Enzymes don't get used up
- The idea
- Predict
- See it
- Why
- Check
The idea
“The reaction slowed down because the enzyme got used up.”
The enzyme is in the reaction, and most things in a reaction get used up. So when the rate drops, it's easy to blame the enzyme.
The short version
The mistake: The reaction slowed down because the enzyme got used up.
What's actually true: An enzyme leaves each reaction unchanged and gets reused. When the rate drops, substrate is running low or conditions changed.
- An enzyme is a catalyst. It speeds up a reaction without being changed. It binds substrate at its active site, lowers the activation energy, and releases the products.
- So one enzyme molecule can run the reaction thousands of times. The substrate is what runs out.
- A reaction that levels off with plenty of enzyme left has usually run out of substrate. Adding more restarts it.
- A pH or temperature change can also slow or stop an enzyme by changing its active site's shape. It still isn't used up.
On the exam: Enzymes are reused. When a rate drops, ask whether the substrate ran low or the conditions changed.