Explain the mechanism of enzyme action including the active site and…

The idea

Enzymes are biological catalysts made of protein. Each enzyme has a region called the active site whose shape is complementary to one particular substrate. In the lock-and-key idea the substrate fits the active site exactly, forming an enzyme–substrate complex; the reaction then occurs and products are released, leaving the enzyme unchanged so it can be reused. Specificity means an enzyme normally catalyses only reactions of substrates whose shape matches its active site — for example, catalase acts on hydrogen peroxide but not on starch. A useful method: name the enzyme, name its substrate, state that the substrate shape is complementary to the active site, describe the complex forming, the reaction occurring and products leaving, then state the enzyme is unchanged and reusable.

What earns the marks

• Enzymes are protein catalysts that speed up reactions without being used up.
• The active site is the specific region of the enzyme where the substrate binds.
• The substrate shape is complementary to the active site, so they fit together.
• An enzyme–substrate complex forms; the reaction occurs and products are released.
• The enzyme is unchanged and can catalyse further reactions.
• Specificity: an enzyme acts only on substrates whose shape matches its active site.

Common mistakes and how to put them right

• Saying the enzyme and substrate have the 'same' shape.

  Put it right: Correct this to complementary shapes that fit together.

• Claiming the enzyme is used up or changed permanently in the reaction.

  Put it right by stating it is unchanged and reusable.

• Describing the active site as changing shape to fit any substrate.

  Put it right by stressing that the active site shape is fixed and specific to particular substrates.

Exam tips

• Always name a specific enzyme and its substrate so the explanation is concrete.
• Use the sequence: substrate approaches → complementary fit at active site → complex → reaction → products released → enzyme unchanged.
• Link the word 'specificity' explicitly to the shape of the active site rather than leaving it implied.