Phosphoenolpyruvate from pyruvate

The Jirst indirect route in glucose synthesis involves the formation of phosphoenolpyruvate from pyruvate without the intervention of pyruvate kinase. This route is catalyzed by two enzymes. At first, pyruvate is converted into oxaloacetate. This reaction occurs in the mitochondria as the pyruvate molecules enter them, and is catalyzed by pyruvate carboxylase according to the scheme... 

The first step in the gluconeogenic direction involves the formation of phosphoenolpyruvate from pyruvate. Reversal of the pyruvate kinase reaction requires at least two ATP-to-ADP conversions. One means by which this is done is shown in figure 12.26. 

In the formation of phosphoenolpyruvate from pyruvate in the mesophyll cells (fig. 15.28), notice the energetics of the reaction. How does this process compare energetically with the formation of phosphoenolpyruvate from pyruvate in the start of gluco-neogenesis ... 

Finally, oxaloacetate is simultaneously decarboxylated and phosphorylated by phosphoenolpyruvate carboxykinase in the cytosol. The CO2 that was added to pyruvate by pyruvate carboxylase comes off in this step. Recall that, in glycolysis, the presence of a phosphoryl group traps the unstable enol isomer of pyruvate as phosphoenolpyruvate (Section 16.1.7). In gluconeogenesis, the formation of the unstable enol is driven by decarboxylation—the oxidation of the carboxylic acid to CO2—and trapped by the addition of a phosphate to carbon 2 from GTP. The two-step pathway for the formation of phosphoenolpyruvate from pyruvate has a AG° of + 0.2 kcal mol ( + 0.13 kj moP ) in contrast with +7.5 kcal mol ( + 31 kj mol ) for the reaction catalyzed by pyruvate kinase. The much more favorable AG° for the two-step pathway results from the use of a molecule of ATP to add a molecule of CO2 in the carboxylation step that can be removed to power the formation of phosphoenolpyruvate in the decarboxylation step. Decarboxylations often drive reactions otherwise highly endergonic. This metabolic motif is used in the citric acid cycle (Section IS.x.x), the pentose phosphate pathway (Section 17.x.x), and fatty acid synthesis (Section 22.x.x). 

Pyruvate is derived from phosphoenolpyruvate may be inter-converted into lactate, alanine, oxaloacetate. Formation ofacetyl-CoA from pyruvate is essentially irreversible... 

Pyruvate kinase the last enzyme in aerobic glycolysis, it catalyzes a substrate-level phosphorylation of ADP using the high-energy substrate phosphoenolpyruvate (PEP). Pyruvate kinase is activated by fructose 1,6-bisphosphate from the PFK-1 reaction (feedforward activation). 

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). 

Figure 10.4 The abolition of positive cooperativity on the binding of allosteric effectors to some enzymes. Note the dramatic increases in activity at low substrate concentrations on the addition of adenosine monophosphate to isocitrate dehydrogenase, of deoxycytosine diphosphate to deoxythymidine kinase, and of fructose 1,6-diphosphate to pyruvate kinase this shows how the activity may be switched on by an allosteric effector (PEP = phosphoenolpyruvate). [From J. A. Hathaway and D. E. Atkinson, J. Biol. Chem. 238,2875 (1963) R. Okazaki and A. Kornbcrg, J. Biol. Chem. 239,275 (1964) R. Haeckel, B. Hess, W. Lauterhom, and K.-H. Wurster, Hoppe-Seyler s Z. Physiol. Chem. 349, 699 (1968).]...

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. 

Phosphoenolpyruvate is formed from pyruvate by way of oxaloacetate through the action of pyruvate carboxylase and phosphoenolpyruvate carboxykinase. 

The Q pathway for the transport of CO2 starts in a mesophyll cell with the condensation of CO2 and phosphoenolpyruvate to form oxaloacetate, in a reaction catalyzed by phosphoenolpyruvate carboxylase. In some species, oxaloacetate is converted into malate by an NADP+-linked malate dehydrogenase. Malate goes into the bundle-sheath cell and is oxidatively decarboxylated within the chloroplasts by an NADP+-linked malate dehydrogenase. The released CO2 enters the Calvin cycle in the usual way by condensing with ribulose 1,5-bisphosphate. Pyruvate formed in this decarboxylation reaction returns to the mesophyll cell. Finally, phosphoenolpyruvate is formed from pyruvate by pyruvate-Pi dikinase. 

A third fate of pyruvate is its carboxylation to oxaloacetate inside mitochondria, the first step in gluconeogenesis. This reaction and the subsequent conversion of oxaloacetate into phosphoenolpyruvate bypass an irreversible step of glycolysis and hence enable glucose to be synthesized from pyruvate. The carboxylation of pyruvate is also important for replenishing intermediates of the citric acid cycle. Acetyl CoA activates pyruvate carboxylase, enhancing the synthesis of oxaloacetate, when the citric acid cycle is slowed by a paucity of this intermediate. 

Because of the very high price of ATP, reaction (5.7) must be coupled with a regenerating system, the transfer of phosphate to ADP starting from the enol phosphate of pyruvic acid (an easily accessible and inexpensive phosphate), catalysed by the enzyme pyruvate kinase (reaction (5.8). In the same flask are mixed glucose, phosphoenolpyruvate, hexokinase, pyruvate kinase, and a catalytic quantity of ATP (about 1% mol) and the system produces D-glucose 6-phosphate until the phosphoenolpyruvate runs out. The kinases are easily accessible and, if they are immobilized on an insoluble support (see Section 10.4.1), they are reusable a certain number of times. In this way glucose 6-phosphate can be easily prepared on a 250 g scale (Poliak et al. 1977). 

Although the carboxylation of the enol adduct of the aromatic ketones [Eqs. (28) and (29)] is a stoichiometric reaction, these reactions are related to biological CO2 fixation, where the phosphoenolpyruvate product from pyruvate is carboxylated into oxaloacetate ... 

Phosphoenolpyruvate can be produced from pyruvate in bacteria by reactions... 

The pathway involved is glycolysis, the conversion of glucose to pyruvate. The initial step is phosphorylation of glucose by the enzyme hexokinase and the final step is formation of pyruvate from phosphoenolpyruvate by pyruvate kinase (Chapter 11). The BPG is made from an intermediate compound, 1,3-bisphosphoglycerate, by the enzyme bisphosphoglycerate mutase. 

Phosphoenolpyruvates 26 are conveniently prepared from pyruvic acid and dimethyl-trimethylsilyl phosphite 24 through the Perkow reaction.23,24... 

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