During prolonged fasting, which of the following liver enzymes has upregulated activity?

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11ecba2d_0eeb_9ec5_a3bf_058c517e3947_MD0008_00 The liver is responsible for maintaining blood glucose levels during periods of fasting. During fasting, glucose consumption and storage are downregulated in the liver and glucose synthesis is upregulated. Newly formed glucose is exported into the blood to keep levels constant. In the early stages of fasting, the liver synthesizes glucose by degrading glycogen (glycogenolysis). However, if fasting is prolonged, glycogen stores are depleted, and the liver must upregulate the synthesis of glucose from smaller precursor molecules (gluconeogenesis). Many of the reactions in gluconeogenesis are glycolysis reactions in reverse, with both processes using the same enzymes. However, a few glycolysis reactions are irreversible: phosphorylation of glucose by glucokinase, phosphorylation of fructose 6-phosphate by phosphofructokinase, and phosphate transfer from phosphoenolpyruvate by pyruvate kinase. The enzymes that catalyze these reactions are tightly regulated, and gluconeogenesis uses different enzymes to bypass irreversible steps. During prolonged fasting, the bypassing enzymes of gluconeogenesis are upregulated, whereas key glycolysis enzymes are downregulated. The first step in gluconeogenesis is carboxylation of pyruvate to form oxaloacetate. This step bypasses the irreversible pyruvate kinase reaction and is catalyzed by the enzyme pyruvate carboxylase. In the well-fed state, pyruvate carboxylase is downregulated so that pyruvate can be used in other pathways, but during prolonged fasting, pyruvate carboxylase activity is upregulated in the liver to increase the rate of gluconeogenesis. (Choice A) Glycogen synthase consumes glucose to produce glycogen. During fasting, the liver must synthesize glucose rather than consume it, so glycogen synthase activity is downregulated. (Choices C and D) Glucokinase and phosphofructokinase are tightly regulated enzymes of glycolysis that phosphorylate glucose and fructose 6-phosphate, respectively. During prolonged fasting, the activities of these enzymes are downregulated in the liver, whereas the gluconeogenesis enzymes-glucose 6-phosphatase and fructose 1,6-bisphosphatase-are upregulated to bypass these irreversible steps and increase gluconeogenesis. Educational objective: During periods of fasting, the liver helps maintain blood glucose levels. The liver initially synthesizes glucose by degrading glycogen. However, after glycogen stores are depleted in prolonged fasting, gluconeogenesis is upregulated, increasing the synthesis of glucose from precursors such as pyruvate. In the fasting state, the liver downregulates key enzymes of glycolysis and upregulates enzymes of gluconeogenesis.

During Prolonged Fasting, Which of the Following Liver Enzymes Has