Glycerophosphate shuttl

The Glycerophosphate Shuttle Ensures Efficient Use of Cytosolic NADH... 

In the glycerophosphate shuttle, two different glycerophosphate dehydrogenases, one in the cytoplasm and one on the outer face of the mitochondrial inner membrane, work together to carry electrons into the mitochondrial matrix (Figure 21.32). NADH produced in the cytosol transfers its electrons to dihydroxyaeetone phosphate, thus reducing it to glyeerol-3-phosphate. This metabolite is reoxidized by the FAD -dependent mitochondrial membrane enzyme to... 

FIGURE 21.33 The glycerophosphate shuttle (also known as the glycerol phosphate shuttle) couples the cytosolic oxidation of NADH with mitochondrial reduction of [FAD]. 

Figure 12-12. Glycerophosphate shuttle for transfer of reducing equivalents from the cytosol into the...

Shuttle pathways for the transport of electrons across the inner mitochondrial membrane. A. Glycerophosphate shuttle. B. Malate-aspartate shuttle. 

DHAP is also employed in the glycerophosphate shuttle, which functions as an electron-transporting mechanism in insect muscle (Figure 15.11a). 

Dihydroxyacetone (No. 1716) is classified chemically as a ketotriose. It is endogenous in animals and is readily phosphorylated in vivo to form dihydroxyacetone phosphate. Dihydroxyacetone phosphate is involved in several common metabolic pathways, glycolysis and gluconeogenesis, and the glycerophosphate shuttle (Michal, 1999). It is also part of the Calvin cycle in plant metabolism (Michal, 1999). 

The glycerophosphate shuttle (Figure 5.12) involves reduction of dihydroxy acetone phosphate to glycerol 3-phosphate in the cytosol (with oxidation of NADH to NAD" ) and oxidation of glycerol 3-phosphate to dihydroxyacetone phosphate inside the mitochondrion. Dihydroxyacetone phosphate and glycerol 3-phosphate are transported in opposite directions by an antiporter in the mitochondrial membrane. 

The malate-aspartate shuttle is sensitive to the NADH/NAD" ratios in the cytosol and mitochondria, and cannot operate if the mitochondrial NADH/NAD" ratio is higher than that in the cytosol. However, because it does not use NAD" in the mitochondrion, the glycerophosphate shuttle can operate even when the mitochondrial NADH/NAD" ratio is higher than that in the cytosol. 

The glycerophosphate shuttle is important in muscle in which there is a very high rate of glycolysis (especially insect flight muscle) the malate-aspartate shuttle is especially important in heart and liver. 

---