Regulation
Three enzymes catalyze the irreversible steps of glycolysis and are the main regulatory points: hexokinase (step 1), PFK-1 (step 3), and pyruvate kinase (step 10). Of these, PFK-1 is the most heavily regulated and is called the master switch of glycolysis.
PFK-1: The Master Switch
PFK-1 is allosterically controlled by multiple effectors:
| Effector | Effect on PFK-1 | Meaning |
|---|---|---|
| ATP | Inhibits | ”Cell has enough energy, slow down” |
| AMP / ADP | Activates | ”Cell is energy-starved, speed up” |
| Citrate | Inhibits | ”TCA cycle is backed up, do not send more pyruvate” |
| Fructose-2,6-bisphosphate (F-2,6-BP) | Activates strongly | Hormonal signal |
| H+ (low pH) | Inhibits | Slows glycolysis during lactic acid accumulation |
Fructose-2,6-Bisphosphate - The Hormonal Link
F-2,6-BP is not a glycolysis intermediate but a dedicated regulator. It is made by PFK-2 (a different enzyme from PFK-1) from F6P. PFK-2 has two activities: a kinase (makes F-2,6-BP) and a phosphatase (removes it). Insulin and glucagon control which activity dominates:
- Insulin (fed state): dephosphorylates PFK-2 → kinase activity dominates → F-2,6-BP goes UP → PFK-1 ACTIVATED → glycolysis speeds up.
- Glucagon (fasted state): phosphorylates PFK-2 → phosphatase activity dominates → F-2,6-BP goes DOWN → PFK-1 inhibited → glycolysis slows. Simultaneously, gluconeogenesis is activated.
This reciprocal regulation at the same control point (PFK-1) lets a single hormone shift metabolism between storage (fed) and mobilization (fasted).
Hexokinase and Pyruvate Kinase
- Hexokinase: inhibited by its product G6P (end-product inhibition). Glucokinase (liver version) has higher Km and is NOT inhibited by G6P, so the liver continues to phosphorylate glucose at high blood-glucose levels.
- Pyruvate kinase: activated by feed-forward signal F-1,6-BP (upstream intermediate); inhibited by ATP and alanine. In liver, pyruvate kinase is also phosphorylated (inactivated) by glucagon-triggered PKA, reducing pyruvate production during fasting.
Which molecule is the strongest allosteric activator of PFK-1?
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Fructose-2,6-bisphosphate (F-2,6-BP). It is not an intermediate of glycolysis but a dedicated regulator made by PFK-2. Insulin increases F-2,6-BP (activates glycolysis); glucagon decreases it (activates gluconeogenesis). AMP also activates PFK-1 but F-2,6-BP has a much larger effect in most tissues.
How does glucagon reduce glycolysis in the liver?
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Glucagon raises cAMP, activating PKA. PKA phosphorylates PFK-2, shifting it from kinase to phosphatase mode - F-2,6-BP levels drop. PFK-1 is less activated, slowing glycolysis. PKA also phosphorylates pyruvate kinase (inactivating it) and glycogen phosphorylase kinase (activating glycogen breakdown). Net: glucose is released into the blood rather than burned in the liver.
Why is hexokinase inhibited by glucose-6-phosphate, but glucokinase is not?
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Hexokinase exists in most cells, which need glucose for routine fuel and should not waste glucose when downstream is backed up. Product inhibition (G6P feedback) stops phosphorylation when glycolysis is slow. Glucokinase is liver-specific, designed to take up excess glucose after a meal - it should NOT be inhibited by G6P so the liver can continue sponging up glucose into glycogen and other pathways when blood glucose is high.