What You Need to Know

Key Concept Diagram

Enzymes are proteins that act as biological catalysts — they speed up reactions without being consumed in the process

The active site of an enzyme has a specific shape complementary to its substrate (lock-and-key model)

Enzyme activity is affected by temperature (optimal temperature, denaturation above it) and pH (each enzyme has an optimal pH)

The induced fit model refines the lock-and-key model: the active site slightly changes shape to better fit the substrate

Key Vocabulary

Enzyme

A biological catalyst — a protein that speeds up a specific chemical reaction without being used up

Active site

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

Denaturation

The irreversible change in an enzyme's shape caused by extreme heat or pH, preventing it from functioning

Substrate

The specific molecule(s) that an enzyme acts on and converts into products

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Why does enzyme activity decrease significantly above the optimum temperature?

The substrate is destroyed by high temperatures The enzyme becomes too small to bind the substrate The reaction happens too fast for products to form The enzyme is denatured — its active site changes shape permanently

Question 2

The enzyme pepsin works best at pH 2 (found in the stomach). What would happen to its activity at pH 7 (neutral)?

Activity would decrease significantly Activity would remain the same Activity would increase The enzyme would replicate itself

Question 3

According to the lock-and-key model, why can an enzyme only catalyse one type of reaction?

Enzymes are too large to interact with multiple substrates The active site has a specific complementary shape to one substrate only Each enzyme contains only one molecule of substrate Enzymes are destroyed after each reaction

Enzymes & Biological Reactions