Malate aspartate shuttle

The malate-aspartate shuttle system is important in transporting NADH, produced during glycolysis in the cytosol into the mitochondria. NADH is required in the TCA cycle and further in the production of ATP, the energy currency of the cell.

1. MALATE-ASPARTATE SHUTTLE BY- AAYUSHI RAMBIA M.Sc. SEM I DEPARTMENT OF MICROBIOLOGY REG. NO. 18308001 PONDICHERRY UNIVERSITY

2. SHUTTLE SYSTEM A shuttle system in general sense is something that goes back and forth in order to transfer things between the two places. In biological systems a shuttle system is used to transfer reducing equivalents across the membrane , more specifically across the inner mitochondrial membrane. A shuttle system is required because there are certain reducing equivalents such as NADH which cannot  cross the membrane, but it can reduce another molecule that can cross the membrane, so that its electrons can reach the electron transport chain , which is the ultimate goal to produce ATP.

3. NEED FOR MALATE ASPARTATE SHUTTLE The malate aspartate shuttle is required to transport the reducing equivalent NADH , produced during glycolysis in the cytosol into the mitochondria as the mitochondrial inner membrane is impermeable to NADH. The rate of transfer through this shuttle is 10-folds higher than any other shutlle systems NADH is required in the TCA cycle which operates in the mitochondria , from where the electrons are finally accepted by oxygen in the ETC to produce ATP , the final energy quotient.

4. SITE AND STEPS INVOLVED The shuttle operates in the mitochondria found mainly in the liver , kidney and heart because these three organs are the most metabolically active organs in the body. The shuttle involves four major enzymes which operate to help transfer NADH into the mitochondria : Malate dehydrogenase Aspartate amino transferase Malate – alphaketoglutarate antiporter Glutamate aspartate antiporter

5. PICTORIAL REPRESENTATION

6. STEP1 The glycolytic pathway produces two molecules of NADH in the payoff phase : 2Glyceraldehyde-3-phosphate 2−  + 2P i 2−  + 2NAD +  → 2(1,3-Bisphosphoglycerate 4−  )+ 2NADH + 2H + The NADH thus formed is used to reduced oxaloacetate present in the cytosol into malate catalysed by the enzyme “ malate dehydrogensae ”. Oxaloacetate + NADH + H + → L- malate + NAD +

7. STEP2 The malate so formed can cross the mitochondrial inner membrane and enter the mitochondria. This is facilitated by the enzyme “ malate - α - ketoglutarate carrier” The malate - α - ketogluatarate carrier helps the entry of malate inside the mitonchomdria in exhange with a molecule of α - ketoglutarate to the cytosol . (CYTOSOL) (MITOCHONDRIAL MATRIX) malate - α - ketoglutarate L- Malate L- Malate carrier α - ketoglutarate α - ketoglutarte

8. STEP3 Inside the mitochondrial matrix the malate is oxidised by NAD + back to oxaloacetate and NADH catalysed by the enzyme “ malate dehydrogenase ” (mitochondrial malate dehydrogenase ) . Malate + NAD + → Oxaloacetate + NADH + H +

9. STEP4 The oxaloacetate so formed needs to be transported back to the cytosol but the mitochondrial inner membrane is impermeable to oxaloacetate , so the oxaloacetate in the matrix is converted into aspartate catalysed by the enzyme “ aspartate aminotransferase ”. Oxaloacetate → Aspartate Simulataneously glutamate is converted into α - ketoglutarate .

10. STEP5 The aspartate so formed is tranported out of the mitochondria into the cytosol via the “glutamate- aspartate carrier” in exchange for cytosolic glutamate . (CYTOSOL) (MITOCHONDRIAL MATRIX) aspartate aspartate L-glutamate L-glutamate

11. STEP6 Upon reaching the cytosol , the aspartate is converted back to oxaloacetate by the enzyme “ aspartate aminotransferase ” ,thus replenishing the cytosolic oxaloacetate . Aspartate → oxaloacetate Simultaneously α - ketoglutarate in the cytosol is converted into glutamate. This step ensures that there is not net movement of substrates between the cytosol and the mitochondria and there is only the movement of electron.

12. REGULATION  There are two antiporter proteins located in the inner membrane of the mitochondria, the glutamate- aspartate transporter (transporter I) and the malate -α- ketoglutarate transporter (transporter II). In transporter I, the efflux of aspartate from the mitochondria is accompanied by the stoichiometric entry of glutamate and proton into the mitochondria. Therefore, this electrogenically driven process is irreversible and the rate-controlling step of the M-A shuttle. The proton gradient created in the cytosol as well as the mitochondrial matrix drives the entry and exit of substrates via the antiporters .

13. REFERENCES Principles of Biochemistry- Lehninger edition 4 th Biochemistry by Voet and Voet The Metabolic Significance of the Malate-Aspartate Cycle in Heart. Circulation Research is published by the American Heart Association. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2572303 https://www.researchgate.net/figure/The-malate-aspartate-shuttle-is-the-principal-mechanism-for-the-movement-of-reducing_fig3_236857315

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