Conversion to phosphoenolpyruvate

This last pair of reactions is complicated by the fact that p)rruvate carboxylase is found in the mitochondria, whereas phosphoenolp)mivate carboxykinase is found in the cytoplasm. As we will see in Chapters 22 and 23, mitochondria are organelles in which the final oxidation of food molecules occurs and large amounts of ATP are produced. A complicated shuttle system transports the oxaloacetate produced in the mitochondria through the two mitochondrial membranes and into the cytoplasm. There, phosphoenolp)mivate carboxykinase catalyzes its conversion to phosphoenolpyruvate. 

Because not all amino acids are glucogenic, and because entry of carbon-skeleton metabolites into the Krebs cycle results in the release of carbon atoms as carbon dioxide during conversion to phosphoenolpyruvate, greater than 3 g of protein is required to make 1 g of glucose. 

For cytoplasmic conversion to phosphoenolpyruvate, the oxaloacetate must leave the matrix as malate. Since the conversion to malate goes against the normal direction of Krebs cycle flux, by involving reduction with NADH, it is more probable that the conversion will occur in the mitochondria. 

Figure 5.13 Conversion of pyruvate to oxaloacetate and then to phosphoenolpyruvate.

Figure 6-8. Conversion of phosphoenolpyruvate to glucose during gluconeogenesis. Except for the indicated enzymes that are needed to overcome irreversible steps of glycolysis, all other steps occur by the reverse reactions catalyzed by the same enzymes as those used in glycolysis.

Figure 13.8 A 25-atom quantum subsystem embedded in an 8863-atom classical system to model the catalytic step in the conversion of D-2-phosphoglycerate to phosphoenolpyruvate by enolase. What factors influence the choice of where to set the boundary between the QM and MM regions Alhambra and co-workers found, using variational transition-state theory with a frozen MM region that was selected from a classical trajectory so as to make the reaction barrier and thermochemistry reasonable, that the breaking and making bond lengths were 1.75 and 1.12 A, respectively, for H, but 1.57 and 1.26 A, respectively, for D...

Conversion of Pyruvate to Phosphoenolpyruvate Requires Two Exergonic Reactions... 

Vertebrates cannot convert fatty acids, or the acetate derived from them, to carbohydrates. Conversion of phosphoenolpyruvate to pyruvate (p. 532) and of pyruvate to acetyl-CoA (Fig. 16-2) are so exergonic as to be essentially irreversible. If a cell cannot convert acetate into phosphoenolpyruvate, acetate cannot serve as the starting material for the gluconeogenic pathway, which leads from phosphoenolpyruvate to glucose (see Fig. 15-15). Without this capacity, then, a cell or organism is unable to convert fuels or metabolites that are degraded to acetate (fatty acids and certain amino acids) into carbohydrates. 

The first "roadblock" to overcome in the synthesis of glucose from pyruvate is the irreversible conversion in glycolysis of pyruvate to phosphoenolpyruvate (PEP) by pyruvate kinase. In gluconeogenesis, pyruvate is first carboxylated by pyruvate carboxylase to oxaloacetate (OAA), which is then converted to PEP by the action of PEP-carboxykinase (Figure 10.3). 

Gluconeogenesis Consumes ATP Conversion of Pyruvate to Phosphoenolpyruvate Requires Two High Energy Phosphates Conversion of Phosphoenolpyruvate to Fructose-1,6-bisphosphate Uses the Same Enzymes as Glycolysis... 

The carboxylation of pyruvate supplies a significant portion of the thermodynamic push for the next step in the sequence. This is because the free energy change for decarboxylation of /3-keto carboxylic acids such as oxaloacetate is large and negative. The oxaloacetate formed from pyruvate by carboxylation is converted to phosphoenolpyruvate in a reaction catalyzed by phosphoenolpyruvate carboxyki-nase. In many species, including mammals, this reaction involves a GTP-to-GDP conversion. 

If we add the equations for the reactions catalyzed by pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and nucleoside diphosphate kinase, we obtain the overall reaction for conversion of pyruvate to phosphoenolpyruvate. 

Pathway of C02 in Gluconeogenesis In the first bypass step of gluconeogenesis, the conversion of pyruvate to phosphoenolpyruvate (PEP), pyruvate is carboxylated by pyruvate carboxylase to oxaloacetate, which is subsequently decarboxylated to PEP by PEP carboxykinase (Chapter 14). Because the addition of C02 is directly followed by the loss of C02, you might expect that in tracer experiments, the 14C of 14C02 would not be incorporated into PEP, glucose, or any intermediates in gluconeogenesis. 

Elimination reactions occur in living organisms also. One important example is the conversion of 2-phosphoglycerate to phosphoenolpyruvate during the metabolism of glucose ... 

Figure 20-4. Biochemical pathways for gluconeogenesis in the liver. Alanine, a major gluconeogenic substrate, is used to synthesize oxaloacetate. The carbon skeletons of glutamine and other glucogenic amino acids feed into the TCA cycle as a-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate and thus also provide oxaloacetate. Conversion of oxaloacetate to phosphoenolpyruvate and ultimately to glucose limits the availability of oxaloacetate for citrate synthesis and thus greatly diminishes flux through the initial steps of the TCA cycle (dashed lines). Concurrent P-oxidation of fatty acids provides reducing equivalents (NADH and FADH2) for oxidative phosphorylation but results in accumulation of acetyl-CoA.

Step 10, the final step in glycolysis, is the irreversible conversion of phosphoenolpyruvate to pyruvate, catalyzed by pyruvate kinase. This is the second energy-yielding step in the glycolytic pathway, and produces ATP Mg2+ is required here, too. 

Step A, the conversion of pyruvate to phosphoenolpyruvate, is accomplished by a circuitous process commencing with pyruvate entering the mitochondrion, which for gluconeogenesis to occur must be in a high-energy state. Under these conditions, the mitochondrial enzyme pyruvate carboxylase catalyzes the conversion of pyruvate to oxaloacetate-. 

Finally, transport can also be driven by the conversion of intracellular substrate to another chemical form. For example, in the case of nucleoside drugs, conversion to the corresponding nucleotides by appropriate kinases may be the limiting factor in cellular uptake and activation. The same principle applies to sulfation, glu euro nidation, prodrug activations, or other metabolic processes that provide a removal of the transported species from the transportable (free) internal pool. In some cases, transport is directly coupled to substrate modification, as in the uptake of sugars into bacterial cells by phosphoenolpyruvate (PEP)-coupled phosphorylation systems. 

Conversion of pyruvate to phosphoenolpyruvate (Figure 5-25) -In the liver, pyruvate is converted to phosphoenolpyruvate. 

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