Gluconeogenesis oxaloacetate

Figure 6-7. Conversion of mitochondrial pyruvate to cytosolic phosphoenolpyruvate to initiate gluconeogenesis. Oxaloacetate cannot pass across the inner mitochondrial membrane, so it is reduced to malate, which can do so.

To initiate gluconeogenesis, oxaloacetate is reduced to malate, which is then transported to the cytosol in the reverse of the malate shuttle. 

Answer Oxaloacetate might be withdrawn for aspartate synthesis or for gluconeogenesis. Oxaloacetate is replenished by the anaplerotic reactions catalyzed by PEP carboxykinase, PEP carboxylase, malic enzyme, or pyruvate carboxylase (see Fig. 16-15, p. 632). 

The answer is a. (Murray, pp 190—198. Sci ivei, pp 1521—1552. Sack, pp 121-138. Wilson, pp 287-317.1 In the formation of phosphoenolpyruvate during gluconeogenesis, oxaloacetate is an intermediate. In the first step, catalyzed by pyruvate carboxylase, pyruvate is carboxylated with the utilization of one high-energy ATP phosphate bond ... 

For PEPCK to function in gluconeogenesis, oxaloacetate produced in the pyruvate carboxylase reaction in the mitochondria, must be transported to the cytoplasm. PEPCK is not under any known allosteric control. Activity of the enzyme is regulated by hormonal control of its transcription. Glucagon stimulates transcription of the structural gene for PEPCK. Insulin inhibits transcription of the enzyme. By inhibiting PEPCK gene transcription, insulin tends to depress gluconeogenesis rates. 

The enzyme that catalyzes this reaction is pyruvate carboxylase, an allosteric enzyme found in the mitochondria. Acetyl-CoA is an allosteric effector that activates pyruvate carboxylase. If high levels of acetyl-GoA are present (in other words, if there is more acetyl-GoA than is needed to supply the citric acid cycle), pyruvate (a precursor of acetyl-GoA) can be diverted to gluconeogenesis. (Oxaloacetate from the citric acid cycle can frequendy be a starting point for gluconeogenesis as well.) Magnesium ion (Mg +) and biotin are also required for effective catalysis. We have seen Mg + as a cofactor before, but we have not seen biotin, which requires some discussion. 

Pyruvate carboxylase is the most important of the anaplerotie reactions. It exists in the mitochondria of animal cells but not in plants, and it provides a direct link between glycolysis and the TCA cycle. The enzyme is tetrameric and contains covalently bound biotin and an Mg site on each subunit. (It is examined in greater detail in our discussion of gluconeogenesis in Chapter 23.) Pyruvate carboxylase has an absolute allosteric requirement for acetyl-CoA. Thus, when acetyl-CoA levels exceed the oxaloacetate supply, allosteric activation of pyruvate carboxylase by acetyl-CoA raises oxaloacetate levels, so that the excess acetyl-CoA can enter the TCA cycle. 

FIGURE 23.5 Pyruvate carboxyl compartmentalized reaction. Pyruva verted to oxaloacetate in the mitoci Because oxaloacetate cannot be trai across the mitochondrial membrant reduced to malate, transported to tl and then oxidized back to oxaloace gluconeogenesis can continue. 

Step 1 of Figure 29.13 Carboxylation Gluconeogenesis begins with the carboxyl-afion of pyruvate to yield oxaloacetate. The reaction is catalyzed by pyruvate carboxylase and requires ATP, bicarbonate ion, and the coenzyme biotin, which acts as a carrier to transport CO2 to the enzyme active site. The mechanism is analogous to that of step 3 in fatty-acid biosynthesis (Figure 29.6), in which acetyl CoA is carboxylated to yield malonyl CoA. 

Succinyl-CoA is converted to succinate by the enzyme succinate thiokinase (succinyl-CoA synthetase). This is the only example in the citric acid cycle of substrate-level phosphorylation. Tissues in which glu-coneogenesis occurs (the hver and kidney) contain two isoenzymes of succinate thiokinase, one specific for GDP and the other for ADP. The GTP formed is used for the decarboxylation of oxaloacetate to phos-phoenolpymvate in gluconeogenesis and provides a regulatory hnk between citric acid cycle activity and the withdrawal of oxaloacetate for gluconeogenesis. Nongluconeogenic tissues have only the isoenzyme that uses ADP. 

