Shuttle systems glycerol phosphate

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. 

Malate-aspartate Shuttle This is the common shuttle in mammalian systems. It is both more complex and more efficient than the glycerol phosphate shuttle, yielding 2.5 ATP/NADH. It can be thought of as occurring in two phases ... 

One carrier system that has been extensively studied in insect flight muscle is the glycerol-phosphate shuttle. This mechanism uses the presence on the outer face of the inner mitochondrial membrane of an FAD-dependent enzyme... 

Under aerobic conditions, a greater energy yield may be derived from the electrons contained within the NADH molecule by their participation in the process of oxidative phosphorylation (Section 10.4). The location of the electron-transport assemblies within the inner mitochondrial membrane necessitates the penetration of the membrane by NADH (Section 9.5). The inner mitochondrial membrane is, however, impermeable to NADH molecules. This obstacle is circumvented by the use of shuttle systems (Section 10.5) which do not transport the NADH molecules across the membrane but transfer the electrons as components of another substance which can transverse the membrane. Two shuttle systems exist for this purpose the glycerol phosphate shuttle and the malate-aspartate shuttle. Their relative activities are tissue dependent, e.g. the glycerol phosphate shuttle predominates in the cells of mammalian skeletal muscle and brain whilst the malate-aspartate shuttle is... 

There are two shuttle systems the glycerol-3-phosphate shuttle found in skeletal muscle and nerve cells and the oxaloacetate-malate shuttle foimd in heart and liver cells. Because skeletal muscle produces the majority of the ATP for the body, it is the glycerol-3-phosphate shuttle that is used most commonly when discussing metabolic energy )delds. In Example 22.1, calculation of the ATP harvest of glycolysis is based on this shuttle. 

It is often necessary to move electrons into mitochondria for disposal via oxidative phosphorylation. However, NADH and FADH2 do not penetrate the inner mitochondrial membrane. Instead, such electrons may first be passed to dihydroxyacetone phosphate or to oxaloacetate to make glycerol-3-phosphate and malate, respectively. These compounds can penetrate the inner mitochondrial membrane via the porters described earlier and oxidized there by mitochondrial NAD+ or FAD. These systems are termed the glycerol-3-phosphate and malate shuttles, respectively, and they are described in greater detail in Chapter 18. 

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