Difficulty: Medium
Correct Answer: Fructose-2,6-bisphosphate
Explanation:
Introduction:Hepatic carbohydrate metabolism must balance glycolysis (glucose utilization) against gluconeogenesis (glucose production). A single allosteric regulator, fructose-2,6-bisphosphate (F-2,6-BP), provides powerful reciprocal control of the two opposing pathways in response to hormonal cues.
Given Data / Assumptions:
Concept / Approach:Identify the metabolite that directly and reciprocally regulates PFK-1 and fructose-1,6-bisphosphatase (FBPase-1). F-2,6-BP is the key: it activates PFK-1 (promoting glycolysis) and inhibits FBPase-1 (suppressing gluconeogenesis), thereby coordinating hepatic glucose flux with the hormonal state that also influences glycogen metabolism.
Step-by-Step Solution:
In fed state (insulin high) → PFK-2 active → F-2,6-BP rises → PFK-1 activated → glycolysis up; FBPase-1 inhibited → gluconeogenesis down.In fasting (glucagon high) → FBPase-2 active → F-2,6-BP falls → PFK-1 inhibited and FBPase-1 relieved → gluconeogenesis up.Verification / Alternative check:Hepatocyte studies show that altering F-2,6-BP levels rapidly shifts glycolytic/gluconeogenic flux independent of changes in total metabolite pools.
Why Other Options Are Wrong:
a) NAD+ is a redox cofactor, not a reciprocal flux signal here.c) Acetyl-CoA activates pyruvate carboxylase but does not directly regulate PFK-1/FBPase-1 pair.d) F-1,6-BP is a glycolytic intermediate and feed-forward activator of pyruvate kinase, not the reciprocal signal.e) PKA is a kinase enzyme (not a metabolite) acting upstream via phosphorylation cascades.Common Pitfalls:Confusing fructose-2,6-bisphosphate (regulator) with fructose-1,6-bisphosphate (intermediate); conflating hormonal kinases with small-molecule allosteric effectors.
Final Answer:Fructose-2,6-bisphosphate.
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