Electrons from Cytosolic NADH Are Imported by Shuttle Systems

Electrons from Cytosolic NADH Are Imported by Shuttle Systems 

The mitochondrial inner membrane has no transport system for NAD+ or NADH. In animal cells, most of the NADH that must be oxidized by the respiratory chain is generated in the mitochondrial matrix by the TCA cycle or the oxidation of fatty acids. However, NADH also is generated by glycolysis in the cytosol. If 02 is available, it clearly is advantageous to reoxidize this NADH by the respiratory chain, rather than by the formation of lactate or ethanol as described in chapter 12. This is evident from the findings that approximately 2.5 molecules of ATP can be formed for each NADH oxidized in the mitochondria, whereas no ATP is made when NADH is oxidized by the cytosolic lactate dehydrogenase or alcohol dehydrogenase. 

Animal cells use several types of shuttle system to transfer electrons from cytosolic NADH to the respiratory chain. These shuttle systems consume energy so that instead of 2.5 only about 1.5 molecules of ATP are produced for every cytosolic NADH. 

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