Needed doesn't mean used up
- The idea
- Predict
- See it
- Why
- Check
The idea
“If adding something makes a reaction go, it must be getting used up to make the product.”
Take the Mg2+ away and the reaction stops, so it looks like an ingredient. A tool is needed too, and a tool can be used again and again.
The short version
The mistake: If adding something makes a reaction go, it must be getting used up to make the product.
What's actually true: Reactants become product. A metal ion cofactor or helper protein comes out unchanged, and an added acid changes the conditions.
- A reactant, also called a substrate, is turned into product. That's why it gets used up.
- A cofactor like Mg2+ or Zn2+ binds the enzyme and helps it work. The ion leaves each reaction unchanged, so one ion helps make thousands of product molecules.
- Helper proteins aren't used up either. Transcription factors set RNA polymerase on the promoter, and none of them ends up in the RNA.
- Some additions change the conditions instead. Adding an acid lowers the pH, which can speed up or slow down an enzyme.
On the exam: Ask if the added thing ends up in the product. If it comes out unchanged, it's a helper.