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Glycerol 3-Phosphate Shuttle
In skeletal muscle cells, the NADH molecules produced in glycolysis must be transported onto the electron transport chain under aerobic conditions. To do this, these cells utilize a process called the glycerol 3-phosphate shuttle. In this shuttle, the NADH molecules are first oxidized back into NAD+ while reducing a dihydroxyacetone phosphate (DHAP) molecule into glycerol 3-phosphate (G3P). This process occurs in the cytoplasm and it allows the cell to transfer the high energy electrons onto a molecule that can readily pass into the mitochondrial intermembrane space. Once G3P moves into the intermembrane space, it is then oxidized back into DHAP by the action of a membrane-bound enzyme called the mitochondrial glycerol 3-phosphate dehydrogenase. An FAD molecule bound to this enzyme accepts the two high energy electrons and the two H+ ions to form FADH2. The FADH2 then gives up the two protons and electrons to a ubiquinone molecule to form ubiquinol. Ubiquinol then moves along the inner mitochondrial membrane to transfer the electrons onto complex III. Since this shuttle bypasses complex I, NADH molecules transported onto the ETC in this manner only produce a net result of 1.5 ATP molecules per NADH.
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