Control Points in Glycolysis and the Committed Step
Only a few reactions in a pathway are far from equilibrium, and those are where regulation acts. In glycolysis the committed step responds to signals about how much energy the cell already has, which is why the pathway speeds up when demand rises.
What a learner can do afterwards
- Identifies the irreversible steps of the pathway and explains why control sits there.
- Predicts the flux response to a rise in the cell's energy charge.
- Explains why regulating a near-equilibrium step would do little.
1 · Read
Glycolysis splits one glucose into two pyruvate and banks a small net gain of ATP plus NADH. Most of its ten steps sit near equilibrium, so they run freely in either direction as conditions shift. Three steps are effectively irreversible under cell conditions. Those are the valves where control lives. The committed step uses the enzyme phosphofructokinase.
Cells tune the pathway to their energy charge, the balance of ATP against ADP and AMP. When ATP is plentiful it slows phosphofructokinase down. When AMP rises it signals shortage and speeds the enzyme up. Citrate, the first product of the next cycle, also damps the enzyme when downstream capacity is full. So the pathway speeds up when demand rises.
Control sits at the irreversible steps because regulating a near equilibrium step achieves little. Such steps adjust on their own and other steps compensate. A step counts as effectively irreversible when its free energy change is strongly negative under real cellular concentrations. Reversing it would need a different route with different enzymes.
Picture a sprinter whose muscles burn ATP fast. AMP climbs, ATP falls, and phosphofructokinase speeds up, so flux through glycolysis rises. Later, at rest with full ATP and plenty of citrate, the same enzyme slows down. Same pathway, different pace, set by the energy charge.
Control lives at the irreversible steps, and phosphofructokinase tunes glycolysis to the energy charge of the cell.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.