Energy-releasing Steps of Glycolysis

Glyceraldehyde 3-phosphate is oxidized by the enzyme glyceraldehyde phosphate dehydrogenase, which transfers a phosphate group to the sugar, forming 1,3-bisphosphoglycerate. During this oxidation, two electrons and a proton are released and picked up by NAD+, forming NADH. This coupled oxidation and phosphorylation releases energy that drives ATP production.

The energy-releasing phase produces four ATP molecules total. Phosphoglycerate kinase transfers a phosphate group from 1,3-bisphosphoglycerate to ADP, creating two ATP molecules. Later, pyruvate kinase removes phosphate groups from phosphoenolpyruvate and transfers them to ADP, generating two additional ATP molecules.

Phosphoglycerate mutase catalyzes the conversion of 3-phosphoglycerate into its isomer, 2-phosphoglycerate. This rearrangement of the phosphate group position prepares the molecule for the next step, where enolase removes a water molecule to form phosphoenolpyruvate, an unstable intermediate that readily donates its phosphate to ADP.

Phosphoenolpyruvate is unstable because it rapidly loses its phosphate group to ADP, forming pyruvate and ATP. This high-energy phosphate bond makes PEP an excellent substrate for pyruvate kinase. The instability ensures efficient energy capture and drives the final step of the energy-releasing phase toward completion.

Glycolysis produces a net of two ATP molecules and two NADH molecules per glucose. Although the energy-releasing phase generates four ATP and two NADH, the energy-requiring steps consume two ATP, resulting in a net gain of two ATP. The two NADH molecules can be converted back to NAD+ for further glycolysis or used in subsequent metabolic pathways.

The energy-requiring phase consumes two ATP molecules to phosphorylate glucose and fructose-6-phosphate, while the energy-releasing phase produces four ATP and two NADH through oxidation and phosphorylation reactions. The energy-releasing phase occurs twice—once for each 3-carbon sugar produced in the first phase—making it the net energy-generating portion of glycolysis.

In the presence of oxygen, pyruvate can be broken down further through pyruvate oxidation and the citric acid cycle, releasing additional ATP molecules. The NADH produced during glycolysis accumulates in the cell and can be converted back to NAD+ for continued glycolysis, or it can be used in the electron transport chain to generate more ATP through oxidative phosphorylation.