When acetyl-CoA accumulates in the mitochondrial matrix (for example, after a big meal), it must be moved to the cytoplasm where it can be used in fatty acid biosynthesis. Acetyl-CoA cannot pass directly through the inner membrane of the mitochondrion, however, and must be shuttled out of the mitochondrion on the back of oxaloacetate (to form citrate). The citrate shuttle system operates as follows (see Figure 18.31):

1. Acetyl-CoA + Oxaloacetate -> Citrate + CoASH (catalyzed by Citrate Synthase)

2. Citrate is transported across the mitochondrial membrane

3. In the cytoplasm, citrate is converted back to oxaloacetate and acetyl-CoA (Catalyzed by citrate lyase, Figure 18.31).

4. To move the oxaloacetate (released after the last step) back into the mitochondrion, Oxaloacetate + NADH + H⁺  -> L-Malate + NAD⁺ (catalyzed by Malate Dehydrogenase)

5. L-Malate +  NADP⁺ -> NADPH + Pyruvate + H⁺ (catalyzed by Malic Enzyme)

6. Pyruvate is tranported into the mitochondrial matrix.

7. Inside the mitochondrial matrix, Pyruvate + ATP + CO2 -> Oxaloacetate + ADP + Pi (catalyzed by Pyruvate Carboxylase).

The net effect of these reactions is that acetyl-CoA is moved out of the mitochondrion and cytosolic NADH is converted to cytosolic NADPH. NADPH is used, in turn, for the biosynthesis of fatty acids. Recall (see here) that the synthesis of 1 mole of palmitate requires 14 moles of NADPH, most of which is generated in the cytosol via the pentose phosphate pathway.

-Oxidation