Enzyme-controlled Reactions (WJEC GCSE Biology: Combined Science)

• Enzymes control chemical reactions, meaning that they convert substrate molecules into products
• The conversion of a substrate into a product occurs when the substrate attaches to a region of the enzyme known as the active site
• Enzymes are specific to one particular substrate; this is because the shape of the active site is complementary to the substrate

Enzyme specificity diagram

Enzymes are specific to one particular substrate as the active site is complementary in shape to the substrate

The 'lock and key' model

• This model of enzyme action states that the substrate fits into the active site of an enzyme very precisely, like a key going into a lock

Lock and key model diagram

The active site and the substrate are complementary to each other, meaning that the substrate fits into the active site like a key into a lock

Molecular collisions

• During an enzyme-catalysed reaction the following sequence of events occurs:
  • Enzymes and substrates randomly move about in solution
  • Enzymes collide with their complementary substrate molecules and the reaction occurs
  • The product forms and is then released from the active site
  • The enzyme is unchanged and will go on to catalyse further reactions

• Enzyme activity will increase or decrease depending on the number of molecular collisions
  • When more collisions occur between enzyme and substrate molecules the rate of reaction will increase and more product will be produced
  • Fewer enzyme and substrate collisions will slow the rate of reaction and result in less product

Higher Tier Only

• When an enzyme and complementary substrate collide the substrate binds to the active site of the enzyme; this forms an enzyme-substrate complex
• The enzyme-substrate complex is a temporary structure; after the enzyme catalyses the reaction then the products are released and the active site is free to form another enzyme-substrate complex

Enzyme-substrate complex diagram