5. Gluconeogenesis converts pyruvate to glucose and is the reverse of glycolysis

Question

5. Gluconeogenesis converts pyruvate to glucose and is the reverse of glycolysis. a. How many steps occur in gluconeogenesis? b. Many of the enzymes used in each pathway are the same. Explain how the sign of Gibbs free energy can change so a reaction is spontaneous in both direction c. Write the net reaction equation for gluconeogenesis. O d. Under what physiologic conditions would gluconeogenesis be activated. 6. a. Briefly describe the Cori cycle b. In which tissue does the anabolic phase occur. c. Can the cycle be run indefinitely? Explain

Explanation / Answer

5. a. Eleven

b. While most steps in gluconeogenesis are the reverse of those found in glycolysis, three regulated and strongly exergonic reactions are replaced with more kinetically favorable reactions. Hexokinase/glucokinase, phosphofructokinase, and pyruvate kinase enzymes of glycolysis are replaced with glucose-6-phosphatase, fructose-1,6-bisphosphatase, and PEP carboxykinase. This system of reciprocal control allows glycolysis and gluconeogenesis to inhibit each other and prevent the formation of a futile cycle.

c.2 Pyruvate + 4 ATP + 2 GTP + 2 NADH+ + 2 H+ + 4 H2O Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+

d. When blood glucose increases then glucogenesis starts.

6. a. It refers to the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is converted back to lactate.

b. Liver

c. Since two molecules of lactate are required for the synthesis of one molecule of glucose, the net cost is 2 x 3 = 6 high energy bonds per molecule of glucose.

Conversely, the glycolytic leg of the cycle produces only 2 ATP per molecule of glucose.
Therefore, more energy is required to produce glucose from lactate than that obtained by anaerobic glycolysis in extrahepatic tissues. This explains why the Cori cycle cannot be sustained indefinitely.

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