10.pdf

IV. REGULATION OF GLUCONEOGENESIS The moment-to-moment regulation of gluconeogenesis is determined primarily by the circulating level of glucagon and by the availability of gluconeogenic substrates. In addition, slow adaptive changes in enzyme activity result from an alteration in the rate of enzyme...

IV. REGULATION OF GLUCONEOGENESIS The moment-to-moment regulation of gluconeogenesis is determined primarily by the circulating level of glucagon and by the availability of gluconeogenic substrates. In addition, slow adaptive changes in enzyme activity result from an alteration in the rate of enzyme synthesis or degradation or both. [Note: Hormonal control of the glucoregulatory system is presented in Chapter 23.] Figure 10.8 Covalent modification of pyruvate kinase results in inactivation of the enzyme. [Note: Only the hepatic isozyme is subject to covalent regulation.] OAA = oxaloacetate; PEP = phosphoenolpyruvate; cAMP = cyclic AMP; PPi = pyrophosphate; P = phosphate. A. Glucagon This peptide hormone from the a cells of pancreatic islets (see p. 313) stimulates gluconeogenesis by three mechanisms. 1. Changes in allosteric effectors: Glucagon lowers the level of fructose 2,6- bisphosphate, resulting in activation of fructose 1,6-bisphosphatase and inhibition of PFK-1, thus favoring gluconeogenesis over glycolysis (see Figure 10.5). [Note: See p. 99 for the role of fructose 2,6-bisphosphate in the regulation of glycolysis.] 2. Covalent modification of enzyme activity: Glucagon binds its G protein–coupled receptor (see p. 95) and, via an elevation in cyclic AMP (cAMP) level and cAMP- dependent protein kinase activity, stimulates the conversion of hepatic PK to its inactive (phosphorylated) form. This decreases the conversion of PEP to pyruvate, which has the effect of diverting PEP to the synthesis of glucose (Figure 10.8). 3. Induction of enzyme synthesis: Glucagon increases the transcription of the gene for PEP-carboxykinase, thereby increasing the availability of this enzyme as levels of its substrate rise during fasting. [Note: Glucocorticoids also increase expression of the gene, whereas insulin decreases expression.] B. Substrate availability The availability of gluconeogenic precursors, particularly glucogenic amino acids, significantly influences the rate of glucose synthesis. Decreased levels of insulin favor mobilization of amino acids from muscle protein and provide the carbon skeletons for gluconeogenesis. The ATP and NADH coenzymes-cosubstrates required for gluconeogenesis are primarily provided by the catabolism of fatty acids. C. Allosteric activation by acetyl coenzyme A Allosteric activation of hepatic pyruvate carboxylase by acetyl CoA occurs during fasting. As a result of increased lipolysis in adipose tissue, the liver is flooded with fatty acids (see p. 330). The rate of formation of acetyl CoA by β-oxidation of these fatty acids exceeds the capacity of the liver to oxidize it to CO2 and H2O. As a result, acetyl CoA accumulates and activates pyruvate carboxylase. [Note: Acetyl CoA inhibits the PDH complex (by activating PDH kinase; see p. 111). Thus, this single compound can divert pyruvate toward gluconeogenesis and away from the TCA cycle (Figure 10.9).] D. Allosteric inhibition by adenosine monophosphate Fructose 1,6-bisphosphatase is inhibited by AMP—a compound that activates PFK-1. This results in a reciprocal regulation of glycolysis and gluconeogenesis seen previously with fructose 2,6-bisphosphate (see p. 121). [Note: Elevated AMP, thus, stimulates pathways that oxidize nutrients to provide energy for the cell.] Figure 10.9 Acetyl coenzyme A (CoA) diverts pyruvate away from oxidation and toward gluconeogenesis. PDH = pyruvate dehydrogenase; TCA = tricarboxylic acid.  V. CHAPTER SUMMARY Gluconeogenic precursors include the intermediates of glycolysis and the tricarboxylic acid cycle, glycerol released during the hydrolysis of triacylglycerols in adipose tissue, lactate released by cells that lack mitochondria and by exercising skeletal muscle, and α-keto acids derived from the metabolism of glucogenic amino acids (Figure 10.10). Seven of the reactions of glycolysis are reversible and are used for gluconeogenesis in the liver and kidneys. Three reactions are physiologically irreversible and must be circumvented. These reactions are catalyzed by the glycolytic enzymes pyruvate kinase, phosphofructokinase, and hexokinase. Pyruvate is converted to oxaloacetate and then to phosphoenolpyruvate (PEP) by pyruvate