Enzymes in Depth

Explain how enzymes speed up reactions and why temperature and pH change their activity.

About 3 min read
  • Describe enzymes as biological catalysts
  • Explain how the active site works
  • Explain the effect of temperature and pH
  • Interpret enzyme activity graphs
Before you start

You know that proteins have specific shapes, and that a catalyst speeds up a reaction without being used up.

Biological catalysts

Enzymes are proteins that speed up chemical reactions in living things. They lower the activation energy, so reactions can happen quickly at body temperature. An enzyme is not used up, so one enzyme molecule can catalyse many reactions.

Definition: enzyme

A protein that acts as a biological catalyst.

Definition: active site

The part of an enzyme where the substrate binds.

Specificity

The active site has a shape that fits a particular substrate. When the substrate binds, an enzyme-substrate complex forms. In the induced fit model, the active site changes shape slightly to fit more closely. The products are released and the enzyme can be used again.

Temperature and pH

Raising temperature increases kinetic energy, so more collisions occur between enzymes and substrates. Above the optimum temperature, bonds holding the enzyme's shape break and the active site changes. The enzyme is denatured. A pH far from the optimum can denature an enzyme in the same way.

Explain a temperature graph

An enzyme's rate rises up to 40°C, then falls quickly. Explain this.

Up to 40°C, particles move faster and successful collisions increase.

Above 40°C, the active site changes shape and the substrate no longer fits.

Answer: The optimum is about 40°C, and the enzyme denatures above it.

Denatured is not killed

Enzymes are molecules, so they are not alive. Say that they are denatured.

Exam focus
Say it like this“The active site changes shape, so the substrate no longer fits.”
Not like this“The enzyme is killed by heat.” — Link denaturing to the active site.
Common mistakeSaying that the substrate and active site are the same shape instead of complementary.

Practice

Name the substance that binds to an enzyme.
The substrate.
Why is an enzyme specific to one substrate?
Its active site has a complementary shape to that substrate.
Why can a small amount of enzyme catalyse many reactions?
The enzyme is not used up and can bind new substrate after products leave.
Going further
Pepsin works in the stomach at about pH 2. Predict and explain what happens if it moves to pH 8.
Its activity falls because the pH disrupts bonds in the protein, changing the active site, so the substrate no longer fits.

Quick check

Enzymes quick check

What type of molecule is an enzyme?

Its folded chain of amino acids creates the specific active site.

What does an enzyme lower?

Less energy is needed for particles to react, so reactions go faster.

What happens to a denatured enzyme?

The substrate can no longer bind, so the enzyme cannot catalyse the reaction.
Key takeaways
  • Enzymes are biological catalysts made of protein.
  • The active site binds a specific substrate.
  • Temperature and pH affect enzyme activity.
  • Denaturing changes the active site and stops it working.
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