Biological organisms require energy to survive. Glycolysis is one of the pathways cells use to transform sugars like glucose into biochemical energy in the form of ATP.
In the cytosol of the cell, glycolysis converts glucose into pyruvate through a series of 10 enzymatic reactions. This process produces ATP, along with other products such as NADH, that can be used later to produce even more ATP for the cell. These enzymes oxidize one glucose molecule into two pyruvate molecules.
First, a kinase reaction adds a phosphate onto glucose to form glucose-6-phosphate. This is one of two energy consumption steps and is an irreversible reaction.
Next, an isomerase reaction converts glucose-6-phosphate into fructose-6-phosphate by rearranging covalent bonds.
Another kinase removes a phosphate group from ATP and gives it to fructose-6-phosphate to form fructose-1,6-bisphosphate. This is the second energy consumption step and is an irreversible reaction.
In the fourth step of glycolysis, a lyase reaction splits the 6-carbon fructose-1,6-bisphosphate into two 3-carbon sugars, glyceraldehyde-3-phosphate and Dihydroxyacetone phosphate. Another isomerase reconfigures the Di hydroxyacetone phosphate to form a second glyceraldehyde-3-phosphate. At this point in glycolysis, glucose has been metabolized into two glyceraldehyde-3-phosphates and two ATP have been consumed.
The next five steps of glycolysis are the energy-producing phase.
In step six, both glyceraldehyde-3-phosphates are oxidized to 1,3-bisphosphoglycerate by a dehydrogenase. This step produces one NADH for each oxidized glyceraldehyde-3-phosphate, for a total of two NADH. This NADH is later used to produce more ATP for the cell.
In step seven, a kinase transfers a phosphate from 1,3-bisphosphoglycerate to ADP to form ATP and 3-phosphoglycerate. This step is reversible, even though ATP is formed.
The next step involves a mutase reaction that moves the phosphate on the third carbon of 3-phosphoglycerate to the second carbon position to form 2-phosphoglycerate.
In step nine, a lyase reaction removes water from 2-phosphoglycerate to form phosphoenolpyruvate.
In the final step of glycolysis, a kinase reaction removes the phosphate group from phosphoenolpyruvate and donates it to ADP to form ATP and pyruvate. This step is irreversible. At this point, two pyruvate molecules, four ATP, and two NADH are formed for each glucose that was broken down in glycolysis. Pyruvates and NADH could be used to produce more energy for the cell during aerobic respiration.
Here we depict glycolysis as a closed process. But in cells, substrates produced by other reactions can enter glycolysis at different points. For example, when an animal breaks down glycogen, glucose 6-phosphate is produced and can then enter the glycolysis pathway at the second step.
Importantly, this means one less ATP is required for the pathway because the first ATP-consuming step is skipped.
Other sugars can also enter the glycolysis pathway at different points, each having a different effect on the net number of ATP that are produced by glycolysis. These ATP are important energy molecules required for many biochemical pathways and, ultimately, life itself.
Glycolysis is a major contributor to the pool of ATP used in these pathways, which are essential to the survival of biological organisms.
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