Lecture Notes: Malate-Aspartate Shuttle

Background: Cellular Respiration

• Stages of Cellular Respiration:
  1. Glycolysis:
     • Occurs in the cytosol.
     • 1 molecule of glucose → 2 molecules of pyruvate.
     • Generates 2 NADH.
  2. Oxidation of Pyruvate:
     • Oxidative decarboxylation of pyruvate → Acetyl Coenzyme A.
     • Occurs in the mitochondrial matrix.
     • Generates 2 NADH (from 2 pyruvate molecules).
  3. TCA Cycle (Krebs/Citric Acid Cycle):
     • Oxidation of Acetyl Coenzyme A to CO2.
     • Electrons passed to coenzymes NAD+ and FAD → NADH and FADH2.
     • Occurs in the mitochondrial matrix.
  4. Electron Transport Chain & Oxidative Phosphorylation:
     • NADH and FADH2 give up electrons to synthesize ATP.
     • NADH from mitochondrial matrix is used, but cytosolic NADH cannot cross into the matrix.

NADH Oxidation and ATP Synthesis Problem

• Cytosolic NADH:
  • Generated during glycolysis.
  • Cannot cross mitochondrial membrane, needs oxidation to regenerate NAD+.
  • Two pathways for NADH oxidation:
    1. Anaerobic Pathway: Pyruvate → Lactate (via lactate dehydrogenase), NADH is oxidized to NAD+.
    2. Aerobic Pathway: NADH should be oxidized via electron transport chain.

Malate-Aspartate Shuttle

• Function: Transfers electrons from cytosolic NADH to mitochondrial NADH without direct transport of NADH.
• Process Overview:
  1. Oxaloacetate in the cytosol acquires electrons from NADH → Reduced to Malate (via malate dehydrogenase enzyme).
  2. Malate transported into the mitochondrial matrix.
  3. Malate converted back to Oxaloacetate in the matrix, electrons transferred to mitochondrial NAD+ → NADH.
  4. NADH then enters electron transport chain, yielding 2.5 ATP per NADH.
• Oxaloacetate Recycling:
  • Cannot cross the mitochondrial membrane.
  • Transaminated to Aspartate (via aspartate aminotransferase enzyme) with amino group donated by glutamate.
  • Aspartate can cross back to the cytosol.
• Glutamate and Alpha-Ketoglutarate Cycle:
  • Glutamate donates an amino group to form aspartate → Converted to alpha-ketoglutarate.
  • Alpha-ketoglutarate and malate are exchanged between cytosol and mitochondria via an antiporter.
  • Alpha-ketoglutarate is transaminated back to glutamate in the cytosol.

Electron Transport and ATP Synthesis

• NADH Regeneration in Mitochondria:
  • Electrons passed through complexes I, III, IV, and cytochrome c.
  • Final electron acceptor is oxygen.
  • Protons pumped into intermembrane space drive ATP synthesis via ATP synthase (complex V).
  • 1 cytosolic NADH yields 2.5 ATP via the shuttle.

Conclusion

• The Malate-Aspartate Shuttle efficiently transfers reducing equivalents from cytosolic NADH to mitochondria, enabling continued ATP production and glycolysis function.
• Operates prominently in the heart, liver, and kidneys.
• Important for maintaining cellular energy balance.