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Glycolysis Detailed Notes

Overview of Glycolysis Steps

  • Glycolysis is a metabolic pathway that converts glucose into pyruvate, producing ATP and NADH in the process.

Key Terms and Enzymes

  • Cofactor: Magnesium is an important cofactor involved in glycolysis.
  • Phosphoglucoisomerase: An enzyme that isomerizes glucose-6-phosphate into fructose-6-phosphate.
  • Aldolase: The enzyme catalyzes the cleavage of fructose-1,6-bisphosphate to yield two 3-carbon products.
  • Triose phosphate isomerase (TPI): Converts dihydroxyacetone phosphate (DHAP) into glyceraldehyde-3-phosphate (GAP).
  • Kinases: Enzymes that transfer phosphate groups.

Major Steps in Glycolysis

Step 1

  • Glucose is phosphorylated to glucose-6-phosphate (G6P) by using ATP.
  • Reaction: Glucose + ATP → G6P + ADP
    • This reaction consumes one ATP.

Step 2

  • G6P is isomerized to fructose-6-phosphate (F6P).
  • This is catalyzed by phosphoglucoisomerase.

Step 3

  • F6P is phosphorylated again to fructose-1,6-bisphosphate (F1,6BP) using another ATP.
  • Reaction: F6P + ATP → F1,6BP + ADP
    • Consumes another ATP.

Step 4 (Aldolase Reaction)

  • The enzyme aldolase cleaves F1,6BP into:
    • Dihydroxyacetone phosphate (DHAP).
    • Glyceraldehyde-3-phosphate (GAP).
  • Both products are 3-carbon units.

Step 5

  • DHAP is converted to GAP by triose phosphate isomerase.
  • This ensures that glycolysis continues with two GAP molecules.

Payoff Phase of Glycolysis

Step 6

  • GAP undergoes oxidation, and NAD+ is reduced to NADH, producing 1,3-bisphosphoglycerate (BPG).
  • Reaction: GAP + NAD+ + Pi → BPG + NADH + H+
    • This does not consume ATP, but produces NADH.

Step 7

  • BPG donates a phosphate to ADP to generate ATP. This is substrate-level phosphorylation.
  • Reaction: BPG + ADP → 3-phosphoglycerate + ATP
    • This produces one ATP.

Step 8

  • 3-phosphoglycerate (3PG) is converted to 2-phosphoglycerate (2PG).
  • Enzyme: Phosphoglycerate mutase; this reaction does not produce ATP.

Step 9

  • Enolase catalyzes the dehydration of 2PG to form phosphoenolpyruvate (PEP).
  • This is a non-hydrolytic dehydration reaction, losing water as a byproduct.

Step 10

  • PEP donates a phosphate to ADP, producing pyruvate and another ATP.
  • Overall Reaction: PEP + ADP → Pyruvate + ATP
    • Results in the final ATP yield.

Summary of ATP Production

  • Glycolysis consumes 2 ATP in the investment phase.
  • Produces a net of 4 ATP in the payoff phase, yielding a net gain of 2 ATP per glucose molecule.
  • Additionally, 2 NADH are produced, which can enter the oxidative phosphorylation stage for further ATP production.

Energetics of Glycolysis

  • The pathway conserves energy in the form of ATP and NADH, which can be used in cellular respiration for ATP synthesis in aerobic conditions.