3.1 Enzyme Structure

Enzymes are biological catalysts

Enzymes are (mostly) proteins that speed up chemical reactions by lowering their activation energy — the energy barrier a reaction must cross. They are catalysts, so they are not used up and can be reused again and again.

The active site

Each enzyme has an active site where a specific substrate binds. The fit is specific — often described as induced fit, because the active site changes shape slightly to grip the substrate. This specificity is why one enzyme usually catalyzes just one type of reaction. (Many enzyme names end in -ase.)

Key terms

Enzyme

A biological catalyst (usually a protein) that speeds reactions by lowering activation energy.

Active site

The region of an enzyme where the substrate binds and the reaction occurs.

Substrate

The reactant molecule an enzyme acts on.

Activation energy

The energy barrier a reaction must overcome; enzymes lower it.

Induced fit

The active site changing shape slightly to grip the substrate snugly.

Try an AP-style question

An enzyme's active site contains a positively charged R group that attracts its substrate. A mutation replaces it with a nonpolar R group. What is the most likely effect?

1. The enzyme binds the substrate more tightly and works faster
2. The substrate binds poorly, so the reaction rate drops
3. The mutation has no effect because the sequence is unchanged elsewhere
4. The enzyme now catalyzes the reverse reaction only