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.
-All of the following statements about PFK-1 and F1,6BP are correct EXCEPT:

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.
-All of the following statements about PFK-1 and F1,6BP are correct EXCEPT:

Quizplus · Accepted Answer

11ecba2d_0f0b_f83e_a3bf_6f3d77fd2277_MD0008_00 Metabolic pathways consist of both reversible and irreversible reactions, each of which is catalyzed by an enzyme. For a reversible process, the relevant enzyme catalyzes (ie, decreases the activation energy of) both the forward and the reverse reactions. In irreversible processes, the reverse reaction cannot occur, so converting the product of an irreversible reaction back to its precursor requires a separate process known as a bypass reaction. Bypass reactions are catalyzed by distinct enzymes. Glycolysis, the conversion of glucose to pyruvate, consists of seven reversible reactions and three irreversible reactions. Gluconeogenesis converts pyruvate back to glucose, and for the reversible steps, it uses the same enzymes (eg, 3-phosphoglycerate kinase) as glycolysis to catalyze the reverse reactions. To bypass the irreversible steps of glycolysis, gluconeogenesis uses different enzymes than those used in glycolysis to catalyze alternate reactions. According to the passage, PFK-1 transfers a phosphate from ATP to F6P during glycolysis to form ADP and F1,6BP. The reverse reaction would transfer the phosphate back to ADP, regenerating ATP and F6P. However, this reaction is too thermodynamically unfavorable to occur. Instead, gluconeogenesis uses F1,6BPase to hydrolyze the phosphate group from F1,6BP, forming F6P and an inorganic phosphate (P_i), not ATP. Therefore, F1,6BPase does not catalyze the reverse reaction of PFK-1. (Choices A, B, and C) Figure 1 shows that both PFK-1 and F1,6BP have sigmoidal (S-shaped) activity curves, indicating that both enzymes are cooperative. Cooperativity occurs when the binding of a substrate to one active site increases the affinity of other active sites for the substrate. Therefore, by definition, cooperative enzymes must have multiple active sites. Educational objective: Metabolic pathways consist of both reversible and irreversible reactions. Opposing metabolic processes such as glycolysis and gluconeogenesis typically use the same enzymes for reversible reactions (going in opposite directions) but must use different enzymes to catalyze distinct reactions for the irreversible steps.

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