Glycolysis: Intermediates & Enzymes (Know in Order)

I. Energy Investment Phase

  • 1. Glucose

    • Enzyme: Hexokinase / Glucokinase
    • Conversion: Glucose → Glucose-6-phosphate (G6P)

  • 2. Glucose-6-phosphate (G6P)

    • Enzyme: Phosphoglucose isomerase
    • Conversion: G6P → Fructose-6-phosphate (F6P)

  • 3. Fructose-6-phosphate (F6P)

    • Enzyme: PFK-1 (Phosphofructokinase-1)
    • Conversion: F6P → Fructose-1,6-bisphosphate (F1,6BP)
    • ATP Used: 2

  • 4. Fructose-1,6-bisphosphate (F1,6BP)

  • Note: ATP used in this phase totals to 2.

II. Cleavage Phase

  • 5. DHAP + GAP

    • Enzyme: Aldolase

  • 6. GAP (Dihydroxyacetone phosphate only)

    • Pathway: DHAP → GAP via Triose phosphate isomerase
    • Note: Everything downstream occurs twice per glucose molecule.

III. Energy Payoff Phase

  • 7. 1,3-Bisphosphoglycerate (1,3-BPG)

  • 8. 3-Phosphoglycerate (3-PG)

  • 9. 2-Phosphoglycerate (2-PG)

  • 10. Phosphoenolpyruvate (PEP)

  • 11. Pyruvate

    • ATP Produced: 4
    • NADH Produced: 2

IV. Net Yield (Per Glucose)

  • ATP: +2 (total ATP produced = 4, total ATP used = 2)
  • NADH: +2
  • Note: Pyruvate produced: 2.

Chemical Logic of Glycolysis (Why the Steps Work)

  • Why phosphorylate glucose?

    • Traps glucose inside the cell.
    • Destabilizes the molecule, making it reactive.

  • Why PFK-1 is the committed step?

    • First irreversible step unique to glycolysis.
    • Commits carbon to energy production.
    • Major regulatory point in the pathway.

  • Why does aldolase cleavage work?

    • Two phosphates stabilize carbanion intermediates.
    • Carbonyl chemistry enables carbon-carbon cleavage.

  • Why does GAPDH produce NADH?

    • Oxidation releases energy during conversion.
    • Energy conserved as NADH and high-energy acyl phosphate.

  • Why does PEP have such high energy?

    • Enol → keto tautomerization of pyruvate.
    • Makes ATP formation essentially irreversible.

Energy Implications (What Matters for Exams)

  • Glycolysis does not require O₂ (anaerobic process).
  • ATP made via substrate-level phosphorylation.
  • NADH must be reoxidized; otherwise, glycolysis stops.
  • The overall pathway is strongly exergonic, indicating that the reactions release energy.

Regulation of Glycolysis (Very High Yield)

PFK-1 Regulation

  • Activated by:
    • AMP
    • ADP
    • Fructose-2,6-bisphosphate
  • Inhibited by:
    • ATP
    • Citrate

Hormonal Control

  • Insulin → Increases Fructose-2,6-bisphosphate → Increases glycolysis.
  • Glucagon → Decreases Fructose-2,6-bisphosphate → Increases gluconeogenesis.

Other Regulatory Enzymes

  • Hexokinase: inhibited by G6P (product inhibition).
  • Pyruvate kinase: inhibited by ATP and alanine (indicates excess energy).

Energy Concepts (Thermodynamics)

  • ΔG′°: Standard biochemical free energy, a constant for specific reactions.
  • ΔG: Actual free energy; depends on substrate concentrations.
  • Cells manipulate ΔG by controlling metabolite levels.

Irreversible Steps in Glycolysis

  • Hexokinase
  • PFK-1
  • Pyruvate kinase

Gluconeogenesis (Opposite of Glycolysis)

Purpose

  • To maintain blood glucose levels.
  • Supplies glucose to the brain and red blood cells (RBCs).

Bypass Enzymes (MEMORIZE)

  • Pyruvate → PEP:

    • Enzymes: Pyruvate carboxylase, PEP carboxykinase.

  • F1,6BP → F6P:

    • Enzyme: Fructose-1,6-bisphosphatase.

  • G6P → Glucose:

    • Enzyme: Glucose-6-phosphatase.

Why Malate Shuttle is Needed

  • Oxaloacetate cannot cross the mitochondrial membrane directly.
  • Malate transports carbon and NADH into the cytosol for gluconeogenesis.

Energy Cost of Gluconeogenesis

  • Costs: 4 ATP, 2 GTP, and 2 NADH.

Feeder Pathways into Glycolysis

  • Glycogen:

    • Enters as G6P and saves 1 ATP compared to glucose.

  • Pentose Phosphate Pathway:

    • Feeds into F6P and GAP, and provides NADPH for biosynthesis.

  • Fructose:

    • Enters as F6P or triose phosphates.

  • Galactose:

    • Converted to G6P, thus feeding into glycolysis.

Fates of Pyruvate (MEMORIZE CONDITIONS)

  • 1. Pyruvate → Acetyl-CoA:

    • Condition: Aerobic
    • Pathway: Enters TCA cycle (Krebs cycle).

  • 2. Pyruvate → Lactate:

    • Condition: Anaerobic
    • Function: Regenerates NAD⁺ for glycolysis to continue.

  • 3. Pyruvate → Oxaloacetate:

    • Function: In gluconeogenesis (anaplerotic reaction)

  • 4. Pyruvate → Alanine:

    • Function: Amino acid synthesis and nitrogen transport.