Mitochondria gluconeogenesis reactions

There is a logic to the route of these reactions through the mitochondrion. The [NADH]/[NAD+] ratio in the cytosol is 8 X 10 4, about 105 times lower than in mitochondria. Because cytosolic NADH is consumed in gluconeogenesis (in the conversion of 1,3-bisphos-... 

FIGURE 14-19 Alternative paths from pyruvate to phospho-enolpyruvate. The path that predominates depends on the glucogenic precursor (lactate or pyruvate). The path on the right predominates when lactate is the precursor, because cytosolic NADH is generated in the lactate dehydrogenase reaction and does not have to be shuttled out of the mitochondrion (see text). The relative importance of the two pathways depends on the availability of lactate and the cytosolic requirements for NADH by gluconeogenesis. 

Pyruvate carboxylase is a mitochondrial enzyme, vhereas the other enzymes of gluconeogenesis are cytoplasmic. Oxaloacetate, the product of the pyruvate carboxylase reaction, is reduced to malate inside the mitochondrion for transport to the cytosol. The reduction is accomplished by an NADH-linked malate dehydrogenase. When malate has been transported across the mitochondrial membrane, it is reoxidized to oxaloacetate by an NAD+-linked malate dehydrogenase in the cytosol (Figure 16.28). 

Pyruvate carboxylase is a mitochondrial enzyme, whereas the other enzymes of gluconeogenesis are present primarily in the cytoplasm. Oxaloacetate, the product of the pyruvate carboxylase reaction, must thus be transported to the cytoplasm to complete the pathway. Oxaloacetate is transported from a mitochondrion in the form of malate oxaloacetate is reduced to malate inside the mitochondrion by an NADH-linked malate dehydrogenase. After malate has been transported across the mitochondrial membrane, it is reoxidized to oxaloacetate by an NAD -linked malate dehydrogenase in the cytoplasm (Figure 16.26). The formation of oxaloacetate from malate also provides NADH for use in subsequent steps in gluconeogenesis. Finally, oxaloacetate is simultaneously decarboxylated and phospho-ry lated by phosphoenolpyruvate carboxy kinase to generate phosphoenol pyruvate. The phosphoryl donor is GTP. The GO2 that was added to pyruvate by pyruvate carboxylase comes off in this step. 

Pyruvate is converted to phosphoenolpyruvate for glucose synthesis by a two-step reaction, with the intermediate formation of oxaloacetate. As shown in Figure 5.31, pyruvate is carboxylated to oxaloacetate in an ATP-dependent reaction in which the vitamin biotin (section 11.12) is the coenzyme. This reaction can also be used to replenish oxaloacetate in the citric acid cycle when intermediates have been withdrawn for use in other pathways, and is involved in the return of oxaloacetate from the cytosol to the mitochondrion in fatty acid synthesis — see Figure 5.26. Oxaloacetate then undergoes a phosphorylation reaction, in which it also loses carbon dioxide, to form phosphoenolpyruvate. The phosphate donor for this reaction is GTP as discussed in section 5.4.4, this provides regulation over the use of oxaloacetate for gluconeogenesis if citric acid cycle activity would be impaired. 

Unlike glycolysis, which occurs strictly in the cell cytosol, gluconeogen-esis involves a complex interaction between the mitochondrion and the cytosol. This interaction is necessitated by the irreversibility of the pyruvate kinase reaction, by the relative impermeability of the inner mitochondrial membrane to oxaloacetate, and by the specific mitochondrial location of pyruvate carboxylase. Compartmentation within the cell has led to the distribution of a number of enzymes (aspartate and alanine aminotransferases, and NAD -malate dehydrogenase) in both the mitochondria and the cytosol. In the classical situation represented by the rat, mouse, or hamster hepatocyte, the indirect "translocation" of oxaloacetate—the product of the pyruvate carboxylase reaction—into the cytosol is effected by the concerted action of these enzymes. Within the mitochondria oxaloacetate is converted either to malate or aspartate, or both. Following the exit of these metabolites from the mitochondria, oxaloacetate is regenerated by essentially similar reactions in the cytosol and is subsequently decarboxylated to P-enolpyruvate by P-enol-pyruvate carboxykinase. Thus the presence of a membrane barrier to oxaloacetate leads to the functioning of the malate-aspartate shuttle as an important element in gluconeogenesis. 

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