Muscles glycerol-phosphate shuttle

The other shuttle is the malate-aspartate shuttle. The advantage of this shuttle is that it gives you 3 ATPs for the oxidation of each cytoplasmic NADH. In red muscle, heart, and brain tissues the malate-aspartate shuttle is the major pathway for shuttling electrons into mitochondria. In white muscle, the a-glycerol phosphate shuttle predominates (Fig. 14-2). 

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... 

How do shuttle mechanisms differ from one another Two shuttle mechanisms—the glycerol—phosphate shuttle and the malate—aspartate shuttle—transfer the electrons, but not the NADH, produced in cytosolic reactions into the mitochondrion. In the hrst of the two shuttles, which is found in muscle and brain, the electrons are transferred to FAD in the second, which is found in kidney, hver, and heart, the electrons are transferred to NAD. With the malate-aspartate shuttle, 2.5 molecules of ATP are produced for each molecule of cytosolic NADH, rather than 1.5 ATP in the glycerol-phosphate shuttle, a point that affects the overall yield of ATP in these tissues. 

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... 

Skeletal muscle and brain use a different NADH shuttle, the glycerol 3-phosphate shuttle (Fig. 19-28). It differs from the malate-aspartate shuttle in that it delivers the reducing equivalents from NADH to ubiquinone and thus into Complex III, not Complex I (Fig. 19-8), providing only enough energy to synthesize 1.5 ATP molecules per pair of electrons. 

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. 

The main drawback of the glycerol-3-phosphate shuttle is that only two ATP are produced for each cytoplasmic NADH. The reason is that the electrons are shuttled to FADHj, which )delds only two ATP by oxidative phosphorylation. (The energy yield of the oxidation of mitochondrial NADH is three ATP.) Thus the total energy yield from glycolysis imder aerobic conditions in muscle and nerve cells is two ATP, produced by substrate level phosphorylation, plus four ATP (two ATP per NADH), produced by oxidative phosphorylation. This provides an energy yield of six ATP per glucose. 

Skeletal muscle and brain use a different NADH shuttle, the glycerol 3-phosphate shuttle (Fig. 

Gly3PDH participates in the dihydroxyacetone phosphate/glycerol-3-phosphate shuttle of insect muscle (Figure 15.11)... 

The malate—aspartate shuttle is the mechanism by which electrons from NADH produced in the cytosol are transported into mitochondria, as the inner membrane is impermeable to NADH itself. Oxaloacetate is reduced to malate in the cytosol by malate dehydrogenase, in the process oxidizing NADH to replenish cytosolic NAD. The malate-aspartate shuttle is found mainly in cardiac muscle and liver cells, while the glycerol 3-phosphate shuttle operates mainly in brain and skeletal muscle cells. Once malate has entered the mitochondria it is oxidized to oxaloacetate, generating NADH within the mitochondrial matrix. Oxaloacetate is then converted to aspartate, which is transported out of the mitochondria in exchange for glutamate. 

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