The Energy-Investing Phase of Glycolysis
Step 1. The enzyme hexokinase is used to phosphorylate glucose using ATP. Phosphorylation is the addition of a phosphate group to a molecule in a chemical reaction. In this case, ATP is the source of the phosphate. Hexokinase causes the phosphorylation reaction of glucose to produce glucose 6-phosphate.
Step 2. Phosphoglucose isomerase is the enzyme used to convert glucose 6-phosphate to fructose 6-phosphate. Fructose 6-phosphate is an isomer (a molecule with the same molecular formula as another molecule but with a different structure) of glucose 6-phosphate. An isomerase is an enzyme that drives the conversion of a molecule into its isomer form.
Step 3. Phosphofructokinase is the enzyme that uses ATP to phosphorylate the molecule on one end of fructose 6-phosphate. Here phosphofructokinase is a rate-limiting enzyme. There is less activity from this enzyme when ADP levels are low and the concentration of ATP is high. It is more active when ADP concentration is high. When there is sufficient ATP in the system, this step of the pathway slows down. At this point along the glycolysis pathway, two ATP molecules have been used, resulting the production of fructose 1,6-bisphosphate.
Step 4. Aldolase breaks fructose 1,6-bisphosphate into two different 3-carbon sugars. These 3-carbon sugars are dihydroxyacetone-phosphate (DAP) and glyceraldehyde 3-phosphate (G3P).
Step 5. Triose phosphate isomerase converts DAP to G3P, which is an isomer of DAP. This reaction is fully reversible but proceeds in this direction because the G3P is immediately used as a reactant for Step 6, the beginning of the energy-harvesting phase.
The Energy-Harvesting Phase of Glycolysis
Step 6. Glyceraldehyde 3-phosphate dehydrogenase catalyzes a two-step reaction. The first reaction oxidizes G3P with the coenzyme NAD+ to make NADH. Energy is produced from this reaction to power the second reaction where a phosphate group is oxidized to form 1,3-bisphosphoglycerate.
Step 7. Phosphoglycerate kinase catalyzes the transfer of a phosphate from 1,3-bisphosphoglycerate to ADP. This phosphorylation reaction is done to make ATP and the product 3-phosphoglycerate.
Step 8. Phosphoglycerate mutase rearranges the phosphate in 3-phosphoglycerate to make 2-phosphoglycerate.
Step 9. Enolase removes a water molecule from 2-phosphoglycerate. This hydrolysis reaction, the process by which the water molecule is removed, results in the formation of a double bond between two carbon atoms. Phosphoenolpyruvate is also produced.
Step 10. Pyruvate kinase catalyzes the transfer of a phosphate from phosphoenolpyruvate to ADP. This results in the formation of pyruvate and ATP.