Q.32 Which one of the following molecules is an allosteric activator of phosphofructokinase–1?
(A) Fructose 1,6–bisphosphate (B) Fructose 2,6–bisphosphate
(C) Glucose 6–phosphate (D) Citrate
Answer: (B) Fructose 2,6-bisphosphate
Fructose 2,6-bisphosphate (F2,6BP) is the most potent allosteric activator of phosphofructokinase-1 (PFK-1), the rate-limiting enzyme of glycolysis. F2,6BP binds at a distinct allosteric site, increasing PFK-1’s affinity for fructose-6-phosphate (F6P) and counteracting ATP inhibition, thus promoting glycolytic flux during fed states.
Option Analysis
(A) Fructose 1,6-bisphosphate: Incorrect—product of PFK-1 reaction; acts as feedforward activator in some organisms but primarily substrate product, not allosteric regulator in mammals.
(B) Fructose 2,6-bisphosphate: Correct—produced by PFK-2; binds allosteric site, stabilizes R-state, decreases K_m for F6P, overrides ATP inhibition.
(C) Glucose 6-phosphate: Incorrect—inhibits hexokinase (first glycolytic enzyme), not PFK-1 activator.
(D) Citrate: Incorrect—potent allosteric inhibitor of PFK-1; signals high TCA cycle activity, downregulates glycolysis.
Introduction: Phosphofructokinase-1 Allosteric Activator
Phosphofructokinase-1 (PFK-1) regulation by allosteric activators controls glycolytic flux in response to cellular energy needs. Fructose 2,6-bisphosphate emerges as the key activator distinguishing this MCQ, critical for biochemical engineering and molecular biology exam preparation.
PFK-1 Regulation Summary Table
| Molecule | Effect on PFK-1 | Mechanism | Physiological Role |
|---|---|---|---|
| Fructose 2,6-BP | Potent activator | ↑F6P affinity, ↓ATP inhibition | Fed state signaling |
| Fructose 1,6-BP | Weak activator | Product activation (feedforward) | Glycolytic intermediate |
| Glucose 6-phosphate | No effect | Inhibits hexokinase instead | Early pathway control |
| Citrate | Strong inhibitor | Stabilizes T-state, ↓V_max | TCA cycle feedback |
Molecular Mechanism
F2,6BP Action: Binds nucleotide-binding allosteric site (distinct from ATP substrate site), induces conformational change from inactive T-state to active R-state, increases catalytic efficiency ~10-600 fold depending on conditions.
Hormonal Control: Insulin → ↑PFK-2 activity → ↑F2,6BP → ↑glycolysis; glucagon → ↓F2,6BP → ↓glycolysis.
This regulatory node integrates energy status (ATP/AMP), carbon availability (F6P), and hormonal signals, making PFK-1 the primary glycolytic control point in mammalian metabolism.
