The human digestive system: Students should be able to use the ‘lock and key theory’ as a simplified model to explain enzyme action.

The idea

Enzymes are biological catalysts made of protein. Each enzyme has a specific active site whose shape is complementary to one substrate. In the lock and key model, the substrate is the key and the enzyme is the lock: only the correctly shaped key fits, forming an enzyme–substrate complex. The substrate is then broken down (or joined) and the products are released, leaving the enzyme unchanged and free to act again. For example, amylase has an active site complementary to starch, so it catalyses starch breakdown, but it cannot catalyse protein digestion. The model explains enzyme specificity and why enzymes are denatured by high temperature or extreme pH: the active site shape changes, so the substrate no longer fits. It is a simplified model because the active site is flexible and can adjust slightly as the substrate binds.

What earns the marks

• Enzymes are proteins that act as biological catalysts, speeding up reactions without being used up.
• The active site is a region of the enzyme with a shape complementary to a specific substrate.
• The substrate binds to the active site to form an enzyme–substrate complex.
• The reaction occurs and products are released; the enzyme is unchanged and can be reused.
• Specificity arises because only a substrate with a complementary shape can fit the active site.
• Denaturation by high temperature or extreme pH alters the active site shape so the substrate no longer fits.
• The model is simplified: real active sites are flexible and can change shape slightly on binding.

Common mistakes and how to put them right

• Saying the enzyme is used up or changed by the reaction.

  Put it right by stating the enzyme is a catalyst and is released unchanged.

• Confusing the substrate with the enzyme, for example calling the active site the 'key'.

  Put it right by stating the substrate is the key and the enzyme is the lock.

• Claiming any substrate can fit any enzyme.

  Put it right by stating the active site and substrate shapes must be complementary, which gives specificity.

Exam tips

• Name the enzyme, its substrate and its products in your answer, for example amylase, starch and maltose, to show the model in action.
• Use the phrase 'complementary shape' rather than 'same shape' when describing the fit between active site and substrate.
• When explaining denaturation, link the change in active site shape directly to the substrate no longer being able to bind.