# Enzyme Catalysis and Active Site

> Enzymes are protein catalysts. A substrate binds in the active site, the enzyme makes the reaction easier, and the products leave so the enzyme can work again.

Web page: https://cellnaut.com/topics/enzyme-catalysis-active-site

![Teaching illustration: Enzyme Catalysis and Active Site](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/enzyme-catalysis-active-site-teaching.jpg)

## Labelled parts

1. **Enzyme surface**: The folded protein; its three-dimensional shape determines what it does.
2. **Active site**: The pocket where the substrate binds and the reaction happens.
3. **Substrate**: The molecule the enzyme acts on.
4. **Catalytic residues**: Amino acids in the site that directly help break or form bonds.
5. **Noncovalent interactions**: Hydrogen bonds and charge attractions hold the substrate in place without permanent bonds.
6. **Enzyme-substrate complex**: The temporary complex in which the reaction is speeded up.
7. **Product release**: Products no longer fit the site well, so they leave.
8. **Product molecules**: The new molecules formed; the enzyme itself is not used up.

## Key points

- The active site fits particular substrates, which is why enzymes are specific.
- Enzymes lower the activation energy, so the reaction runs faster without being used up.
- Temperature and pH change an enzyme's shape, so they affect its activity.

## Check yourself

**1. What does an enzyme do to the activation energy of a reaction?**

- A. Raises it
- B. Lowers it
- C. Removes the need for substrates
- D. Changes the products into reactants

**Answer:** B. Lowers it - A lower energy barrier means the reaction runs faster.

**2. What happens to an enzyme after the reaction?**

- A. It is used up
- B. It is released unchanged and can be reused
- C. It becomes the product
- D. It turns into DNA

**Answer:** B. It is released unchanged and can be reused - Catalysts are not consumed.

## Related lessons

- [Mitochondria and Chloroplasts](https://cellnaut.com/learn/energy-organelles)

## See it in 3D

- [Animal Cell](https://cellnaut.com/studio/animal)