Pyruvate to phosphoenolpyruvate

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

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... 

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. 

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-. 

These four reactions convert pyruvate to phosphoenolpyruvate (Step A, Fig. 11-8) and bypass the irreversible step in glycolysis that is catalyzed by pyruvate kinase (Step 10). 

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

Figure 5-25. The conversion of pyruvate to phosphoenolpyruvate (PEP). Follow the diagram by starting with the precursors alanine and lactate. OAA = oxaloacetate FA = fatty acid TG = triacylglycerol PDH = pyruvate dehydrogenase PC = pyruvate carboxylase PEPCK = phosphoenolpyruvate PK = pyruvate kinase PK-P = phos- phorylated pyruvate kinase.

Fig. 31.5. Conversion of pyruvate to phosphoenolpyruvate (PEP). Follow the shaded circled numbers on the diagram, starting with the precursors alanine and lactate. The first step is the conversion of alanine and lactate to pyruvate. Pyruvate then enters the mitochondria and is converted to OAA (circle 2) by pyruvate carboxylase. Pyruvate dehydrogenase has been inactivated by both the NADH and acetyl-CoA generated from fatty acid oxidation, which allows oxaloacetate production for gluconeogenesis. The oxaloacetate formed in the mitochondria is converted to either malate or aspartate to enter the cytoplasm via the malate/aspartate shuttle. Once in the cytoplasm the malate or aspartate is converted back into oxaloacetate (circle 3), and phosphoenolpyruvate carboxykinase will convert it to PEP (circle 4). The white circled numbers are alternate routes for exit of carbon from the mitochondrion using the malate/aspartate shuttle. OAA = oxaloacetate FA = fatty acid TG = triacylglycerol.

The conversion of pyruvate to phosphoenolpyruvate in gluconeogenesis takes place in two steps. The first step is the reaction of pyruvate and carbon dioxide to give oxaloacetate. This step requires energy, which is available from the hydrolysis of ATP. 

The successive carboxylation and decarboxylation reactions are both close to equilibrium (they have low values of their standard free energies) as a result, the conversion of pyruvate to phosphoenolpyruvate is also close to equilibrium (AG° = 2.1 kj mol = 0.5 kcalmoh ). A small in crease in the level of oxaloacetate can drive the equilibrium to the right, and a small increase in the level of phosphoenolpyruvate can drive it to the left. A concept well known in general chemistry, the law of mass action, relates the concentrations of reactants and products in a system at equilibrium. Changing the concentration of reactants or products causes a shift to reestablish equilibrium. A reaction proceeds to the right on addition of reactants and to the left on addition of products. 

Describe the enzymatic steps in the conversion of pyruvate to phosphoenolpyruvate. Name the enzymes, intermediates, and cofactors involved in these reactions. 

Pyruvate Kinase ATP pyruvate 2-O-phosphotransferase. A phosphotransferase that catalyzes reversibly the phosphorylation of pyruvate to phosphoenolpyruvate in the presence of ATP. It has four isozymes (L, R, Ml, and M2). Defidency of the enzyme results in hemolytic anemia. EC 2.7.1.40. [nih]... 

Under conditions unfavorable for transcarboxylation, CO2 fixation onto PEP becomes vitally important for bacteria, but the availability of PEP can limit the production of C4-compounds. Nevertheless, for this situation Nature supplied propionibacteria with the enzyme pyruvate-phosphate dikinase (Evans and Wood, 1968). The enzyme is induced by growth on lactate, it generates pyrophosphate and catalyzes the conversion of pyruvate to phosphoenolpyruvate in vivo. 

The equilibrium of pyruvate kinase is also strongly in the direction of glycolysis, because the immediate product of the reaction is enolpyruvate, which is chemically unstable. As shown in Figure 5.31, enolpyruvate undergoes a non-enzymic reaction to yield pyruvate. This means that little of the product of the enzymic reaction is available to undergo the reverse reaction in the direction of gluconeogenesis. The conversion of pyruvate to phosphoenolpyruvate in gluconeogenesis is discussed in section 5.7. 

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