8.3
Energy-releasing Steps of Glycolysis
Glycolysis is divided into two phases based on whether energy is utilized or released. While the first phase consumes ATP, the second phase produces e…
The second phase of glycolysis begins with the oxidation of two molecules of glyceraldehyde 3-phosphate. It is catalyzed by the enzyme glyceraldehyde phosphate dehydrogenase, which transfers a phosphate group to the sugar, forming 1, 3- bisphosphoglycerate.
Two electrons and a proton released during the reaction are picked up by NAD plus, forming NADH.
Phosphoglycerate kinase then transfers a phosphate group from each 1,3- bisphosphoglycerate to ADP, creating two molecules of ATP and 3-phosphoglycerate.
Next, the enzyme phosphoglycerate mutase converts 3-phosphoglycerate into its isomer 2-phosphoglycerate.
Further, the enzyme enolase removes a water molecule from each 2-phosphoglycerate to form two molecules of phosphoenolpyruvate or PEP.
With the help of pyruvate kinase, the phosphate groups are removed from PEP and taken up by ADP, generating two more molecules of ATP, along with the end product, pyruvate.
In this way, at the end of the energy releasing phase of glycolysis– four ATPs, two NADHs, and two pyruvate molecules are produced.
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Q1: What happens during the oxidation of glyceraldehyde 3-phosphate in 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.
Q2: How many ATP molecules are produced in the energy-releasing phase of glycolysis?
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.
Q3: What is the role of phosphoglycerate mutase in glycolysis?
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.
Q4: Why is phosphoenolpyruvate considered an unstable molecule?
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.
Q5: What is the net energy yield from one glucose molecule after glycolysis?
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.
Q6: How does the energy-releasing phase differ from the energy-requiring phase of glycolysis?
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.
Q7: What happens to pyruvate after glycolysis in the presence of oxygen?
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.