Glycolysis Payoff Phase

Glycolysis Payoff Phase

The video explains the final five reactions of glycolysis, known as the payoff phase. The two glyceraldehyde-3-phosphate (GAP) molecules produced earlier are each converted into pyruvate. These reactions produce NADH and ATP.

Because each reaction occurs twice per glucose molecule, the payoff phase yields 2 NADH and 4 ATP. Since 2 ATP were used in the investment phase, glycolysis has a net gain of 2 ATP per glucose, along with 2 NADH and 2 pyruvate.

The Five Reactions

1. GAP is oxidized and phosphorylated to 1,3-bisphosphoglycerate (1,3-BPG).

   • The enzyme is glyceraldehyde-3-phosphate dehydrogenase.
   • Inorganic phosphate is added; the reaction does not use ATP for this phosphate.
   • GAP is oxidized, while NAD⁺ is reduced to NADH by accepting a hydride ion (hydrogen with two electrons).
   • The enzyme’s active-site histidine and cysteine help form a temporary enzyme-bound intermediate. Phosphate then attacks this intermediate, releasing 1,3-BPG and restoring the active site.
   • This is the redox step in the payoff phase.

2. 1,3-BPG transfers a phosphate to ADP, making ATP and 3-phosphoglycerate.

   • The enzyme is phosphoglycerate kinase.
   • Despite the enzyme’s name, in glycolysis it transfers a phosphate from 1,3-BPG to ADP.
   • This is substrate-level phosphorylation: ATP is made directly by transferring a phosphate from a high-energy intermediate to ADP.
   • Since there are two 1,3-BPG molecules per glucose, this step produces 2 ATP total.

3. The phosphate moves from the third carbon to the second carbon.

   • Phosphoglycerate mutase converts 3-phosphoglycerate to 2-phosphoglycerate.
   • A phosphorylated histidine in the enzyme temporarily donates a phosphate to the substrate, forming a two-phosphate intermediate. The enzyme then removes the phosphate from the third carbon.
   • This leaves the phosphate on the second carbon and regenerates the enzyme’s active-site components.

4. 2-phosphoglycerate loses water to form phosphoenolpyruvate (PEP).

   • The enzyme is enolase.
   • This elimination reaction removes a water molecule and creates PEP, a high-energy phosphate-transfer compound.

5. PEP transfers its phosphate to ADP, making ATP and pyruvate.

   • The enzyme is pyruvate kinase.
   • Phosphate transfer to ADP produces ATP, while the remaining molecule becomes pyruvate, an alpha-keto acid.
   • This is another example of substrate-level phosphorylation.
   • With two PEP molecules per glucose, this step produces 2 more ATP.

Key Takeaway

The payoff phase converts the two GAP molecules into two pyruvate molecules. It generates 2 NADH and 4 ATP gross. After accounting for the 2 ATP spent earlier in glycolysis, the pathway’s net ATP gain is 2 ATP per glucose.

Speakers and Sources

• One unnamed instructor/narrator from Sparks Chemistry.
• The instructor refers to earlier videos in the glycolysis series, an earlier discussion of chymotrypsin, and a later discussion of ATP formation through the electron transport chain. These are references, not additional speakers featured in the subtitles.