Glycolysis: what it is and reactions

What is glycolysis

Glycolysis or glycolysis is a series of chemical reactions that cells use to obtain energy from the breakdown of glucose. It is also called the Embden-Meyerhof road, after the scientists who discovered it.

Glucose enters the cell by crossing the cell membrane, and glycolysis takes place in the cytoplasm, both in prokaryotic and eukaryotic cells. It is a catabolic pathway, because the glucose molecule is broken down to produce energy in the form of ATP (adenosine triphosphate).

In the glycolytic process, glucose oxidation occurs without the participation of oxygen. Glycolysis consists of 10 chemical reactions, which transform a six-carbon compound, glucose, into two three-carbon molecules, pyruvate.

Simplified scheme of glycolysis. NAD+: oxidized nicotinamide adenine dinucleotide; NADH: reduced nicotinamide adenine dinucleotide; ADP: adenosine diphosphate; ATP: adenosine triphosphate.

The general equation for glycolysis is as follows:

This means that one molecule of glucose plus 2 NAD +, plus 2 ADP and 2 phosphate groups results in 2 molecules of pyruvate, plus 2 NADH, 2 H+ protons, 2 ATP molecules and 2 water molecules.

In glycolysis, the following are consumed:

1 molecule of glucose.2 molecules of adenosine triphosphate (ATP).4 molecules of adenosine diphosphate (ADP)2 molecules of oxidized nicotinamide adenine dinucleotide (NAD+).2 phosphates (PO4).

In glycolysis the following are produced:

4 molecules of ATP, 2 molecules of reduced nicotinamide adenine dinucleotide (NADH). 2 molecules of pyruvate.

Pyruvate can then pass into the Krebs cycle in aerobic respiration or fermentation, ATP will be used in other cellular reactions, and NADH can pass into the mitochondria to produce more ATP.

See also:

Glycolysis reactions

Step 1 Glucose phosphorylation: In this reaction, the enzyme glucokinase (or hexokinase in some tissues) promotes the addition of a phosphate group from ATP to carbon 6 of glucose, forming glucose 6-phosphate and ADP.

Step 2 Conversion of glucose 6-phosphate: The enzyme phosphohexose isomerase transforms glucose 6-phosphate into fructose 6-phosphate.

Step 3 Fructose 6-phosphate phosphorylation: Phosphofructokinase transfers a phosphate group from ATP to carbon 1 of fructose 6-phosphate, forming fructose 1,6-bisphosphate and ADP.

Step 4 Breakdown of fructose 1,6-bisphosphate: Fructose 1,6-bisphosphate is broken down into two different three-carbon molecules, dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, thanks to aldolase.

Step 5 Conversion of dihydroxyacetone phosphate: to continue the glycolysis process, dihydroxyacetone phosphate has to be transformed into glyceraldehyde 3-phosphate. From this step, there are now two molecules of glyceraldehyde 3-phosphate that continue along with the following reactions.

Step 6 Phosphorylation and oxidation of glyceraldehyde 3-phosphate: Glyceraldehyde 3-phosphate is oxidized by losing two hydrogens, which pass to NAD+, forming NADH + H+. At the same time, it gains a phosphate group from inorganic phosphate (PO43-). The enzyme that catalyzes this step is Glyceraldehyde phosphate dehydrogenase and the product that continues in the next step is 1,3-bisphosphoglycerate.

Step 7 ATP Formation: 1,3-bisphosphoglycerate is a molecule with a high energy level as it has two phosphate groups. A phosphate group is transferred to adenosine diphosphate (ADP) to form the first ATP of the glycolysis process by the action of Phosphoglycerate kinase.

Step 8 Change of place of phosphate group: 3-phosphoglycerate has its phosphate group on carbon 3, so that phosphate group must be moved to carbon 2. This is done by the enzyme Phosphoglycerate mutase and the product is 2-phosphoglycerate.

Step 9 Dehydration of 2-phosphoglycerate: Enolase is the enzyme that catalyzes the loss of a water molecule from 2-phosphoglycerate to form phosphoenolpyruvate, which has a higher potential to transfer phosphate group.

Step 10 ATP Formation: This is the last step of glycolysis, where pyruvate kinase transfers the phosphate group of phosphoenolpyruvate to an ADP, to form ATP and pyruvate.

In the following table we summarize the ten steps of glycolysis with their enzymes and products.

Step Reactants Enzyme Products 1 Glucose + ATP Glucokinase
(hexokinase) Glucose 6-phosphate + ADP 2 Glucose 6-phosphate Phosphohexose isomerase Fructose 6-phosphate 3 Fructose 6-phosphate + ATP Phosphofructokinase Fructose 1,6-bisphosphate 4 Fructose 1,6-bisphosphate Aldolase

Dihydroxyacetone phosphate

Glyceraldehyde 3-phosphate

5 Dihydroxyacetone phosphate Triose phosphate isomerase Glyceraldehyde 3-phosphate 6 Glyceraldehyde 3-phosphate + NAD+ Glyceraldehyde 3-phosphate dehydrogenase 1,3-bisphosphoglycerate +NADH + H+ 7 1,3-biphosphoglycerate + ADP Phosphoglycerate kinase 3-phosphoglycerate + ATP 8 3- phosphoglycerate Phosphoglycerate mutase 2-phosphoglycerate 9 2-phosphoglycerate Enolase Phosphoenolpyruvate + H2O 10 Phosphoenolpyruvate + ADP Pyruvate kinase Pyruvate + ATP

References

Chaudhry, R.,Varacallo, M. (2021) Biochemistry, Glycolysis. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Available at: https://www.ncbi.nlm.nih.gov/books/NBK482303/

Nelson, DL, Cox, MM, Hoskins, AA (2021) Lehninger Principles of Biochemistry. 8th ed. Macmillan Learning. Boston.