The human digestive system: Enzymes catalyse specific reactions in living organisms due to the shape of their active site.

The idea

Enzymes are biological catalysts made of protein. Each enzyme has a region called the active site, whose shape is complementary to the shape of its substrate. The substrate binds to the active site, forming an enzyme-substrate complex, and the reaction is catalysed to form products. Because the active site shape is specific, each enzyme catalyses only one reaction or a small group of similar reactions. This explains why digestive enzymes such as amylase, protease and lipase act on different nutrients. If the enzyme is denatured, for example by high temperature or extreme pH, the active site shape changes and the substrate can no longer bind, so the rate of reaction falls.

What earns the marks

• Describe enzymes as proteins that act as biological catalysts and speed up reactions without being used up.
• Explain that the active site has a specific shape that is complementary to the substrate.
• Describe the formation of an enzyme-substrate complex and the release of products.
• Explain specificity in terms of only a complementary substrate being able to bind to the active site.
• Explain that denaturation changes the shape of the active site so the substrate cannot bind and the reaction is not catalysed.

Common mistakes and how to put them right

• Saying that enzymes are killed by high temperature.

  Put it right by stating that enzymes are denatured, meaning the active site shape changes.

• Stating that the active site changes shape to fit any substrate.

  Put it right by explaining that the substrate must be complementary to the active site.

• Confusing the enzyme with the substrate.

  Put it right by naming the enzyme and its substrate clearly, for example amylase and starch.

Exam tips

• Use the terms active site, substrate and enzyme-substrate complex accurately in your answers.
• Link specificity to the complementary shape of the active site rather than saying enzymes are simply specific.
• When explaining denaturation, state clearly that the active site shape changes and the substrate can no longer bind.