Energy yield and ATP production

Glycolysis - The Net Score

Energy Investment Phase (ATP Used):

• -2 ATP total
  • Hexokinase / Glucokinase
  • Phosphofructokinase-1 (PFK-1)

Energy Payoff Phase (ATP/NADH Gained):

• +4 ATP total (via substrate-level phosphorylation)
  • Phosphoglycerate Kinase (x2)
  • Pyruvate Kinase (x2)
• +2 NADH total
  • Glyceraldehyde-3-Phosphate Dehydrogenase (x2)

Net Yield per Glucose:

• 2 ATP
• 2 NADH
• 2 Pyruvate

⭐ Arsenate inhibits glyceraldehyde-3-P dehydrogenase, preventing ATP production via substrate-level phosphorylation without stopping glycolysis, resulting in a net yield of 0 ATP.

ATP Payout - Substrate-Level Wins

• Glycolysis generates ATP directly via substrate-level phosphorylation, where a high-energy phosphate group is transferred from a substrate to ADP.
• This occurs at two key steps in the payoff phase, each happening twice per glucose molecule, yielding a total of 4 ATP.

• Phosphoglycerate Kinase

  • Transfers a phosphate from 1,3-Bisphosphoglycerate (1,3-BPG) to ADP.
  • Yields: 2 ATP (1 per 1,3-BPG).

• Pyruvate Kinase

  • Transfers a phosphate from Phosphoenolpyruvate (PEP) to ADP.
  • Yields: 2 ATP (1 per PEP).

⭐ High-Yield: Pyruvate Kinase Deficiency, an autosomal recessive disorder, causes hemolytic anemia. Mature RBCs rely exclusively on glycolysis for ATP to power ion pumps; enzyme failure leads to membrane instability and lysis.

📌 Mnemonic: The two "P" Kinases Pay you back: Phosphoglycerate Kinase & Pyruvate Kinase.

Electron Shuttles - Banking NADH's Power

• Problem: Cytosolic NADH from glycolysis cannot directly enter the mitochondria for the electron transport chain (ETC).
• Solution: Shuttles transport electrons from NADH across the inner mitochondrial membrane.

• Malate-Aspartate Shuttle
  • More efficient; regenerates mitochondrial NADH.
  • Net Yield: ~32 ATP per glucose molecule.
• Glycerol-3-Phosphate Shuttle
  • Less efficient; generates mitochondrial FADH₂.
  • Net Yield: ~30 ATP per glucose molecule.

⭐ The choice of shuttle reflects tissue-specific energy needs. The heart prioritizes maximum ATP yield (Malate-Aspartate), while skeletal muscle may prioritize rapid, albeit less efficient, ATP production during intense activity (Glycerol-3-Phosphate).

Oxygen Not Included - The Lactate Detour

• When O₂ is absent, pyruvate converts to lactate, a process crucial for regenerating $NAD^+$ from NADH, allowing glycolysis to continue.
• Enzyme: Lactate dehydrogenase (LDH).
• Net Energy Yield: 2 ATP per glucose molecule.
• Primary Sites:
  • Red blood cells (lack mitochondria).
  • Intensely exercising muscle, creating an "oxygen debt."
• Cori Cycle: Lactate is transported to the liver and converted back to glucose (gluconeogenesis), which can return to the muscles.

⭐ RBCs rely exclusively on anaerobic glycolysis for all their ATP requirements.

• Glycolysis yields a net of 2 ATP per glucose molecule via substrate-level phosphorylation.
• It also produces a net of 2 NADH, which can be used in the electron transport chain for more ATP.
• The two ATP-producing enzymes are Phosphoglycerate Kinase and Pyruvate Kinase.
• In anaerobic conditions, this is the only source of ATP, yielding a net 2 ATP.
• The rate-limiting enzyme is Phosphofructokinase-1 (PFK-1).
• Arsenic inhibits glyceraldehyde-3-phosphate dehydrogenase, leading to 0 net ATP production.