Ho

Figure 13.9. Gluconeogenesis allows glucose to be made from pyruvate.

oxaloacetate to feed gluconeogenesis. However, in passing around the Krebs cycle from citrate to oxaloacetate, the two carbons delivered by the acetyl-CoA have already been lost as CO2.

IN DEPTH 13.2 The Glyoxylate Shunt_

Many plants store triacylglycerol oils in their seeds. We have emphasized that mammals cannot make glucose from acetyl-CoA as two carbon dioxides are lostfor every two carbons on acetyl-CoA that enter the Krebs cycle. Plants, however, can convert acetyl-CoA into glucose. Special organelles called glyoxysomes carry out a series of reactions called the glyoxylate pathway or glyoxylate shunt, which effectively short-circuits the Krebs cycle.

As in the Krebs cycle acetyl-CoA combines with oxaloacetate to give citrate, which is converted to isocitrate. However, rather than undergoing oxidation and loss of CO2, the isocitrate is split to give succinate and the two carbon glyoxylate. Glyoxylate is then combined with another acetyl-CoA to yield malate, which is oxidized to oxaloacetate. In parallel the succinate is also converted to oxaloacetate using the standard Krebs cycle reactions. The overall effect of one complete cycle is therefore to generate one oxaloacetate that can enter gluconeogenesis (page 295).

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