Glucose is the most preferred source of energy for many cells. A common pathway for the catabolism of glucose is glycolysis, which breaks down glucose into pyruvate. Glycolysis is also called Embden-Meyerhof-Parnas (EMP) pathway for its major discoverer.
Glycolysis is the pathway that breaks down glucose into two pyruvates (product) and produces 4 ATP (net gain of 2 ATP) and 2 NADH (nicotinamide adenosine dinucleotide+hydrogen) in the process. The pathway occurs in the cytoplasm of the cell.
Glycolysis can take place in the presence (oxidation) and absence of oxygen (fermentation). In the presence of oxygen, the pyruvate is changed into Acetyl CoA. It enters the citric acid (TCA or Krebs) cycle, whereas, in anaerobic conditions, pyruvate is converted into lactate or ethanol.
The general reaction of glycolysis is;
Glucose+ 2 ATP+ 2 NAD → 2 Pyruvate + 4 ATP + 2 NADH
Enzymes of Glycolysis
There are ten different enzymes involved in glycolysis. These are listed below with their roles in each step:
- Hexokinase: It activates glycolysis by phosphorylating glucose. This is the rate-limiting step in glucose metabolism.
- Phosphohexose isomerase: It changes glucose-6-phosphate to its isomer fructose-6-phosphate.
- Phosphofructokinase: It adds another phosphate group to fructose-6-phosphate with the help of ATP (Adenosine Triphosphate).
- Aldolase: Breaks down the six-carbon fructose-6-phosphate to 3 carbon dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP).
- Isomerase: This is another enzyme that converts dihydroxyacetone phosphate to its isomer glyceraldehyde-3-phosphate.
- Glyceraldehyde-3-phosphate dehydrogenase: It has two uses; it dehydrogenates GAP and adds phosphate group into GAP and converts it into 1,3-biphosphoglycerate.
- Phosphoglycerate kinase: Helps transfer one phosphate molecule into ADP (adenosine diphosphate).
- Phosphoglycerate mutase: Helps change phosphate’s position from third to second; that is, 3-phosphoglycerate is transformed into 2-phosphoglycerate.
- Enolase: Helps remove a molecule of water from 2-phosphoglycerate to form phosphoenolpyruvate (PEP).
- Pyruvate kinase: It works in transferring phosphate of phosphoenolpyruvate to form pyruvate.
Note: Isomers are those compounds that have the same molecular formula but different atomic arrangements.
Did you know: The suffix “-ase” is used to signify an enzymes.
Steps of Glycolysis
The process of glycolysis has the following steps:
- Firstly, the glucose (reactant) is converted into glucose-6-phosphate (G-6-P). The hexokinase enzyme is involved in this step, and an ATP molecule is utilized and converted into ADP. The conversion of ATP to ADP generates energy, and a phosphate molecule that is necessary for the conversion of glucose to G-6-P. That is to say, it is one of the regulatory steps of glycolysis.
- Then, the G-6-P is changed into fructose-6-phosphate, an isomer of G-6-P in the presence of phosphohexose isomerase.
- Thirdly, the conversion of fructose-6-phosphate to fructose-1,6-biphosphate occurs with the help of phosphofructokinase. The phosphate molecule of ATP is added to the F-6-P. This step is another regulatory step.
- Similarly, the fructose-1,6-biphosphate now breaks down into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate due to the involvement of the enzyme aldolase.
- In the presence of enzyme isomerase, the dihydroxyacetone phosphate is changed into glyceraldehyde-3-phosphate.
- The glyceraldehyde-3-phosphate is converted into 1,3-biphosphoglycerate in the presence of NAD, enzyme glyceraldehyde-3-phosphate dehydrogenase, and two molecules of phosphate.
- The 1,3-biphosphoglycerate changes into 3-phosphoglycerate with the help of phosphoglycerate kinase and two molecules of ATP are generated in this process.
- With the help of phosphoglycerate mutase and Magnesium ion, 3-phosphoglycerate changes into 2-phosphoglycerate.
- Phosphoenolpyruvate is produced by the change in 2-phosphoglycerate with the help of enzyme enolase.
- Finally, this step is the generation of pyruvate by conversion into phosphoenolpyruvate in the presence of the enzyme pyruvate kinase, and 2 ATP molecule is also developed in this step. At last , it is the final regulatory step of glycolysis.
In anaerobic conditions, pyruvate converts into lactate in the presence of lactate dehydrogenase.
In aerobic conditions, the pyruvate converts into Acetyl CoA, which enters Krebs cycle (TCA or Citric Acid Cycle) in the presence of the pyruvate dehydrogenase complex (PDC).
Products of Glycolysis
The final products of glycolysis are:
- Pyruvate: A molecule of glucose converts into two molecules of pyruvate.
- ATP: A total of four molecules of ATP are produced during glycolysis. Since two molecules of ATP are utilized in the reaction, the net of ATP produced is two.
- NADH+H+: Two molecules of NADH+H+ are produced as a result of glycolysis.
Regulation of Glycolysis
There are three enzymes of the glycolysis that regulate the pathway:
- Glucose-6-phosphate formation is the first step of glycolysis. The enzyme involved here is hexokinase. It is inhibited if the amount of product (glucose-6 phosphate) is high in the cell.
- Phosphofructokinase is an essential regulatory enzyme of glycolysis. The energy change of the cell regulates it. ATP inhibits the reaction, whereas AMP (adenosine monophosphate) activates the third step. So glycolysis is activated when energy is in shortage. In addition, the product of the third step (fructose-1,6-biphosphate) also plays a role in regulating this enzyme. And finally, citrate (from the TCA cycle) inhibits the enzyme. The amount of citrate in the cell determines the need for energy.
- The final regulatory enzyme is pyruvate kinase. It is inhibited by ATP, acetyl-Coenzyme A, and fatty acids. Pyruvate kinase is activated by fructose-1,6-biphosphate (an example of feed-forward activation).
References and Further Reading
- Bailey, R. (2022). Learn About The 10 Steps of Glycolysis. ThoughtCo. Retrieved 2 May 2022, from https://www.thoughtco.com/steps-of-glycolysis-373394.
- Nelson, D., Lehninger, A., Cox, M., & Nelson, D. (2005). Lecture notebook for Lehninger principles of biochemistry, fourth edition (pp. 522-535). W.H. Freeman.
- (2022). Retrieved 3 May 2022, from https://www.cliffsnotes.com/study-guides/biology/biochemistry-i/glycolysis/glycolysis-regulation.