Passage
Glycolysis and gluconeogenesis are tightly regulated, opposing metabolic pathways that help control blood glucose levels. Glycolysis converts glucose to two pyruvate molecules, whereas gluconeogenesis consumes 6 ATP equivalents to convert two pyruvate molecules back to glucose. When glycolysis is upregulated, gluconeogenesis is downregulated, and vice versa.As shown in Figure 1, glycolysis and gluconeogenesis in the liver are largely regulated by the allosteric action of the small molecule fructose-2,6-bisphosphate (F2,6BP) on the enzymes phosphofructokinase-1 (PFK-1) and fructose-1,6-bisphosphatase (F1,6BPase) . PFK-1 is a kinase that uses ATP to phosphorylate fructose-6-phosphate (F6P) in an irreversible step of glycolysis, forming fructose-1,6-bisphosphate (F1,6BP) and ADP. During gluconeogenesis, F1,6BPase removes the phosphate group by hydrolysis.
Figure 1 Activities of (A) PFK-1 and (B) F1,6BPase in the presence (solid lines) and absence (dashed lines) of F2,6BPA bifunctional enzyme that contains a phosphofructokinase-2 (PFK-2) domain and a fructose-2,6-bisphosphatase (F2,6BPase) domain controls F2,6BP levels in the liver. The PFK-2 domain converts F6P to F2,6BP, and the F2,6BPase domain converts F2,6BP back to F6P. When blood glucose levels are low, the enzyme becomes phosphorylated. This phosphorylation event simultaneously activates the F2,6BPase domain and inactivates the PFK-2 domain. Under high blood glucose conditions, the enzyme becomes dephosphorylated, activating the PFK-2 domain and inactivating the F2,6BPase domain.
-Which metabolic process most likely provides the energy necessary for sustained gluconeogenesis?

Question

Passage
Glycolysis and gluconeogenesis are tightly regulated, opposing metabolic pathways that help control blood glucose levels. Glycolysis converts glucose to two pyruvate molecules, whereas gluconeogenesis consumes 6 ATP equivalents to convert two pyruvate molecules back to glucose. When glycolysis is upregulated, gluconeogenesis is downregulated, and vice versa.As shown in Figure 1, glycolysis and gluconeogenesis in the liver are largely regulated by the allosteric action of the small molecule fructose-2,6-bisphosphate (F2,6BP) on the enzymes phosphofructokinase-1 (PFK-1) and fructose-1,6-bisphosphatase (F1,6BPase) . PFK-1 is a kinase that uses ATP to phosphorylate fructose-6-phosphate (F6P) in an irreversible step of glycolysis, forming fructose-1,6-bisphosphate (F1,6BP) and ADP. During gluconeogenesis, F1,6BPase removes the phosphate group by hydrolysis.

Figure 1 Activities of (A) PFK-1 and (B) F1,6BPase in the presence (solid lines) and absence (dashed lines) of F2,6BPA bifunctional enzyme that contains a phosphofructokinase-2 (PFK-2) domain and a fructose-2,6-bisphosphatase (F2,6BPase) domain controls F2,6BP levels in the liver. The PFK-2 domain converts F6P to F2,6BP, and the F2,6BPase domain converts F2,6BP back to F6P. When blood glucose levels are low, the enzyme becomes phosphorylated. This phosphorylation event simultaneously activates the F2,6BPase domain and inactivates the PFK-2 domain. Under high blood glucose conditions, the enzyme becomes dephosphorylated, activating the PFK-2 domain and inactivating the F2,6BPase domain.
-Which metabolic process most likely provides the energy necessary for sustained gluconeogenesis?

Quizplus · Accepted Answer

11ecba2d_0f0f_2c91_a3bf_d9326db20235_MD0008_00 Gluconeogenesis is an anabolic process that synthesizes glucose from smaller precursors such as pyruvate. This pathway is used to increase blood glucose levels during times of fasting. According to the passage, gluconeogenesis requires energy input in the form of 6 ATP equivalents (ATP or GTP) for each glucose molecule synthesized. The energy required to produce these ATP equivalents must be provided by a catabolic process. Because the result of gluconeogenesis is the synthesis of carbohydrates, it is unlikely that the necessary energy is provided by carbohydrate degradation. Instead, the needed ATP equivalents are typically produced by fatty acid oxidation. Fatty acid degradation to acetyl-CoA yields NADH and FADH₂, both of which can enter the electron transport chain to produce ATP. In addition, the resulting acetyl-CoA can enter the citric acid cycle to produce more NADH and FADH₂, as well as GTP (an ATP equivalent). Therefore, of the choices given, fatty acid oxidation most likely provides the energy needed for sustained gluconeogenesis. (Choice B) Glycogenolysis produces glucose, so when glycogen is present, gluconeogenesis is not required. Accordingly, gluconeogenesis generally occurs after glycogen stores have been depleted, so glycogenolysis would not be able to provide the needed energy. (Choice C) In most eukaryotes, fermentation is the conversion of pyruvate to lactate. The resulting lactate can enter gluconeogenesis after being converted back to pyruvate but does not provide the needed energy. (Choice D) The pentose phosphate pathway converts glucose and glycolysis intermediates to ribose-5-phosphate. This process neither consumes nor produces any ATP, so it does not provide the energy for gluconeogenesis. Educational objective: Gluconeogenesis is an anabolic process that requires energy input from ATP equivalents. The necessary energy is provided by catabolic processes, most commonly by fatty acid oxidation.

Passage Glycolysis and Gluconeogenesis Are Tightly Regulated, Opposing Metabolic Pathways That