Sometimes tested B8.2

Mechanism of Enzyme Action

Enzymes are protein catalysts that dramatically speed up specific biochemical reactions. Their function depends on a unique region called the active site, which binds to a specific reactant molecule.

Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.

Key points

  • Enzymes are biological catalysts, meaning they increase the rate of a reaction without being consumed in the process.
  • Each enzyme has a unique, three-dimensional structure which includes a specific region called the active site.
  • The reaction occurs when a specific molecule, the substrate, binds to the active site, forming an enzyme-substrate complex.
  • Enzyme action is highly specific; the shape of the active site is complementary to the shape of its specific substrate, often described by the 'lock and key' model.
  • After the reaction, products are released from the active site, and the enzyme is free to catalyse another reaction.
  • The presence and action of specific enzymes determine which metabolic pathways can occur within a cell.

Formulae

Enzyme + Substrate ≤> Enzyme-Substrate Complex → Enzyme + Product(s)

This represents the general sequence of an enzyme-catalysed reaction. Use it to understand the stages of enzyme action from binding to product release.

Definitions

Enzyme
A protein that acts as a biological catalyst to speed up a specific chemical reaction.
Active Site
The specific 3D region on the surface of an enzyme where the substrate binds and the catalytic reaction occurs.
Substrate
The reactant molecule that an enzyme acts upon.
Enzyme Specificity
The principle that an enzyme will only catalyse one specific reaction or a very limited range of reactions, due to the complementary shapes of its active site and substrate.

Worked example

A solution contains two enzymes, E1 and E2, and three potential substrates, S1, S2, and S3, represented by the shapes below. E1's active site is a deep V-shape. E2's active site is a shallow, wide curve. S1 is a small, spherical molecule. S2 is a large molecule with a sharp, pointed end. S3 is a large, bulky molecule with a rounded end. Which enzyme is likely to act on which substrate, and why is no reaction expected with one of the substrates? Assume conditions are optimal.

  1. 1

    Step 1:

    Analyse the active site of enzyme E1.

    It is a deep V-shape.

  2. 2

    Step 2:

    Compare the shapes of the substrates to E1's active site.

    Substrate S2 has a sharp, pointed end which is complementary to the deep V-shape of E1.

    Therefore, E1 will act on S2.

  3. 3

    Step 3:

    Analyse the active site of enzyme E2.

    It is a shallow, wide curve.

  4. 4

    Step 4:

    Compare the shapes of the substrates to E2's active site.

    Substrate S3 has a large, rounded end which is complementary to the shallow, wide curve of E2.

    Therefore, E2 will act on S3.

  5. 5

    Step 5:

    Consider substrate S1.

    It is small and spherical.

    Its shape is not complementary to the active site of either E1 (V-shape) or E2 (wide curve).

    Therefore, no reaction is expected with S1.

Answer: E1 acts on S2, and E2 acts on S3. No reaction occurs with S1 because its shape is not complementary to the active site of either enzyme, demonstrating enzyme specificity.

Common mistakes

  • ×Misinterpreting diagrams of enzyme action. Students sometimes incorrectly match a substrate that only partially fits the active site, forgetting that a precise complementary shape is required for binding.
  • ×Confusing the substrate and the product. Remember the enzyme acts *on* the substrate to *create* the product.
  • ×Overlooking constraints mentioned in the question. For example, if a question states an enzyme is outside its optimal pH or temperature range, its shape may be altered (denatured), preventing any substrate binding even if the shapes would normally be complementary.

No-calculator tips

  • ✓For 'shape-matching' problems, treat the active site as a lock and the substrate as a key. Systematically check each key against each lock to find the single correct fit.
  • ✓When analysing enzyme action, always follow the sequence: Substrate approaches enzyme → Binds to active site → Reaction occurs → Products are released. This helps structure your reasoning.

Read this topic in the official UAT-UK ESAT guide →

All Biology topics