Glycolysis Lecture Notes

Introduction to Glycolysis

• Definition: Glycolysis is the process of splitting a glucose molecule.
  • Glyco: Sugar
  • Lysis: To split apart
• Glucose: A 6-carbon molecule that splits into two 3-carbon molecules of pyruvate.
• Energy Capture: Energy from glucose converting to pyruvate is captured as ATP and NADH.

Net Reaction of Glycolysis

• Takes place in the cytosol of the cell.
• Reactants: 1 glucose, 2 NAD⁺, 2 ADP, 2 inorganic phosphate ions.
• Products: 2 pyruvate, 2 NADH, 2 ATP, 2 H⁺, 2 H₂O.
• NAD⁺ to NADH: Reduction process (adding hydrogen).

Phases and Steps of Glycolysis

Phase 1: Investment Phase (Steps 1-5)

• Concept: Invest ATP to get more ATP later (endothermic).
• Energy Input: 2 ATP molecules.

Step 1: Glucose to Glucose 6-Phosphate (G6P)

• Process: Phosphorylation (adding a phosphate group).
• Enzyme: Hexokinase (assisted by Mg²⁺).
• Irreversibility: Large negative ΔG (ΔG' = -16.7 kJ/mol).

Step 2: G6P to Fructose 6-Phosphate (F6P)

• Isomerization: Conversion to a different isomer.
• Enzyme: Phosphohexose isomerase (assisted by Mg²⁺).
• Reversibility: ΔG' = 1.7 kJ/mol (reversible).

Step 3: F6P to Fructose 1,6-Bisphosphate (F1,6BP)

• Phosphorylation: Addition of another phosphate group.
• Enzyme: Phosphofructokinase-1 (PFK-1).
• Irreversibility: ΔG' = -14.2 kJ/mol.

Step 4: Cleavage of F1,6BP

• Products: Glyceraldehyde 3-Phosphate (G3P) and Dihydroxyacetone Phosphate (DHAP).
• Enzyme: Aldolase.
• Reversibility: ΔG' = 23.8 kJ/mol (reversible under cellular conditions).

Step 5: DHAP to G3P

• Isomerization: Conversion of DHAP to G3P.
• Enzyme: Triose phosphate isomerase.

Phase 2: Payoff Phase (Steps 6-10)

• Concept: Generate more ATP than invested.
• Net Gain: 2 ATP and 2 NADH.

Step 6: G3P to 1,3-Bisphosphoglycerate (1,3BPG)

• Oxidation: G3P is oxidized; NAD⁺ reduced to NADH.
• Enzyme: Glyceraldehyde 3-phosphate dehydrogenase.

Step 7: 1,3BPG to 3-Phosphoglycerate (3PG)

• ATP Formation: Substrate-level phosphorylation.
• Enzyme: Phosphoglycerate kinase.

Step 8: 3PG to 2-Phosphoglycerate (2PG)

• Mutase Reaction: Movement of phosphate group.
• Enzyme: Phosphoglycerate mutase.

Step 9: 2PG to Phosphoenolpyruvate (PEP)

• Dehydration: Water molecule removed.
• Enzyme: Enolase.

Step 10: PEP to Pyruvate

• ATP Formation: Final substrate-level phosphorylation.
• Enzyme: Pyruvate kinase.
• Cofactors: Mg²⁺ and K⁺ ions.

Overview of Irreversible Steps

• Steps 1, 3, and 10 are irreversible.
• Other steps are reversible, based on ΔG values.

Conclusion

• Glycolysis is a crucial metabolic pathway for energy production.
• Involves investment and payoff phases, leading to net gain of ATP and NADH.