Glucose Oxidation: Steps, Equation & Products

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Glucose oxidation breaks down glucose to create energy. In this lesson we will learn the overall outcome of glucose oxidation, what the net products are, and the individual steps.

Overall Reaction and Equation

Many things that you eat (from candy to bread to broccoli) contains glucose. Glucose is one of the main forms of energy that our body uses. This energy is created by oxidizing the glucose in a process called glycolysis.

There are 10 steps in glycolysis, named based upon the enzymes used in the reaction:

  1. Hexokinase
  2. Isomerase
  3. Phosphofructokinase
  4. Aldolase
  5. Trisphosphate isomerase
  6. Glyceraldehyde phosphate dehydrogenase
  7. Phosphoglycerate kinase
  8. Phosphoglycerate mutase
  9. Enolase
  10. Pyruvate Kinase

Overall glycolysis has 10 steps
Overall glycolysis

Overall the equation for glycolysis is:

Glucose + 2 NAD + 2 ADP + 2P → 2 pyruvic acid + 2 ATP + 2 NADH + 2H

So, each molecule of glucose creates 2 pyruvic acid molecules. It also creates 2 ATP and 2NADH molecules (energy). Now, really 4 ATP molecules are created, but the process also uses up 2 ATP molecules, so the net ATP created is 2.

Glycolysis Steps, Stage 1

Glycolysis is broken into two parts or stages, the investment stage and the harvesting stage. The investment stage is steps one through four and is the stage where ATP is required. This stage is aptly named the investment stage, because we haven't got any energy from glucose yet, but we need to first invest a little energy into the oxidation of glucose in order to then get energy from the oxidation of glucose.

Step 1, Hexokinase

Hexokinase uses an ATP molecules to put a phosphate onto glucose, creating Glucose-6-phosphate. This step is irreversible, meaning that we can't simply use the hexokinase enzyme to remove the phosphate from glucose. This step does not require much energy, so the phosphate can easily be added onto the glucose. In fact it typically happens almost as soon as glucose enters the cell.

Step 1
Step 1

Step 2, Isomerase

The isomerase step isomerizes the glucose molecule into a fructose molecule. But, since the glucose already has a phosphate on it the actual reaction changes glucose-6-phosphate into fructose-6-phosphate. We are simply turning the aldose sugar into the keto form. This reaction is reversible, so in order to ensure that the reaction moves forward the fructose-6-phosphate is quickly used up so that it cannot revert back into glucose-6-phosphate.

Step 2
Step 2

Step 3, Phosphofructokinase

Phosphofructokinase adds another phosphorus onto the molecule, which means it uses another ATP creating Fructose-1,6-biphosphate. It is irreversible and readily happens (as mentioned in step 2 it needs to happen quickly so that the fructose-6-phosphate doesn't simply change back into glucose-6-phosphate). This step is also called the commitment step, because prior products can be used from other processes other than glycolysis, but once fructose-1,6-biphosphate has been created the products can only be used for glycolysis.

Step 3
Step 3

Step 4, Aldolase

The aldolase step breaks the molecule into two, three carbon molecules. The fructose-1,6-biphosphate is broken into glyceraldehyde-3-phosphate (an aldehyde) and dihydroxyacetone phosphate (a ketone). This step is reversible.

Step 4
Step 4

Glycolysis Steps, Stage 2

The second stage of glycolysis includes steps five through ten and is called the harvesting stage, in the harvesting stage ATP and NADH is created. This is the stage where we actually get energy from the oxidation of glucose.

Step 5, Trisphosphate isomerase

Trisphosphate isomerase is similar to a backwards step 2, in that we are changing the keto form of the molecule into the aldehyde form of the molecule. Recall that in step 4 we created two products, an aldehyde and a ketone. The ketone cannot move forward in the process until it is converted into the aldehyde. This step converts the dihydroxyacetone phosphate into glyceraldehyde-3-phosphate. So, we actually end up creating two molecules of glyceraldehyde-3-phosphate. This step is reversible. Since we now have 2 molecules of glyceraldehyde-3-phosphate all of the remaining steps actually occurs twice.

Step 5
Step 5

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