Glycolysis Regulation: Irreversible Steps, PFK-1 and Hexokinase

Glycolysis is controlled at its three irreversible steps, the ones with a large negative delta G. Those are the only places a cell can throttle flux, and the places gluconeogenesis must bypass.

The three control points

PFK-1 is the committed step

Glucose 6-phosphate can still go to glycogen or the pentose phosphate pathway. After PFK-1 the carbon is committed to glycolysis, so PFK-1 is the main rate-limiting step. High ATP and citrate signal an energy-rich cell and slow it. High AMP signals a low-energy cell and speeds it. Fructose 2,6-bisphosphate is the most potent activator and is raised by insulin.

Hexokinase vs glucokinase

• Hexokinase: most tissues, low Km, high affinity, inhibited by its product. Works even at low blood glucose.
• Glucokinase: liver and pancreatic beta cells, high Km, not inhibited by glucose 6-phosphate. It acts mainly after a meal, when glucose is high.

Liver vs muscle pyruvate kinase

Glucagon phosphorylates and inactivates liver pyruvate kinase, so the liver spares glucose for the brain during fasting. Muscle pyruvate kinase is not controlled this way.

Common traps

• Calling hexokinase the committed step. G6P has other fates, so PFK-1 is.
• Mixing PFK-1 with PFK-2. PFK-2 makes fructose 2,6-bisphosphate, the activator.
• Forgetting citrate. It inhibits PFK-1 because plenty of citrate means the citric acid cycle is full.

One-minute recap

• Three irreversible steps: hexokinase, PFK-1, pyruvate kinase.
• PFK-1 is committed and rate-limiting. ATP and citrate inhibit; AMP and F2,6BP activate.
• Glucokinase has high Km and is not product-inhibited.