carboxylase and PEP- carboxykinase. The carboxylase requires biotin and ATP and is allosterically activated by acetyl coenzyme A. PEP-carboxykinase requires GTP. The transcription of its gene is increased by glucagon and the glucocorticoids and decreased by insulin. Fructose 1,6-bisphosphate is converted to fructose 6- phosphate by fructose 1,6-bisphosphatase. This enzyme is inhibited by elevated levels of AMP and activated when ATP levels are elevated. The enzyme is also inhibited by fructose 2,6-bisphosphate, the primary allosteric activator of glycolysis. Glucose 6-phosphate is converted to glucose by glucose 6- phosphatase. This enzyme of the endoplasmic reticular membrane is required for the final step in gluconeogenesis as well as hepatic and renal glycogen degradation. Its deficiency results in severe, fasting hypoglycemia. Figure 10.10 Key concept map for gluconeogenesis. TCA = tricarboxylic acid. CoA = coenzyme A; cAMP = cyclic adenosine monophosphate; P = phosphate; PG = phosphoglycerate; BPG = bisphosphoglycerate. Study Questions Choose the ONE best answer. 10.1 Which one of the following statements concerning gluconeogenesis is correct? A. It is an energy-producing (exergonic) process. B. It is important in maintaining blood glucose during a fast. C. It is inhibited by a fall in the insulin-to-glucagon ratio. D. It occurs in the cytosol of muscle cells. E. It uses carbon skeletons provided by fatty acid degradation. Correct answer = B. During a fast, glycogen stores are depleted, and gluconeogenesis maintains blood glucose. Gluconeogenesis is an energy-requiring (endergonic) pathway (both ATP and GTP get hydrolyzed) that occurs in liver, with kidney becoming a major glucose- producing organ in prolonged fasting. It utilizes both mitochondrial and cytosolic enzymes. Gluconeogenesis is stimulated by a fall in the insulin/glucagon ratio. Fatty acid degradation yields acetyl coenzyme A (CoA), which cannot be converted to glucose. This is because there is no net gain of carbons from acetyl CoA in the tricarboxylic acid cycle, and the pyruvate dehydrogenase reaction is physiologically irreversible. It is the carbon skeletons of most amino acids that are gluconeogenic. 10.2 Which reaction in the diagram below would be inhibited in the presence of large amounts of avidin, an egg white protein that binds and sequesters biotin? Correct answer = C. Pyruvate is carboxylated to oxaloacetate by pyruvate carboxylase, a biotin-requiring enzyme. B (PDH complex) requires thiamine pyrophosphate, lipoic acid, FAD, coenzyme A, NAD; D (transaminase) requires pyridoxal phosphate; E (lactate dehydrogenase) requires NADH. 10.3 Which one of the following reactions is unique to gluconeogenesis? A. 1,3-Bisphosphoglycerate → 3-phosphoglycerate  B. Lactate → pyruvate C. Oxaloacetate → phosphoenolpyruvate D. Phosphoenolpyruvate → pyruvate Correct answer = C. The other reactions are common to both gluconeogenesis and glycolysis. 10.4 Use the chart below to show the effect of adenosine monophosphate (AMP) and fructose 2,6-bisphosphate on the listed enzymes of gluconeogenesis and glycolysis. Both fructose 2,6-bisphosphate and adenosine monophosphate downregulate gluconeogenesis through inhibition of fructose 1,6- bisphosphatase and upregulate glycolysis through activation of phosphofructokinase-1. This results in reciprocal regulation of the two pathways. 10.5 The metabolism of ethanol by alcohol dehydrogenase produces reduced nicotinamide adenine dinucleotide (NADH). What effect is the change in the NAD+/NADH ratio expected to have on gluconeogenesis? Explain. The increase in NADH as ethanol is oxidized will decrease the availability of oxaloacetate (OAA) because the reversible oxidation of malate to OAA by malate dehydrogenase of the tricarboxylic acid cycle is driven in the reverse direction by the high availability of NADH. Additionally, the reversible reduction of pyruvate to lactate by lactate dehydrogenase of glycolysis is driven in the forward direction by NADH. Thus, two important gluconeogenic substrates, OAA and pyruvate, are decreased as a result of the increase in NADH during ethanol metabolism. This results in a decrease in gluconeogenesis. 10.6 Given that acetyl coenzyme A cannot be a substrate for gluconeogenesis, why is its production in fatty acid oxidation essential for gluconeogenesis? Acetyl coenzyme A inhibits the pyruvate dehydrogenase complex and activates pyruvate carboxylase, pushing pyruvate to gluconeogenesis and away from oxidation.