Dihydroxyacetone phosphate transport

The fatty acids released on triacylglycerol hydrolysis are transported to mitochondria and degraded to acetyl CoA, while the glycerol is carried to the liver for further metabolism. In the liver, glycerol is first phosphorylated by reaction with ATP. Oxidation by NAD+ then yields dihydroxyacetone phosphate (DHAP), which enters the carbohydrate metabolic pathway. We ll discuss this carbohydrate pathway in more detail in Section 29.5. 

Glycerol may be picked up by liver and converted to dihydroxyacetone phosphate (DHAP) for gluconeogenesis, and the fetty adds are distributed to tissues that can use them. Free fatty acids are transported through the blood in association with serum albumin. 

FIGURE 20-15 The Pi-triose phosphate antiport system of the inner chloroplast membrane. This transporter facilitates the exchange of cytosolic P for stromal dihydroxyacetone phosphate. The products of photosynthetic carbon assimilation are thus moved into the cytosol... 

The FADH2 enters the electron-transport chain at coenzyme Q, while the dihydroxyacetone phosphate can return to the cytoplasm. Although this shuttle is generally inefficient, in the sense that only two ATP molecules are produced per FADH2 molecule oxidized, compared with three for NADH oxidation, it provides a mechanism for regeneration of NAD+ in the cytosol. The presence of cytosolic NAD+ is essential for continued glycolysis (see Fig. 11-20). 

Borchert, S., Grosse, H., and Heldt, H. W. 1989. Specific transport of inorganic phosphate, glucose 6-phosphate, dihydroxyacetone phosphate and 3-phospho-glycerate into amy-loplasts from pea roots. Fed. Eur. Biochem. Soc. 253,183-186. 

Free fatty acids may be absorbed directly by tissues, or bound to albmnin for transport human serum albmnin possesses multiple fatty acid binding sites of various affinities. Glycerol is returned via the blood to the liver (and kidneys), where it is converted to the glycolytic intermediate dihydroxyacetone phosphate (glycerol is an important source of glucose in gluco-... 

Figure 18.37. Glycerol 3-Phosphate Shuttle. Electrons from NADH can enter the mitochondrial electron transport chain by being used to reduce dihydroxyacetone phosphate to glycerol 3-phosphate. Glycerol 3-phosphate is reoxidized by electron transfer to an FAD prosthetic group in a membrane-bound glycerol 3-phosphate dehydrogenase. Subsequent electron transfer to Q to form QH2 allows these electrons to enter the electron-transport chain.

Glycerol 3-phosphate shuttle for the transport of cytoplasmic reducing equivalents to the inner mitochondrial membrane. Glycerol 3-phosphate, which carries the reducing equivalents, is oxidized to dihydroxyacetone phosphate by an FAD-linked dehydrogenase located on the C side of the inner membrane. 

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). 

After Its formation In the chloroplast stroma, glyceraldehyde 3-phosphate Is transported to the cytosol In exchange for phosphate. The final steps of sucrose synthesis occur In the cytosol of leaf cells. In these reactions, one molecule of glyceraldehyde 3-phosphate Is Isomerized to dihydroxyacetone phosphate. This compound condenses with a second molecule of glyceraldehyde 3-phosphate to form fructose 1,6-bIs-phosphate, which Is the reverse of the aldolase reaction In glycolysis (see Figure 8-4, step 0)). Fructose 1,6-bIsphosphate Is converted primarily to sucrose by the reactions shown In the bottom portion of Figure 8-42. 

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