Glycolysis Lecture Notes

Introduction to Glycolysis

• Definition: Glycolysis is the process of splitting glucose, a six-carbon sugar, into two three-carbon molecules of pyruvate.
• Energy Capture: During glycolysis, energy is captured in the form of ATP and NADH.

Net Reaction of Glycolysis

• Location: Takes place in the cytosol of the cell.
• Reactants:
  • 1 Glucose
  • 2 NAD+ molecules
  • 2 ADP + 2 Inorganic phosphate ions
• Products:
  • 2 Pyruvate molecules
  • 2 NADH molecules
  • 2 ATP molecules
  • 2 H+ ions
  • 2 Water molecules
• Type of Reaction: NAD+ is reduced to NADH.

Phases of Glycolysis

• Investment Phase (Steps 1-5):
  • Energy-consuming phase.
  • 2 ATP molecules are used.
• Payoff Phase (Steps 6-10):
  • Energy-producing phase.
  • Produces 4 ATP and 2 NADH, net gain of 2 ATP and 2 NADH.

Detailed Steps of Glycolysis

Step 1: Phosphorylation of Glucose

• Substrate: Glucose
• Product: Glucose-6-phosphate (G6P)
• Enzyme: Hexokinase
• Cofactor: Magnesium ion (Mg2+)
• Energy: Irreversible reaction (ΔG' = -16.7 kJ/mol)

Step 2: Isomerization to Fructose-6-phosphate

• Substrate: Glucose-6-phosphate
• Product: Fructose-6-phosphate (F6P)
• Enzyme: Phosphohexose isomerase
• Energy: Reversible reaction (ΔG = +1.7 kJ/mol)

Step 3: Phosphorylation to Fructose-1,6-bisphosphate

• Substrate: Fructose-6-phosphate
• Product: Fructose-1,6-bisphosphate
• Enzyme: Phosphofructokinase-1 (PFK-1)
• Cofactor: Mg2+
• Energy: Irreversible reaction (ΔG' = -14.2 kJ/mol)

Step 4: Cleavage into G3P and DHAP

• Substrate: Fructose-1,6-bisphosphate
• Products: Glyceraldehyde-3-phosphate (G3P) and Dihydroxyacetone phosphate (DHAP)
• Enzyme: Aldolase
• Energy: Reversible reaction (ΔG = +23.8 kJ/mol)

Step 5: Isomerization of DHAP to G3P

• Substrate: Dihydroxyacetone phosphate
• Product: Glyceraldehyde-3-phosphate
• Enzyme: Triose phosphate isomerase
• Energy: Reversible reaction

Payoff Phase

Step 6: Oxidation and Phosphorylation of G3P

• Substrate: Glyceraldehyde-3-phosphate
• Product: 1,3-Bisphosphoglycerate
• Enzyme: Glyceraldehyde 3-phosphate dehydrogenase
• Cofactor: NAD+ (reduced to NADH)

Step 7: Phosphate Transfer to ADP

• Substrate: 1,3-Bisphosphoglycerate
• Product: 3-Phosphoglycerate
• Enzyme: Phosphoglycerate kinase
• Energy: Produces 2 ATP

Step 8: Isomerization to 2-Phosphoglycerate

• Substrate: 3-Phosphoglycerate
• Product: 2-Phosphoglycerate
• Enzyme: Phosphoglycerate mutase

Step 9: Formation of Phosphoenolpyruvate

• Substrate: 2-Phosphoglycerate
• Product: Phosphoenolpyruvate
• Enzyme: Enolase
• Energy: Produces 2 H2O

Step 10: Conversion to Pyruvate

• Substrate: Phosphoenolpyruvate
• Product: Pyruvate
• Enzyme: Pyruvate kinase
• Energy: Produces 2 ATP

Summary

• Irreversible Steps: 1, 3, and 10
• Net Gains: 2 ATP and 2 NADH per glucose molecule

Additional Information

• Cofactors: Magnesium and Potassium ions required for certain steps.
• Purpose: Detailed steps provide a comprehensive understanding of glycolysis and the biochemical rationale behind it.