Gluconeogenesis pyruvate carboxylase activation

Bannister DW (1976a) The biochemistry of fatty liver and kidney syndrome. Biotin-mediated restoration of hepatic gluconeogenesis in vitro and its relationship to pyruvate carboxylase activity. Biochemical Journal 156, 167-73. 

The four key enzymes in gluconeogenesis (pyruvate carboxylase, PEP car-boxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase) are affected to varying degrees by allosteric modulators. For example, fructose-1,6-bisphos-phatase is activated by ATP and inhibited by AMP and fructose-2,6-bisphosphate. Acetyl-CoA activates pyruvate carboxylase. (The concentration of acetyl-CoA, a product of fatty acid degradation, is especially high during starvation.)... 

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. 

Acetyl-CoA is a potent allosteric effector of glycolysis and gluconeogenesis. It allosterically inhibits pyruvate kinase (as noted in Chapter 19) and activates pyruvate carboxylase. Because it also allosterically inhibits pyruvate dehydrogenase (the enzymatic link between glycolysis and the TCA cycle), the cellular fate of pyruvate is strongly dependent on acetyl-CoA levels. A rise in... 

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. 

Between meals when fatty acids are oxidized in the liver for energy, accumulating acetyl CoA activates pyruvate carboxylase and gluconeogenesis and inhibits PDH, thus preventing conversion of lactate and alanine to acetyl CoA. 

The FADHj and NADH are oxidized in the electron transport chain, providing ATP. In musde and adipose tissue, the acetyl CoA enters the citric acid cyde. In liver, the ATP may be used for gluconeogenesis, and the acetyl CoA (which cannot be converted to glucose) stimulates gluco-neogenesis by activating pyruvate carboxylase. 

Answer B. Acetyl CoA activates pyruvate carboxylase and gluconeogenesis during fasting. 

Gluconeogenesis is regulated at the level of pyruvate carboxylase (which is activated by acetyl-CoA) and FBPase-1 (which is inhibited by fructose 2,6-bisphosphate and AMP). 

Allosteric activation of hepatic pyruvate carboxylase by acetyl CoA occurs during fasting. As a result of excessive lipolysis in adipose tis sue, the liver is flooded with fatty acids (see p. 328). The rate of for mation of acetyl CoA by p-oxidation of these fatty acids exceeds the capacity of the liver to oxidize it to C02 and H20. As a result, acetyl CoA accumulates and leads to activation of pyruvate carboxylase. [Note Acetyl CoA inhibits pyruvate dehydrogenase (see p. 108). Thus, this single compound can divert pyruvate toward gluconeogenesis and away from the TCA cycle.]... 

During a fast, the liver is flooded with fatty acids mobilized from adipose tissue. The resulting elevated hepatic acetyl CoA produced primarily by fatty acid degradation inhibits pyruvate dehydrogenase (see p. 108), and activates pyruvate carboxylase (see p. 117). The oxaloacetate thus produced is used by the liver for gluconeogenesis rather than for the TCA cycle. Therefore, acetyl Co A is channeled into ketone body synthesis. 

Eight enzyme-catalyzed reactions are involved in the conversion of acetyl-CoA into fatty acids. The first reaction is catalyzed by acetyl-CoA carboxylase and requires ATP. This is the reaction that supplies the energy that drives the biosynthesis of fatty acids. The properties of acetyl-CoA carboxylase are similar to those of pyruvate carboxylase, which is important in the gluconeogenesis pathway (see chapter 12). Both enzymes contain the coenzyme biotin covalently linked to a lysine residue of the protein via its e-amino group. In the last section of this chapter we show that the activity of acetyl-CoA carboxylase plays an important role in the control of fatty acid biosynthesis in animals. Regulation of the first enzyme in a biosynthetic pathway is a strategy widely used in metabolism. 

Oxaloacetate, the product of the pyruvate carboxylase reaction, functions both as an important citric acid cycle intermediate in the oxidation of acetyl CoA and as a precursor for gluconeogenesis. The activity of pyruvate carboxylase depends on the presence of acetyl CoA so that more oxaloacetate is made when acetyl CoA levels rise. 

Thus pyruvate carboxylase generates oxaloacetate for gluconeogenesis but also must maintain oxaloacetate levels for citric acid cycle function. For the latter reason, the activity of pyruvate carboxylase depends absolutely on the presence of acetyl CoA the biotin prosthetic group of the enzyme cannot be carboxy-lated unless acetyl CoA is bound to the enzyme. This allosteric activation by acetyl CoA ensures that more oxaloacetate is made when excess acetyl CoA is present. In this role of maintaining the level of citric acid cycle intermediates, the pyruvate carboxylase reaction is said to be anaplerotic, that is filling up. ... 

In addition, hepatic fatty acid oxidation is also required to sustain gluconeogenesis. These fatty acids may be obtained from exogenous feeding or metabolism of fatty acids released from endogenous lipid stores. (3-Oxidation of fatty acids provides the acetyl-CoA needed to activate mitochondrial pyruvate carboxylase and the NADH used as the substrate in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase in the direction of gluconeogenesis (see Fig. 10-1). 

Pyruvate Carboxylase Pyruvate carboxylase catalyzes the car-boxylation of pyruvate to oxaloacetate - both the first committed step of gluconeogenesis from pyruvate and also an important anaplerotic reaction, permitting repletion of tricarboxylic acid cycle intermediates and hence fatty acid synthesis. The mammalian enzyme is activated aUosterically by acetyl CoA, which accumulates when there is a need for increased activity of pyruvate carboxylase to synthesize oxaloacetate to permit increased citric acid cycle activity or for gluconeogenesis (Attwood, 1995 Jitrapakdee and Wallace, 1999). 

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. 

Pyruvate carboxylase is an important enzyme in the gluconeogenic pathway converting pyruvate and bicarbonate into oxaloacetate. AcCoA is required for allosteric activation of the enzyme, and in the diabetic state where fatty acid oxidation is elevated, the high concentrations of AcCoA result in significant activation and gluconeogenesis overactivity. Phenylalkanoic... 

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