Aminotransferase (transaminase) reactions form pymvate from alanine, oxaloacetate from aspartate, and a-ketoglutarate from glutamate. Because these reactions are reversible, the cycle also serves as a source of carbon skeletons for the synthesis of these amino acids. Other amino acids contribute to gluconeogenesis because their carbon skeletons give rise to citric acid cycle... 

In pigeon, chicken, and rabbit liver, phospho-enolpymvate carboxykinase is a mitochondrial enzyme, and phosphoenolpyruvate is transported into the cytosol for gluconeogenesis. In the rat and the mouse, the enzyme is cytosolic. Oxaloacetate does not cross the mitochondrial inner membrane it is converted to malate, which is transported into the cytosol, and convetted back to oxaloacetate by cytosolic malate dehydrogenase. In humans, the guinea pig, and the cow, the enzyme is equally disttibuted between mitochondria and cytosol. 

The main source of GTP fot phosphoenolpytuvate catboxykinase inside the mitochondrion is the teaction of succinyl-CoA synthetase (Chaptet 16). This provides a hnk and hmit between cittic acid cycle activity and the extent of withdtawal of oxaloacetate fot gluconeogenesis. 

This enzyme, similar to all C02 assimilating enzymes, contains biotin for a cofactor. Oxaloacetate is released from the mitochondria into the cytoplasm to enter gluconeogenesis. In the cytoplasm, oxaloacetate converts to phosphoenolpyruvate via a reaction catalyzed by phosphoenolpyruvate carboxylase ... 

There are two unusual aspects to the regulation of gluconeogenesis. The first step in the reaction, the formation of oxaloacetate from pyruvate, requires the presence of acetyl-CoA. This is a check to make sure that the TCA cycle is adequately fueled. If there s not enough acetyl-CoA around, the pyruvate is needed for energy and gluconeogenesis won t happen. However, if there s sufficient acetyl-CoA, the pyruvate is shifted toward the synthesis of glucose. 

A problem for gluconeogenesis is that pyruvate carboxylase, which produces oxaloacetate from pyravate, is present in the mitochondria but phosphoenolpyruvate carboxylase, at least in human liver, is present in the cytosol. For reasons given in Chapter 9, oxaloacetate cannot cross the mitochondrial membrane and so a transporter is not present in any cells. Hence, oxaloacetate is converted to phosphoenolpyruvate which is transported across the membrane (Figure 6.25). 

The degradation of most amino acids is anaplerotic, because it produces either intermediates of the cycle or pyruvate glucogenic amino acids see p. 180). Gluconeogenesis is in fact largely sustained by the degradation of amino acids. A particularly important anaplerotic step in animal metabolism leads from pyruvate to oxaloacetic acid. This ATP-dependent reaction is catalyzed by pyruvate... 

OVERSATURATION OXALOACETATE DECARBOXYLASE Oxaloacetate, synthesis in gluconeogenesis, PYRUVATE CARBOXYLASE PHOSPHOENOLPYRUVATE CARBOXYKI-NASE (PYROPHOSPHATE)... 

A. The ability to synthesize new oxaloacetate from pyruvate is essential to maintain activity of the TCA cycle for cell growth and for gluconeogenesis. 

C. Oxaloacetate can also be converted to malate and transported to the cytoplasm for gluconeogenesis under fasting conditions (see Chapter 6). 

This shunts oxaloacetate toward gluconeogenesis and leaves acetyl CoA available for formation of ketone bodies. 

The standard free-energy change for this reaction is quite high, but under physiological conditions (including a very low concentration of oxaloacetate) AG 0 and the reaction is readily reversible. Mitochondrial malate dehydrogenase functions in both gluconeogenesis and the citric acid cycle, but the overall flow of metabolites in the two processes is in opposite directions. 

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