Enzymes speed reactions without changing the energy released
- The idea
- Predict
- See it
- Why
- Check
The idea
“An enzyme changes how much energy a reaction releases, making it more favorable.”
Add an enzyme and a crawling reaction races, so it seems the enzyme added energy. It only lowered the hill the reaction has to climb.
The short version
The mistake: An enzyme changes how much energy a reaction releases, making it more favorable.
What's actually true: An enzyme lowers only the activation energy. The free-energy change, and so the energy released, stays the same.
- Activation energy is the climb to the unstable transition state at the top of the hill. An enzyme's active site lowers it, so more molecules get over each second.
- ΔG, the free-energy change, is the gap between products and reactants. The path between them doesn't matter, so an enzyme can't change it.
- The lower hill works both ways, so the reverse reaction speeds up too. Equilibrium comes sooner, at the same point.
- To run an uphill, endergonic reaction, a cell couples it to an energy-releasing one like ATP hydrolysis. The enzyme makes the link. ATP supplies the energy.
On the exam: An enzyme lowers the hill, never the drop. Same reactants and products, same energy released.