Medicine & USMLE

Glycolysis - Payoff Phase

  1. Citric Acid Cycle (TCA, Krebs)
  2. Glycolysis - Investment Phase
  3. Glycolysis - Payoff Phase
  4. Pentose Phosphate Pathway - Oxidative Phase
  5. Pentose Phosphate Pathway - Non-Oxidative Phase
  6. Glycogenesis
  7. Glycogenolysis
  8. Gluconeogenesis
  9. Electron Transport Chain (ETC)


The payoff phase is the second phase of glycolysis, producing an energy ‘payoff’ in the form of the high-energy molecules ATP and NADH.

For every molecule of glucose that enters glycolysis, two G3P molecules enter the payoff phase. This G3P is catalyzed by glyceraldehyde 3-phosphate dehydrogenase to form the high-energy intermediate 1,3-BPG, as well as an NADH. 1,3-BPG is then catalyzed by phosphoglycerate kinase to form 3-PG. This reaction also produces ATP through substrate-level phosphorylation. The 3PG then gets catalyzed by PGM to form 2PG. Enolase then catalyzes the conversion of 2PG to PEP, in the process removing water. Finally, pyruvate kinase irreversibly converts PEP to pyruvate, in the process forming our final ATP. Since this reaction is irreversible, it serves an important regulatory point in the pathway.

It’s also crucial to keep in mind that every molecule of glucose gives you two G3P molecules, so we produce two of every product– four ATP, two NADH, and two pyruvate! But since we invested two ATP already in the investment phase, this comes out to a net total of two ATP, two NADH, and two pyruvate molecules.

Key Points

  • Glycolysis - Payoff Phase
    • Etymology 
      • Glyco- = glucose = sugar
      • -lysis = to cut/split
    • Summary
      • Glycolysis is a metabolic pathway breaks down glucose to produce energy
        • Other carbohydrates are metabolized by the pathway as well
      • Occurs in the cytosol of the cell
        • part of anaerobic respiration
          • Can function with or without the presence of oxygen
        • Ancient pathway
          • highly conserved across different forms of life
          • Present in aerobic and anaerobic organisms
      • two phases
        • Investment phase
          • 2 ATP are ‘invested’ into glucose metabolism
        • Payoff phase
          • 4 ATP and 2 NADH are produced
        • Net payoff: 2 ATP and 2 NADH (per glucose)
    • Reaction Steps
      • Glyceraldehyde-3-phosphate (G3P) + Pi → [glyceraldehyde-3-phosphate dehydrogenase] 1,3-Bisphosphoglycerate
        • For every 1 molecule of glucose that enters glycolysis, 2 G3P enter the payoff phase
          • Matter is conserved
            • Glucose has  a 6 carbon chain and G3P has a 3 carbon chain
        • Reduces NAD+ to NADH
          • Redox reaction: G3P is oxidized
        • Redox reaction is exergonic, fueling the endergonic phosphorylation of G3P
        • Product 1,3-bisphosphoglyercate is unstable
      • 1,3-Bisphosphoglycerate + ADP → [phosphoglycerate kinase] 3-phosphoglycerate + ATP
        • Forms 1 ATP
          • substrate-level phosphorylation
          • energy from the exergonic redox reaction that was ‘stored’ in the phosphate bond formation is now used to transfer a phosphate group to ATP
      • 3-phosphoglycerate → [Mutase] 2-phosphoglycerate
        • Mutase is an isomerase (isomerization reaction)
        • This isomerization reaction makes the molecule less stable (think higher energy)
      • 2-phosphoglycerate → [Enolase] Phosphoenolpyruvate
      • Phosphoenolpyruvate + ADP → [Pyruvate Kinase] Pyruvate + ATP
        • Forms 1 ATP 
          • substrate-level phosphorylation
        • Pyruvate is the final product of glycolysis
          • has multiple possible metabolic fates, including TCA cycle
        • Irreversible
    • Regulation
      • Regulation is based on energy need and blood glucose levels
      • Pyruvate Kinase reaction is a regulated step
        • PFK is still the overall rate-limiting step for glycolysis
      • Glycolysis speeds up (activation)
        • High blood glucose levels
          • Insulin
            • Covalent modification
              • Triggers cascade that dephosphorylates pyruvate kinase
            • Gene expression
              • Insulin → upregulation
          • F1,6BP (feed forward activation)
      • Glycolysis slows down (inhibition)
        • Low blood glucose levels
          • Glucagon
            • Covalent modification
              • Triggers cascade that phosphorylates pyruvate kinase
            • Gene expression
              • Glucagon → downregulation
          • Alanine
        • Low energy need
          • ATP