Shuttle Pathways or systems: Glycerophosphate shuttle and Malate-aspartate shuttle

Shuttle Pathways or Systems

(Transport of reducing equivalents)

• The inner mitochondrial membrane is impermeable to NADH.       
• NADH produced in the cytosol cannot directly enter the mitochondria.
• NADH produced in the glycolysis is extramitochondrial, whereas the electron transport chain, where NADH has to be oxidized to NAD+ is in the mitochondrion.
• NADH produced in cytosol transfer the reducing equivalents through the mitochondrial membrane via substrate pairs, linked by suitable dehydrogenases by shuttle systems.
• It is important that the specific dehydrogenases which act as “shuttle” be present on both sides of the mitochondrial membrane.
• Two such shuttle systems are there:
  1. Glycerophosphate shuttle
  2. Malate-aspartate shuttle

Glycerophosphate shuttle:

• This shuttle system is not much common to be used in humans. It is present in insect flight muscle and in white muscle.
• This alternative means of moving reducing equivalents from the cytosol to the respiratory chain operates in skeletal muscle and the brain.
• It delivers the reducing equivalents from NADH through FAD in
  glycerol 3-phosphate dehydrogenase to ubiquinone and thus into Complex III, not Complex I
• Cytosolic glycerol 3-phosphate dehydrogenase oxidizes NADH to NAD+.
• The reducing equivalents are transported through glycerol 3-phosphate into the mitochondria.
• An isozyme of  Glycerol 3-phosphate dehydrogenase—present on the outer surface of the inner mitochondrial membrane— reduces FAD to FADH₂.
• Dihydroxyacetone phosphate escapes into the cytosol and the shuttling continues.
• FADH₂ gets oxidized via ETC to generate 2 ATP.
• Note that this shuttle does not involve membrane transport systems.

 Malate-aspartate shuttle (Malate shuttle):

• This shuttle system is more common and universal.
• This shuttle for transporting reducing equivalents from cytosolic NADH into the mitochondrial matrix is used in the liver, kidney, and heart.
• Reduced NADH + H+ is reformed in the mitochondrion, which is oxidized in respiratory chain produces 3 ATP.
• The system is rather a little complex as OAA is impermeable to mitochondrial membrane; whereas malate, aspartate, glutamate, and α-ketoglutarate are permeable to the mitochondrial membrane.
• So OAA reformed in mitochondrion has to be transaminated to form aspartate.
• In the cytosol again OAA is regenerated by transamination.
• This shuttle involves membrane transport systems.
• In the cytosol, oxaloacetate accepts the reducing equivalents (NADH) and becomes malate.
• Malate then enters mitochondria where it is oxidized by mitochondrial malate dehydrogenase.
• In this reaction, NADH and oxaloacetate are regenerated.
• NADH gets oxidized via electron transport chain and 3 ATP are
  produced.
• This is in contrast to glycerolphosphate shuttle where only 2 ATP are
  produced.
• In the mitochondria, oxaloacetate participates in transamination reaction with glutamate to produce aspartate and α-ketoglutarate.
• The aspartate enters the cytosol and transaminates with α-ketoglutarate to give oxaloacetate and glutamate.

Steps of Malate-aspartate shuttle:

1. NADH in the cytosol enters the intermembrane space through openings in the outer membrane (porins), then passes two reducing equivalents to oxaloacetate, producing malate. 2
2. Malate crosses the inner membrane via the malate–α-ketoglutarate transporter.
3. In the matrix, malate passes two reducing equivalents to NAD+, and the resulting NADH is oxidized by the respiratory chain; the oxaloacetate formed from
   malate cannot pass directly into the cytosol.
4. Oxaloacetate is first transaminated to aspartate, and
5. Aspartate can leave via the glutamate-aspartate transporter.
6. Oxaloacetate is regenerated in the cytosol, completing the cycle, and glutamate produced in the same reaction enters the matrix via the glutamate-aspartate transporter.

Note:

• When body utilises α-glycero-P-shuttle, net ATP produced by glycolysis—TCA cycle per molecule glucose oxidised will be 36 ATP (2 ATP less) and NOT 38 ATP.
• Use of Malate shuttle will form 38 ATP

Shuttle pathways and tissues

• Liver, kidney, and heart utilize malate-aspartate shuttle, and yield 3 ATP per mole of NADH.
• Skeletal muscle and the brain utilize glycerol-phosphate shuttle and liberate 2 ATP from NADH.

References:

• Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2000). Lehninger principles of biochemistry. New York: Worth Publishers.
• John W. Pelley, Edward F. Goljan (2011). Biochemistry. Third edition. Philadelphia: USA.
• Smith, C. M., Marks, A. D., Lieberman, M. A., Marks, D. B., & Marks, D. B. (2005). Marks’ basic medical biochemistry: A clinical approach. Philadelphia: Lippincott Williams & Wilkins.
• https://en.wikipedia.org/wiki/Malate-aspartate_shuttle
• https://en.wikipedia.org/wiki/Glycerol_phosphate_shuttle