Carbohydrate Metabolism and Energy Production

Aug 21, 2024

Tutorial 28: Carbohydrate Metabolism and Glycolysis

Overview of Carbohydrate Metabolism

  • Carbohydrates from the diet are digested, breaking glycosidic bonds in polysaccharides and disaccharides to free monosaccharides.
  • Main monosaccharide from digestion is glucose.
  • Glucose (a 6-carbon molecule) undergoes catabolism through glycolysis to form two 3-carbon molecules of pyruvate.

Glycolysis

  • Location: Occurs in the cytosol of cells.
  • Process: 10-step metabolic pathway.
    • Steps 1-5: Glucose is cleaved into two 3-carbon molecules (D-glyceraldehyde 3-phosphate).
    • Steps 6-10: Converts D-glyceraldehyde 3-phosphate into pyruvate twice.

Detailed Steps

  1. Step 1: Glucose is phosphorylated using ATP.
  2. Step 2: Isomerization of glucose 6-phosphate to fructose 6-phosphate.
  3. Step 3: Second phosphorylation using another ATP.
  4. Step 4: Cleavage into two 3-carbon isomers, dihydroxyacetone phosphate, and D-glyceraldehyde 3-phosphate.
  5. Step 5: Isomerization of dihydroxyacetone phosphate into D-glyceraldehyde 3-phosphate.
  6. Steps 6-10: Occur twice due to two D-glyceraldehyde 3-phosphate molecules.
    • Step 6: Oxidation and phosphorylation without ATP.
    • Step 7: Transfer phosphate to ADP, forming ATP.
    • Step 8: Isomerization.
    • Step 9: Dehydration forming a double bond.
    • Step 10: Transfer of phosphate forming ATP and yielding pyruvate.

ATP Balance

  • ATP Investment: -2 ATP (first half)
  • ATP Yield: +4 ATP (second half, 2 ATP per D-glyceraldehyde 3-phosphate)
  • Net Gain: +2 ATP
  • NADH Production: 2 NADH (reoxidized in ETC to produce ATP)

Pyruvate Oxidation

  • Process: Pyruvate (3-carbon) converts to acetyl CoA (2-carbon) and CO2.
  • Coenzyme Involvement: NAD+ reduced to NADH.
  • Anaerobic Conditions: Pyruvate reduces to lactate maintaining glycolysis.

Krebs Cycle

  • Entry: Acetyl CoA enters cycle.
  • Products:
    • 3 NADH, 1 FADH2, 1 GTP per cycle turn.
  • ATP Calculation:
    • Each NADH produces 3 ATP.
    • FADH2 produces 2 ATP.
    • GTP counted as ATP.
  • Total Yield Per Glucose: 2 turns per glucose molecule leading to 24 ATP.

Total ATP Yield from Glucose

  • Glycolysis: +8 ATP
  • Pyruvate Oxidation: +6 ATP
  • Krebs Cycle: +24 ATP
  • Total: 38 ATP per glucose.

Alternative ATP Counting

  • Max yields: 3 ATP per NADH, 2 ATP per FADH2.
  • More actual yields: 2.5 ATP per NADH, 1.5 ATP per FADH2 (may result in 32 ATP total).

Conclusion

  • Ensure consistency with textbook or instructor's method for ATP yield calculation.

Note

  • Focus on glycolysis and pyruvate oxidation today; Krebs cycle and ETC discussed previously.
  • Different pathways may use alternative monosaccharides entering as glucose 6-phosphate or fructose 6-phosphate.
  • Anabolic pathways like glycogenesis, glycogenolysis, and lipogenesis are part of carbohydrate metabolism.