Glycolysis: The Reactions
Glycolysis is a metabolic pathway involving 10 enzymatic reactions that oxidize glucose, a six-carbon sugar, into two three-carbon molecules known as pyruvate.
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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.
Let’s watch as 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 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.
The dihydroxyacetone phosphate is rearranged by another isomerase to form a second glyceraldehyde-3-phosphate. At this point in glycolysis, glucose has been metabolized into two glyceraldehyde-3-phosphates, and two ATPs 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 NADHs. These NADHs are 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. Like reactions one and three, this step is irreversible.
At this point, two pyruvate molecules, four ATPs, and two NADHs are formed for each glucose that was broken down in glycolysis. The pyruvates and NADHs could be used in aerobic respiration to produce more energy for the cell.
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 ATPs that are produced by glycolysis.
These ATPs 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, pathways that are essential to the survival of biological organisms.
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