What is glycolysis?
Highlights of glycolysis
- Energy-requiring phase. In this phase, the starting molecule of glucose gets rearranged, and two phosphate groups are attached to it. The phosphate groups make the modified sugar—now called fructose-1,6-bisphosphate—unstable, allowing it to split in half and form two phosphate-bearing three-carbon sugars. Because the phosphates used in these steps come from , two molecules get used up.
- Energy-releasing phase. In this phase, each three-carbon sugar is converted into another three-carbon molecule, pyruvate, through a series of reactions. In these reactions, two molecules and one molecule are made. Because this phase takes place twice, once for each of the two three-carbon sugars, it makes four and two overall.
Detailed steps: Energy-requiring phase
- Glucose is converted to glucose-6-phosphate by hexokinase. This step converts one ATP to ADP.
- Glucose-6-phosphate is converted into fructose-6-phosphate by phosphoglucose isomerase.
- Fructose-6-phosphate is converted into fructose-1,6-bisphosphate by phosphofructokinase. This step converts one ATP to ADP.
- Fructose-1,6-bisphosphate is split into two three-carbon molecules, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (DHAP). The enzyme that catalyzes this step is fructose bisphosphate aldolase.
- The DHAP is converted into glyceralde-3-phosphate by an enzyme called triose phosphate isomerase. This reaction can go in either direction, but because glyceraldehyde-3-phosphate is continually being used up in the rest of the pathway, the equilibrium favors conversion of DHAP to glyceraldehyde-3-phosphate.
Detailed steps: Energy-releasing phase
- Glyceraldehyde-3-phosphate is converted into 1,3-bisphosphoglycerate. This is a redox reaction in which NAD+ is converted to NADH (with the release of an H+ ion). An inorganic phosphate is also a reactant for this reaction, which is catalyzed by glyceraldehyde-3-phosphate dehydrogenase.
- 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by phosphoglycerate kinase. This step converts an ADP to an ATP.
- 3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycerate mutase.
- 2-phosphoglycerate is converted to phosphoenolpyruvate (PEP) by enolase. This reaction releases a water molecule.
- Phosphoenolpyruvate (PEP) is converted to pyruvate by pyruvate kinase. An ADP is converted to an ATP in this reaction.