Glycolysis

The biochemical breakdown of glucose.

Glycolysis

Glycolysis is the initial pathway of metabolism. Glycolysis is an anaerobic process, meaning this pathway does not require oxygen. As a result, this pathway occurs in both prokaryotic organisms and eukaryotic organisms. In glycolysis, the simple sugar glucose is transformed into several substances that can be used in other metabolic pathways. The primary goal of metabolism is to produce ATP, which the human body uses to produce its energy.

KEY POINTS

  • Glycolysis starts with glucose and produces pyruvate
  • Glycolysis costs 2 ATP to start
  • Glycolysis produces 4 ATP
  • The NET gain of ATP is 2 ATP
  • Energy in the form of ATP is input at step 1 and step 3



Glycolysis Occurs in 10 Different Steps

The glycolysis pathway is extremely complex. The first step begins with glucose. Because the pathway occurs in steps, at each of the steps exists an intermediate. The intermediate products of glycolysis enable other metabolic pathways to insert compounds into glycolysis. In this way, the first step of glycolysis is not necessarily always the first step.

Take note of what is involved in each step of glycolysis. Also, keep in mind the enzymes that are required for the completion of each step.

Step 1: Glucose is converted into Glucose 6-Phosphate

Glucose is converted into Glucose 6-phosphate through the use of the enzyme hexokinase (glucokinase). This is the very first step in glycolysis. However, it is reversible, meaning that Glucose 6-phosphate can be converted back to glucose via gluconeogenesis. The first step of glycolysis requires the input of one ATP molecule.

The first step in glycolysis is the rate-limiting step. This step relies on the concentration of ATP available to continue in the subsequent reactions. If there is more ATP than ADP, the reaction rate will decrease. Alternatively, if there is more ADP than ATP, then the reaction rate will increase.

Step 2: Glucose 6-Phosphate is converted into Fructose 6-Phosphate

Glucose 6-phosphate is converted to Fructose 6-phosphate using the enzyme Phosphoglucose isomerase.

Step 3: Fructose 6-Phosphate is converted into Fructose 1,6-Bisphosphate

In the third step of glycolysis, Fructose 6-phosphate is converted into Fructose 1,6-Bisphosphate using the enzyme Phosphofructinase. This is the committed step of glycolysis. The committed step is the step of the pathway which controls its overall efficiency. The third step is the second and last step that requires input of energy in the form of ATP.

Step 4: Fructose 1,6-Bisphosphate is converted into DHAP and/or Glyceraldehyde 3-Phosphate

The fourth step of glycolysis is the fork in the road. In this step, Fructose 1,6-Bisphosphate is converted into DHAP and/or Glyceraldehyde 3-Phosphate via the enzyme aldolase.

Step 5: DHAP is converted into Glyceraldehyde 3-Phosphate

DHAP can be converted to Glyceraldehyde 3-Phosphate and vice versa. These two molecules are potential inputs from products of the metabolism of fructose. However, in a forward-moving progression, more DHAP is converted to Glyceraldehyde 3-phosphate than the alternative.



Step 6: Glyceraldehyde 3-Phosphate is converted into 1,3-Bisphosphoglycerate

In the sixth step, Glyceraldehyde 3-Phosphate is converted into 1,3-Bisphosphoglycerate using the enzyme Glyceraldehyde Phosphate Dehydrogenase. In this step, an electron is removed and added to NAD+, resulting in NAD+ becoming NADH.

Step 7: 1,3-Bisphosphoglycerate is converted into 3-Phosphoglycerate

In this step 1,3-Bisphosphoglycerate is converted into 3-Phosphoglycerate via the enzyme Phosphoglycerate Kinase. In this step, two ADP are converted to two ATP. Also, note that 2 molecules of 3-phosphoglycerate are produced.

Step 8: 3-Phosphoglycerate is converted into 2-Phosphoglycerate

In the eighth step of glycolysis, 2 molecules of 3-Phosphoglycerate are converted into 2 molecules of 2-Phosphoglycerate using the enzyme Phosphoglyceromutase.

Step 9: 2-Phosphoglycerate is converted into Phosphoenolpyruvate

In step 9, the 2 molecules of 2-Phosphoglycerate are converted into 2 molecules of Phosphoenolpyruvate via the enzyme enolase and water.

Step 10: Phosphoenolpyruvate is converted into Pyruvate

In the last step of glycolysis, 2 molecules of Phosphoenolpyruvate are converted into 2 molecules of Pyruvate. In this step, 2 more ADP are converted to ATP, yielding a total of 4 ATP molecules.

Biochemical Steps of Glycolysis

Glycolysis for the MCAT

For the MCAT, it is unlikely that you will need to remember the chemical composition of each of the intermediate products of glycolysis. However, knowing the structure and compositions of each intermediate may help you get a better understanding of the underlying mechanism.

On the other hand, you should be able to recite the key points mentioned above. Also, memorization of the steps, especially the rate-limiting steps, will help you prepare for medical school. Once in medical school, you will see and will need to be able to memorize the basics of this metabolic process.

To study the mechanism of glycolysis, try rewriting the steps in order. Then, go back and add the enzymes necessary for each step. Lastly, be able to label the rate-limiting and committed steps of the glycolytic pathway. Know the steps where electrons and phosphate groups are transferred. Once you have learned all of the basic metabolic pathways, be able to determine the steps in which all of the different pathways meet.

See also: Enzymes and Metabolism.



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