3.1 Mode of Action of Enzymes

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. State that enzymes are globular proteins that catalyse reactions inside cells (intracellular enzymes) or are secreted to catalyse reactions outside cells (extracellular enzymes).
  2. Explain the mode of action of enzymes in terms of an active site, enzyme–substrate complex, lowering of activation energy, and enzyme specificity — including the lock-and-key hypothesis and the induced-fit hypothesis.
  3. Investigate the progress of enzyme-catalysed reactions by measuring rates of formation of products using catalase, and rates of disappearance of substrate using amylase.
  4. Outline the use of a colorimeter for measuring the progress of enzyme-catalysed reactions that involve colour changes.

Section 1: What Are Enzymes?

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Enzymes as Biological Catalysts

An enzyme is a substance that speeds up the rate of a chemical reaction without being used up or permanently changed in the process. This is called being a catalyst — something that makes reactions happen faster.

The word biological simply means "to do with living things." So enzymes are catalysts that work inside living organisms. Almost every chemical reaction that keeps you alive — digesting food, breathing, growing — is controlled by an enzyme. Without enzymes, these reactions would happen far too slowly to support life.

Enzymes Are Globular Proteins

Enzymes are a type of protein. Specifically, they are globular proteins, which means they fold into a rounded, roughly ball-like shape (as opposed to fibrous proteins, which are long and thread-like).

Proteins are made from long chains of smaller molecules called amino acids, joined together by peptide bonds. The order of these amino acids determines how the protein folds. This folding gives each enzyme its own unique, precise three-dimensional shape — and that shape is critical to how the enzyme works.

Intracellular vs. Extracellular Enzymes

Enzymes can work in two locations:

  • Intracellular enzymes — these are produced inside a cell and do their job inside that same cell.

    • Example: Catalase — this enzyme is found inside cells and breaks down a toxic waste product called hydrogen peroxide into harmless water and oxygen.

  • Extracellular enzymes — these are produced inside a cell but are then secreted (released) so they can work outside the cell.

    • Example: Amylase — this is a digestive enzyme released into the gut, where it breaks down starch (a large carbohydrate molecule) into smaller sugars. Food molecules are too large to enter cells, so they must be broken down outside first.

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