Electron transport chain malate-aspartate shuttle

The second electron shuttle system, called the malate-aspartate shuttle, is shown in Figure 21.34. Oxaloacetate is reduced in the cytosol, acquiring the electrons of NADH (which is oxidized to NAD ). Malate is transported across the inner membrane, where it is reoxidized by malate dehydrogenase, converting NAD to NADH in the matrix. This mitochondrial NADH readily enters the electron transport chain. The oxaloacetate produced in this reaction cannot cross the inner membrane and must be transaminated to form aspartate, which can be transported across the membrane to the cytosolic side. Transamination in the cytosol recycles aspartate back to oxaloacetate. In contrast to the glycerol phosphate shuttle, the malate-aspartate cycle is reversible, and it operates as shown in Figure 21.34 only if the NADH/NAD ratio in the cytosol is higher than the ratio in the matrix. Because this shuttle produces NADH in the matrix, the full 2.5 ATPs per NADH are recovered. 

E. There are two shuttle mechanisms, the malate-aspartate shutde and the glycerol 3-phosphate shuttle, that transport electrons to the inner mitochondrial matrix to be used in the electron transport chain. 

Note that, under aerobic conditions, the two NADH molecules that are synthesized are reoxidized via the electron transport chain generating ATP. Given the cytoplasmic location of these NADH molecules, each is reoxidized via the glycerol 3-phosphate shuttle (see Topic L2) and produces approximately two ATPs during oxidative phosphorylation or via the malate-aspartate shuttle (see Topic L2) and produces approximately three ATPs during oxidative phosphorylation. 

Electron shuttles Enzymatic processes whereby electrons from NADH can be transferred across the mitochondrial barrier. The glycerol 3-phosphate shuttle uses the reduction of dihydroxyacetone phosphate to glycerol 3-phosphate and reoxidation to transfer electrons from cytosolic NADH to coenzyme Q in the electron transport chain. The malate-aspartate shuttle uses malate and aspartate in a two-member transfer exchange to transfer electrons from cytosolic NADH to mitochondrial NADH (see Figures 27-2 and 27-3). 

Fig. 22.8. Malate-aspartate shuttle. NADH produced by glycolysis reduces oxaloacetate (OAA) to malate, which crosses the mitochondrial membrane and is reoxidized to OAA. The mitochondrial NADH donates electrons to the electron transpwrt chain, with 2.5 ATPs generated for each NADH. To complete the shuttle, oxaloacetate must return to the cytosol, although it cannot be directly transported on a translocase. Instead, it is transaminated to aspartate, which is then transported out to the cytosol, where it is transaminated back to oxaloacetate. The translocators exchange compounds in such a way that the shuttle is completely balanced. TA = transamination reaction. a-KG = a-ketoglutarate.

The NADH that is produced in the mitochondrion thus passes electrons to the electron transport chain. With the malate-aspartate shuttle, 2.5 moles of ATP are produced for each mole of cytosolic NADH rather than 1.5 moles of ATP in the glycerol-phosphate shutde, which uses FADHg as a carrier. The Biochemical Connections box on page 600 discusses some practical applications of our understanding of the catabolic pathways. 

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