofructokinase (one of the complex known enzyme) helps to change it with co-factor magnesium. This reaction is irreversible in various cellular conditions.
This step is very much similar to the first step where ATP is turned into ADP and Phospholyration is involved too. Here, a phosphate group is added to the carbon. Like, in step 1, magnesium is involved to shield negative charges. That, helps the compound to change from fructose-6-phosphate to fructose 1,6-biphosphate. An enzyme, named “photofructokinases” catalyzes the reaction to facilitate its activities. The reaction that the enzyme facilitates is irreversible. The mechanism of the enzyme photofructokinases accelerates when the cell lacks ATP. And, the mechanism decelerates when there is excess/enough ATP. Thus the affinity of the enzyme is lowered for the process. This is a more important reaction since it helps in metabolism rather than storing or converting glucose into another form.
10: Transfer of phosphate from PEP to ADP :
This is the last step of glycolysis. In this process, ATP is regenerated, just like step 7. In this step and the reaction, Phosphoenolpyruvate is turned or converted into pyruvate, the required product. The reaction is facilitated with the help of an enzyme which is known as pyruvate kinase. This is one of the physiologically irreversible reactions in the process of glycolysis.
In the first step of this reaction, Phosphoenolpyruvate reacts with ADP in presence of catalysts like pyruvate kinase and it’s co-factor Magnesium (a metal ion) and thus the reaction produces the final product known as pyruvate and ATP.
The reaction proceeds when enol pyruvate changes its arrangement at a quick pace and thus takes a new form. The new form is known as ketopyruvate. The pyruvate at first comes in the enol form then, after the reaction, it transforms itself into keto form.
The reaction stands, Phosphoenolpyruvate in addition to ADP and also hydrogen, in presence of pyruvate kinase and metal ions like Mg, Zn, etc produces the required product ketopyruvate. Here, ADP is transformed and converted into ATP. The reaction that is facilitated above is just another example of the substrate-level phosphorylation of glycolysis. This is the transfer of Phosphoryl group from Phosphoenolpyruvate to ADP.
Pyruvate kinase, the enzyme, can be of three major different forms; M, L and A type which can be found on different parts of the body of different organisms. These come in a lot of help in different ways in the process of glycolysis. The reaction produces a lot of free energy which is released as heat later.
Now in aerobic glycolysis, Oxidation happens when pyruvate goes to the citric acid cycle.
Whereas, when in anaerobic glycolysis, lactate dehydrogenase converts pyruvate to lactate.
10 steps of glycolysis where three steps are irreversible:
See the figure to have a clear understanding or you can read the following article.
Fig: 10 steps of glycolysis.
Irreversible steps 1, 3, and 10 are indicated in the yellow color arrow.