Biochemistry · Metabolic pathway
Malate–aspartate shuttle !
NADH cannot cross the inner mitochondrial membrane. This shuttle carries its electrons inside on malate, so liver and heart get the full ATP yield.
Updated 2026-11-01
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Key facts
- Cytosolic NADH reduces oxaloacetate to malate; malate enters the matrix and is reoxidised, giving NADH back inside.
- The electrons enter the chain at complex I, so each cytosolic NADH is worth about 2.5 ATP (about 1.5 with the glycerol-3-phosphate shuttle).
- It needs two antiporters and two pairs of isoenzymes (MDH1/MDH2 and AST1/AST2); nitrogen travels on aspartate and glutamate.
- The aspartate–glutamate carrier is electrogenic and uses the proton gradient, which makes the shuttle run one way.
- It is most active in liver, heart and kidney; skeletal muscle and brain rely more on the glycerol-3-phosphate shuttle.
Regulation
- Cytosolic Ca²⁺ activates the aspartate–glutamate carriers (aralar, citrin).
- A high NADH/NAD⁺ ratio in the matrix slows the shuttle.
- Aminooxyacetate blocks the aminotransferases and stops the shuttle (experimental).
The steps, one by one
- Step 1 · Malate dehydrogenase 1 (cytosolic, MDH1) oxaloacetate + NADH ⇌ malate + NAD⁺In the cytosol it uses NADH from glycolysis to reduce oxaloacetate to malate, giving NAD⁺ back to glycolysis.If it fails: If this step cannot keep up, cytosolic NAD⁺ runs short and cells turn more pyruvate into lactate. Lactic acidosis →
- Step 2 · 2-Oxoglutarate/malate carrier (OGC, SLC25A11) malate + α-Ketoglutarate (matrix) ⇌ Malate (matrix) + α-ketoglutarateSwaps cytosolic malate for matrix α-ketoglutarate across the inner membrane without spending energy.If it fails: Inherited SLC25A11 variants predispose to paraganglioma (UniProt: PPGL6). Hereditary paraganglioma and pheochromocytoma (SDH) →
- Step 3 · Malate dehydrogenase 2 (mitochondrial, MDH2) Malate (matrix) + NAD⁺ ⇌ Oxaloacetate (matrix) + NADHIn the matrix it reoxidises malate and gives NADH that feeds complex I; it is the same reaction as the last step of the Krebs cycle.If it fails: Biallelic MDH2 variants cause an early-onset developmental and epileptic encephalopathy (DEE51).
- Step 4 · Aspartate aminotransferase 2 (mitochondrial, GOT2) Oxaloacetate (matrix) + Glutamate (matrix) ⇌ Aspartate (matrix) + α-Ketoglutarate (matrix)Moves the amino group of glutamate onto oxaloacetate to make aspartate, which can leave the matrix (oxaloacetate cannot).If it fails: Mitochondrial AST leaks into the blood in severe liver injury; an AST/ALT ratio above 2 suggests alcohol-related liver disease.
- Step 5 · Aspartate–glutamate carrier (aralar SLC25A12, citrin SLC25A13) irreversible Aspartate (matrix) + Glutamate → Aspartate (cytosol) + Glutamate (matrix)Exports aspartate in exchange for glutamate plus a proton; because it uses the proton gradient, the shuttle cannot run backwards.If it fails: Citrin deficiency (SLC25A13): neonatal cholestasis, and in adults citrullinaemia type II with hyperammonaemia and a liking for protein-rich, low-carbohydrate food. Hyperammonaemia →
- Step 6 · Aspartate aminotransferase 1 (cytosolic, GOT1) Aspartate (cytosol) + α-ketoglutarate ⇌ oxaloacetate + GlutamateIn the cytosol it turns aspartate back into oxaloacetate, ready to take the next NADH, and makes glutamate again.If it fails: Cytosolic AST is most of the AST measured in serum; it rises in liver and muscle injury.
Clinical notes
- Citrin deficiency (SLC25A13): neonatal intrahepatic cholestasis, and in adults citrullinaemia type II with hyperammonaemia.
- Ethanol oxidation floods the cytosol with NADH, which depends on the shuttles to be cleared.
- Glycolysis needs cytosolic NAD⁺; when shuttles cannot keep up, cells reduce pyruvate to lactate instead.
- AST leaks from damaged liver and heart cells; the mitochondrial isoenzyme rises with severe hepatocellular injury.
Pathways
Related conditions
Related
- Mitochondrial NADH shuttles
- Electron transport chain and oxidative phosphorylation
- Complex I (NADH dehydrogenase)
- Aspartate aminotransferase (AST, GOT)
- Glutamate (glutamic acid)
- NAD⁺ (nicotinamide adenine dinucleotide)
- Glycolysis
- Gluconeogenesis
- Urea cycle
- Ethanol metabolism
Test yourself
Think of the answer, then tap the question to see it.
Why does the cell need a shuttle for cytosolic NADH?
The inner mitochondrial membrane has no NADH carrier, so the shuttle moves the electrons on malate instead.
About how many ATP does one cytosolic NADH give through this shuttle?
About 2.5, because NADH is remade in the matrix and enters at complex I; the glycerol-3-phosphate shuttle gives about 1.5 through FADH₂.
Why must oxaloacetate be converted to aspartate in the matrix?
There is no oxaloacetate carrier; aspartate leaves on the aspartate–glutamate carrier and is turned back into oxaloacetate in the cytosol.
Which inherited disorder of this shuttle causes hyperammonaemia?
Citrin deficiency (SLC25A13), the liver aspartate–glutamate carrier; less aspartate reaches the urea cycle.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: SLC25A13 gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Malate dehydrogenase, mitochondrial (MDH2) - Homo sapiens (UniProt, 2026); Mitochondrial 2-oxoglutarate/malate carrier protein (SLC25A11) - Homo sapiens (UniProt, 2026); Malate dehydrogenase, cytoplasmic (MDH1) - Homo sapiens (UniProt, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.