Glycolysis Steps

Glycolysis is a vital metabolic pathway that breaks down glucose to produce energy. This series of ten steps transforms glucose into pyruvate, generating ATP and NADH, which are essential for cellular functions and energy production in living organisms.

1. Glucose phosphorylation by hexokinase

   • Hexokinase catalyzes the conversion of glucose to glucose-6-phosphate.
   • This reaction requires ATP, which donates a phosphate group.
   • It is the first step of glycolysis and is crucial for trapping glucose inside the cell.

2. Isomerization of glucose-6-phosphate to fructose-6-phosphate

   • The enzyme phosphoglucose isomerase converts glucose-6-phosphate into fructose-6-phosphate.
   • This step rearranges the molecular structure, preparing it for further phosphorylation.
   • It is a reversible reaction, allowing for metabolic flexibility.

3. Phosphorylation of fructose-6-phosphate by phosphofructokinase

   • Phosphofructokinase (PFK) adds a second phosphate group to fructose-6-phosphate, forming fructose-1,6-bisphosphate.
   • This is a key regulatory step and is considered the "committed" step of glycolysis.
   • ATP is used in this reaction, and PFK is allosterically regulated by various metabolites.

4. Cleavage of fructose-1,6-bisphosphate into two triose phosphates

   • Aldolase cleaves fructose-1,6-bisphosphate into two three-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
   • This step is essential for continuing the glycolytic pathway.
   • DHAP can be converted to G3P, ensuring both molecules can enter the next steps.

5. Oxidation of glyceraldehyde-3-phosphate and phosphorylation

   • Glyceraldehyde-3-phosphate dehydrogenase oxidizes G3P, reducing NAD+ to NADH.
   • A phosphate group is added, forming 1,3-bisphosphoglycerate.
   • This step is crucial for energy capture in the form of NADH.

6. Substrate-level phosphorylation producing 1,3-bisphosphoglycerate

   • The high-energy compound 1,3-bisphosphoglycerate is formed, which contains a high-energy acyl phosphate bond.
   • This step prepares the substrate for the next phosphorylation reaction.
   • It is a key point for energy conservation in glycolysis.

7. Transfer of phosphate group to ADP, forming 3-phosphoglycerate

   • Phosphoglycerate kinase catalyzes the transfer of a phosphate from 1,3-bisphosphoglycerate to ADP, producing ATP and 3-phosphoglycerate.
   • This is an example of substrate-level phosphorylation, generating ATP directly.
   • It marks the first ATP generation in glycolysis.

8. Isomerization of 3-phosphoglycerate to 2-phosphoglycerate

   • The enzyme phosphoglycerate mutase converts 3-phosphoglycerate into 2-phosphoglycerate.
   • This rearrangement is necessary for the subsequent dehydration step.
   • It is a reversible reaction, allowing for metabolic flexibility.

9. Dehydration of 2-phosphoglycerate to phosphoenolpyruvate

   • Enolase catalyzes the removal of a water molecule from 2-phosphoglycerate, forming phosphoenolpyruvate (PEP).
   • PEP has a high-energy phosphate bond, making it a key substrate for ATP production.
   • This step is crucial for the final ATP-generating reaction in glycolysis.

10. Transfer of phosphate from phosphoenolpyruvate to ADP, forming pyruvate

    • Pyruvate kinase catalyzes the transfer of a phosphate group from PEP to ADP, producing ATP and pyruvate.
    • This is the final step of glycolysis and results in the net gain of ATP.
    • Pyruvate can enter the citric acid cycle or be converted to lactate, depending on oxygen availability.