3.2 Enzyme Catalysis

How catalysis happens

The substrate binds the active site to form an enzyme-substrate complex. The enzyme positions reactants, may stress bonds, and creates a favorable microenvironment — so the reaction happens faster. Then products are released and the enzyme is free to catalyze again.

Reaction rate

Rate rises as you add more substrate — up to a point. When every active site is busy, the enzyme is saturated and rate levels off (adding more enzyme is then needed to go faster). Enzymes stabilize the transition state, which is how they lower activation energy.

Key terms

Catalyst

A substance that speeds a reaction without being consumed.

Enzyme-substrate complex

The temporary structure formed when a substrate binds an enzyme's active site.

Saturation

When all active sites are occupied and rate no longer rises with more substrate.

Transition state

The high-energy state of a reaction; enzymes stabilize it to lower activation energy.

Product

The molecule formed by the reaction, released so the enzyme can be reused.

Try an AP-style question

How does an enzyme increase the rate of a reaction?

1. It supplies extra kinetic energy to the reactants
2. It makes the ΔG of the overall reaction more negative
3. It raises the free energy of the reactant molecules
4. It lowers the activation energy required for the reaction